Prof. Dr.-Ing. Andreas Ostendorf
Institute of Product Engineering
- Effect of laser shock peening with square laser spot on hardness and residual stress of Ti6Al4V alloy
Wang, H. and Kaufman, J. and Brajer, J. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11994 (2022)
view abstract 10.1117/12.2611090
- Effect of laser shock peening without protective coating on the surface mechanical properties of NiTi alloy
Wang, H. and Keller, S. and Chang, Y. and Kashaev, N. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
Journal of Alloys and Compounds 896 (2022)We study the effect of laser shock peening (LSP) without protective coating on the surface mechanical property of NiTi alloy. The Vickers microhardness and wear resistance are measured to determine the mechanical property of NiTi samples treated with different LSP parameters (3 J with 10 ns and 5 J with 20 ns). From the electron backscatter diffraction (EBSD) analysis, it can be found that the laser shock peening does not induce obvious grain refinement in the surface region of NiTi alloy. Both compressive and tensile residual stress in the top layer are determined using the hole drilling method. The results show that the LSP treatment without a protective coating increases the roughness and enhances the surface mechanical properties of NiTi alloy. © 2021 Elsevier B.V.
view abstract 10.1016/j.jallcom.2021.163011
- Ferromagnetic Cobalt Disulfide: A CVD Pathway Toward High-Quality and Phase-Pure Thin Films
Wree, J.-L. and Glauber, J.-P. and Zanders, D. and Rogalla, D. and Becher, M. and Griffiths, M.B.E. and Ostendorf, A. and Barry, S.T. and Ney, A. and Devi, A.
ACS Applied Electronic Materials 4 (2022)
view abstract 10.1021/acsaelm.2c00685
- GOAT: a multipurpose optical simulation tool
Weigel, T. and Schweiger, G. and Ostendorf, A.
Journal of the Optical Society of America B: Optical Physics 39 (2022)Many solutions exist for optical simulations. Many of them are isolated solutions, are complex to use due to their wide range of applications, or are difficult to adapt to new problems. Therefore, there is a need for easy-to-use, flexibly adaptable program packages. For this reason, we present here a programming library for the simulation of optical problems, which is based on geometrical optics due to its high flexibility. Special attention was paid to a flexible adaptability to different problems and an easy usability. The program package is freely available as an open-source project implemented in C++ and can be downloaded from the GitHub platform (Weigel, GitHub, 2021). © 2022 Optica Publishing Group.
view abstract 10.1364/JOSAB.459574
- Impact of cobalt content and grain growth inhibitors in laser-based powder bed fusion of WC-Co
Schwanekamp, T. and Marginean, G. and Reuber, M. and Ostendorf, A.
International Journal of Refractory Metals and Hard Materials 105 (2022)Processing of tungsten carbide‑cobalt (WC-Co) by laser-based powder bed fusion (PBF-LB) can result in characteristic microstructure defects such as cracks, pores, undesired phases and tungsten carbide (WC) grain growth, due to the heterogeneous energy input and the high thermal gradients. Besides the processing conditions, the material properties are affected by the initial powder characteristics. In this paper, the impact of powder composition on microstructure, phase formation and mechanical properties in PBF-LB of WC-Co is studied. Powders with different cobalt contents from 12 wt.-% to 25 wt.-% are tested under variation of the laser parameters. Furthermore, the impact of vanadium carbide (VC) and chromium (Cr) additives is investigated. Both are known as grain growth inhibitors for conventional sintering processes. The experiments are conducted at a pre-heating temperature of around 800 °C to prevent crack formation in the samples. Increasing laser energy input reduces porosity but leads to severe embrittlement for low cobalt content and to abnormal WC grain growth for high cobalt content. It is found that interparticular porosity at low laser energy is more severe for low cobalt content due to poor wetting of the liquid phase. Maximum bending strength of σB > 1200 MPa and Vickers hardness of approx. 1000 HV3 can be measured for samples generated from WC-Co 83/17 powder with medium laser energy input. The addition of V and Cr leads to increased formation of additional phases such as Co3W3C, Co3V and Cr23C6 and to increased lateral and multi-laminar growth of the WC grains. In contrast to conventional sintering, a grain growth inhibiting effect of V and Cr in the laser molten microstructure is not achieved. © 2022 Elsevier Ltd
view abstract 10.1016/j.ijrmhm.2022.105814
- Laser performance of LD side-pumped Er:YSGG crystal rods with different diameters
Hu, L. and Sun, D. and Hou, Y. and Gurevich, E.L. and Ostendorf, A. and Guo, Q.
Proceedings of SPIE - The International Society for Optical Engineering 12310 (2022)
view abstract 10.1117/12.2641626
- Layer thickness controlling in Direct Energy Deposition process by adjusting the powder flow rate
Behlau, F. and Thiele, M. and Maack, P. and Esen, C. and Ostendorf, A.
Procedia CIRP 111 (2022)
view abstract 10.1016/j.procir.2022.08.033
- Low-temperature ALD process development of 200 mm wafer-scale MoS2 for gas sensing application
Neubieser, R.-M. and Wree, J.-L. and Jagosz, J. and Becher, M. and Ostendorf, A. and Devi, A. and Bock, C. and Michel, M. and Grabmaier, A.
Micro and Nano Engineering 15 (2022)
view abstract 10.1016/j.mne.2022.100126
- Machine-learning based analysis of time sequences for multiplexed microresonator sensor
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 12139 (2022)This paper discusses an application of machine-learning solution for processing of the dynamical sensing responses collected with a multiplexed microresonator detector. Performance of a long short-term memory network (LSTM) out of bidirectional and dropout layers is analyzed on example of the experimental data collected for a temporal gradient of the local refractive index. We experimentally demonstrate the possibility for analyte parameters prediction with accuracy of > 99% based on a set of complex non-linear highly specific time sequences of the intensities radiated by the microcavities which is obtained within a timescale 4 times shorter than required to reach the steady state. Optimization possibilities in terms of the number of microresonator signals to consider for the LSTM network training along with the complexity of its architecture are analyzed. © 2022 SPIE.
view abstract 10.1117/12.2621383
- Nucleation and growth studies of large-area deposited WS2 on flexible substrates
Berning, T. and Becher, M. and Wree, J.-L. and Jagosz, J. and Kostka, A. and Ostendorf, A. and Devi, A. and Bock, C.
Materials Research Express 9 (2022)
view abstract 10.1088/2053-1591/ac9bd0
- Plasma-Enhanced Atomic Layer Deposition of Molybdenum Oxide Thin Films at Low Temperatures for Hydrogen Gas Sensing
Wree, J.-L. and Rogalla, D. and Ostendorf, A. and Schierbaum, K.D. and Devi, A.
ACS Applied Materials and Interfaces (2022)
view abstract 10.1021/acsami.2c19827
- Surface modification of silicon by femtosecond laser ablation in liquid
Maack, P. and Kanitz, A. and Hoppius, J. and Köhler, J. and Esen, C. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11989 (2022)Pulsed laser ablation is steadily gaining popularity in micromachining to keep pace with the increasing demand for precision manufacturing and functional surfaces. However, efficient laser processing under atmospheric conditions primarily suffers from particle redeposition and therefore requires additional cleaning steps to obtain high surface quality. To reduce additional cleanings steps after manufacturing, laser ablation in liquid allows for a significant reduction in particle redeposition as particles rapidly cool down and penetrate into the liquid without stitching to the surface. However, laser ablation in liquid is accompanied by the complex interaction between the hot molten material, the generated plasma and the over-critical liquid in the ablation zone. During this interaction, chemical reactions at the surface can take place and cause a persistent change of surface chemistry. Since the surface chemistry is a key aspect for micromachining, the interaction has to be studied to determine whether laser processing in liquids can be a feasible alternative to laser processing under ambient atmospheric conditions while reducing the problem of redeposition. Here, we present the results on the change of surface chemistry by laser ablation in liquid of a pristine silicon substrate. The micromachining process is either performed in an aqueous or gaseous environment and studied in dependence of laser intensity. The changes in surface chemistry are evaluated by micro-Raman spectroscopy and EDX. Copyright © 2022 SPIE.
view abstract 10.1117/12.2608708
- Theoretical simulation and experimental verification of dynamic caustic manipulation using a deformable mirror for laser material processing
Smarra, M. and Gurevich, E.L. and Ostendorf, A.
Optics and Laser Technology 149 (2022)The influence of a deformable mirror on spatial light modulation in ultrafast lasers processing is demonstrated. The deformable mirror was integrated into an optical setup which contains an additional lens for generating a nearly linear focus shift in the focal plane behind the f-theta lens. The deformation of the mirror surface can be described by the Zernike terms Defocus, Astigmatism, and a combination of both, resulting in a cylindric lens behavior. The influence of the mirror surface deformation in this optical setup on the intensity distribution in the focal plane was simulated. From the simulation results, the caustic in the focal plane was calculated. The simulation results were compared to experiments using a picosecond laser with a maximum pulse energy of about 60 µJ. We demonstrate that the initial astigmatism of the raw beam can be reduced using the deformable mirror. A linear focus shift (R2=98.7%) and the generation of elliptical/ line intensity distributions are shown. Line intensity distribution was used to demonstrate slit drilling application in thin metal foils. © 2021
view abstract 10.1016/j.optlastec.2021.107814
- Additive manufacturing of PA12 carbon nanotube composites with a novel laser polymer deposition process
Wencke, Y.L. and Kutlu, Y. and Seefeldt, M. and Esen, C. and Ostendorf, A. and Luinstra, G.A.
Journal of Applied Polymer Science 138 (2021)The facile manufacture of PA12 MWCNT/silica (50/50 by weight) nanocomposite powders through a high energy mixing process is presented, which are useful to generate 3D objects by a novel Laser Polymer Deposition (LPD) process. The mixing as well as the LPD process led to no discernible changes in the material properties (DSC, SEM, LD) of the core-shell nanocomposites, enabling the recycling of unconverted powder. The built parts yield ultimate tensile stresses and Young's modulus at 10%–20% of the bulk material. Partially unmolten particles and voids were identified as the main mechanical failure mechanism in the built parts. The mechanical properties are better with low additive content (Young's modulus: 89.8 ± 5.4 MPa; UTS: 12.9 ± 5.3 MPa with 0.25 wt% additives). Electronic conductivity up to the region of moderate conductivity could be achieved by multiwalled carbon nanotube (MWCNT) network formation (8 × 10−4 S cm−1 at 1.25 wt% of additives). A variant of the processing strategy revealed that a higher mechanical strength can be achieved by a laser induced remelting of the traces following their initial construction. © 2020 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC.
view abstract 10.1002/app.50395
- Application of nanoindentation technique to test surface hardness and residual stress of NiTi alloy after femtosecond laser shock peening
Wang, H. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11679 (2021)Laser shock peening is a new and important surface treatment technique that can enhance the mechanical properties of metal materials. Normally, the nanosecond laser with pulse-width between 5 ns and 20 ns is used to induce a high-pressure shock wave that can generate plastic deformation in the top layer of metals. The femtosecond laser shock peening in the air has been studied recently, which can induce higher pressure shock wave than that of traditional nanosecond laser shock peening in a very short time. The NiTi alloy is processed by femtosecond laser shock peening, then a nanoindentation device is used to measure its surface hardness and residual stress. The hardness results of NiTi alloy before and after treatment show that the femtosecond laser shock peening can increase the hardness of NiTi alloy, which also shows that the femtosecond laser can be used to perform laser shock peening on NiTi alloy without coating. © 2021 SPIE.
view abstract 10.1117/12.2593092
- Direct generation of 3D structures by laser polymer deposition
Thiele, M. and Kutlu, Y. and Dobbelstein, H. and Petermann, M. and Esen, C. and Ostendorf, A.
Journal of Laser Applications 33 (2021)Additive manufacturing with polymers is typically performed using techniques such as stereolithography, selective laser sintering (SLS), or fused deposition modeling. SLS of unmodified powders with CO2 lasers represents the state of the art in powder-based polymer additive manufacturing. In the presented work, thermoplastic polyurethane was successfully processed for the first time with a powder feed technique, which is similar to the well-known laser metal deposition. The powder material was doped with carbon black in order to increase the absorptivity of the powder material for laser radiation in the near-infrared range. Various geometries were produced using a standard laser cladding setup with a modified powder feeding system and an Nd:YAG laser. The powder material and the generated structures were characterized by scanning electron microscopy. Structural properties, e.g., porosity, were controlled by different fabrication strategies and process parameters. Furthermore, hybrid structures consisting of metal and polymer parts were successfully produced in the same experimental setup by using two different powder feeders. © 2021 Author(s).
view abstract 10.2351/7.0000166
- Intelligent imaging sensor out of two-photon polymerized microcavities with self-sensing boosting
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2021 (2021)The recent advances of the artificial intelligence (AI) technologies are pushing forward the development of the novel concepts for sensing data interpretation wherein the knowledge on the exact physical models for transformation of the collected raw signal into the desired parameters becomes optional. Compared to other types of sensors, the optical ones have demonstrated a rush progress as well as a great amount of innovations over the last years. Despite the main emphasis here on the utilization of the camera-based solutions  , the emerging AI-inspired solutions cover various optical sensing platforms, where the flexibility of the machine learning methods is combined with the highly sensitive mechanisms of the optical field interaction with the ambient environment. © 2021 IEEE.
view abstract 10.1109/CLEO/Europe-EQEC52157.2021.9542158
- Life cycle strengthening of high-strength steels by nanosecond laser shock
Bai, Y. and Wang, H. and Wang, S. and Huang, Y. and Chen, Y. and Zhang, W. and Ostendorf, A. and Zhou, X.
Applied Surface Science 569 (2021)Laser shock peening is a surface treatment technology inducing high-pressure shock waves on metallic materials and structures. In this study, the life cycle performance improvement in high-strength steel by nanosecond laser shock peening is investigated. It is found that microstructures formed by laser shock peening lead to higher microhardness, corrosion resistance, and fatigue life, which are significantly beneficial for preventing life cycle failure of mega-scale engineering structures in critical environments. The residual stress is also measured on the surface of samples, which shows that the compressive residual stress can be found in the treated area. © 2021 Elsevier B.V.
view abstract 10.1016/j.apsusc.2021.151118
- Optimization of processing parameters of ultrashort (100 fs-2 ps) pulsed laser shock peening of stainless steel
Kukreja, L.M. and Hoppius, J.S. and Elango, K. and Macias Barrientos, M. and Pöhl, F. and Walther, F. and Gurevich, E. and Ostendorf, A.
Journal of Laser Applications 33 (2021)The authors optimized the processing parameters of laser shock peening (LSP) of stainless steel, taken as a representative metal, using a Ti:sapphire laser with pulse durations in the range of 100 fs-2 ps. It was found that direct exposure of the metal surface to these laser pulses invariably resulted in the formation of laser induced periodic surface structures (LIPSS) on the metal. If LSP was carried out under an aqueous confinement medium, the stainless steel surface was observed to get oxidized without the protective role of the sacrificial layer. Various sacrificial layers were optimized to prevent LIPSS and surface oxidation to achieve maximum peening efficiency. Attenuation of the laser energy due to filamentation and white light generation in the confinement medium of de-ionized water was studied. It was found that 100 fs laser pulses produced much earlier and longer filamentation than those with a pulse duration of 2 ps at the same pulse energy of about 1 mJ. The energy lost in the attenuation mechanisms of filamentation and white light generation was found to be about 60% at the laser pulse duration of 100 fs and only about 20% at 2 ps. These effects are explained in terms of self-focusing and self-phase modulation of the laser light. Keeping filamentation-free length of different confinement media, peening efficiency on stainless steel was investigated for 2 ps laser pulses at different laser fluences. It was found that the maximum achievable hardness of stainless steel increased proportionately with acoustic impedance of the used confinement medium. © 2021 Author(s).
view abstract 10.2351/7.0000475
- Self-learning-based detection via multiple microresonator imaging
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11782 (2021)This paper discusses an intelligent sensing solution based on the phenomenon of the whispering gallery modes in the optical microcavities realized within an affordable instrument configuration featuring simultaneous excitation of multiple microresonators by a single frequency laser along with parallel collection of their signal. Supplemented with a machine learning engine for complex signal interpretation the sensor demonstrates the accuracy of ∼ 10−6 for refractive index prediction and more than 98% for protein concentration classification. © 2021 SPIE · CCC code: 0277-786X/21/$21
view abstract 10.1117/12.2592365
- The effect of laser nitriding on surface characteristics and wear resistance of niti alloy with low power fiber laser
Wang, H. and Nett, R. and Gurevich, E.L. and Ostendorf, A.
Applied Sciences (Switzerland) 11 (2021)The laser nitriding was performed in nitrogen gas at room temperature (20◦ C) and low temperature (−190◦ C) by a low power fiber laser to modify the wear and abrasion resistance of NiTi alloy. The surface roughness and element composition were analyzed by roughness device and energy-dispersive X-ray spectroscopy respectively. The results of roughness show that laser treatment can change the surface roughness due to the laser remelting. The effect of laser nitriding on the microhardness, friction coefficient, and worn scars of NiTi alloy was also studied, which shows that the microhardness of the NiTi alloy increases after laser nitriding. The optical microscope and scanning electron microscope were used to characterize the surface of NiTi alloy after wear testing to observe the microstructure of worn scars. The results show that the laser nitriding with different parameters can induce a nitride layer with different thicknesses and the higher energy deposition is the key factor for the formation of the nitride layer, which can decrease the friction coefficient and reduce wear loss during the application of NiTi alloy. The improvement of wear resistance can be attributed to the hard nitriding layer. © 2021 by the authors. Li-censee MDPI, Basel, Switzerland.
view abstract 10.3390/app11020515
- A Laser Written 4D Optical Microcavity for Advanced Biochemical Sensing in Aqueous Environment
Saetchnikov, A.V. and Tcherniavskaia, E.A. and Saetchnikov, V.A. and Ostendorf, A.
Journal of Lightwave Technology 38 (2020)The topic of 4D printing, that is based on 3D printing of structures capable to response on external disturbances over the time, has evolved enormous interest in science and engineering since its first statement in 2013. Besides multi-material printing originally intended to provide self-assembly function, the interest in use of smart materials capable for self-sensing and/or self-actuating rises. Here the necessity in precise 3D fabrication techniques and solutions for structural response control are current challenges that 4D printing faces. In this paper we report on design, fabrication and characterization of a first example of 4D optical microcavity that shows advanced sensing performance boosted by self-sensing in the form of interaction of the polymer matrix with sensed molecules in aqueous environment. Distinctive features of 4D microcavity spectral response for bulk refractive index variations and sensitivity enhancement for more than one order of magnitude have been experimentally observed and discussed. Possibility for parallel interrogation with a single coupling and detection unit for a set of 4D microresonators fabricated with two-photon polymerization out of quencher-doped SZ2080 material with quality factor of $4\times 10^4$ at 685 nm without any post-processing steps has been experimentally demonstrated. The proposed 4D microcavity-based sensor opens a new avenue for designing of a multiplexed 4D printed optical sensing platform with advanced performance compared to conventional optical detection techniques for broad variety of detection tasks. © 1983-2012 IEEE.
view abstract 10.1109/JLT.2020.2973933
- A new metalorganic chemical vapor deposition process for MoS2with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur
Wree, J.-L. and Ciftyurek, E. and Zanders, D. and Boysen, N. and Kostka, A. and Rogalla, D. and Kasischke, M. and Ostendorf, A. and Schierbaum, K. and Devi, A.
Dalton Transactions 49 (2020)Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2films. The new MOCVD process developed for MoS2is highly promising due to its moderate process conditions, scalability and controlled targeted composition. © The Royal Society of Chemistry 2020.
view abstract 10.1039/d0dt02471f
- Application of dispersed microresonator based sensor for aerospace-related tasks
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
2020 IEEE International Workshop on Metrology for AeroSpace, MetroAeroSpace 2020 - Proceedings (2020)A possible application of the dispersed optical microresonator based sensor in aerospace-related tasks is discussed. Configuration of the proposed sensor as a set of spherical glass microparticles together with its working principle is discussed. Sensing capabilities have been verified on examples of detection of temperature variations together with monitoring of the small microparticles adsorption. Thermal sensitivity is determined on the level of 8.6 pm/°C. The possibility for detection of the adsorption of polesterene microparticles (0.9 and 3.27 µ m size) in frequency sweeping and spectrally unresolved illumination schemes have been shown. © 2020 IEEE.
view abstract 10.1109/MetroAeroSpace48742.2020.9160079
- Benchmarking and validation of a combined CFD-optics solver for micro-scale problems
Münster, R. and Mierka, O. and Turek, S. and Weigel, T. and Ostendorf, A.
OSA Continuum 3 (2020)In this work, we present a new approach for coupled CFD-optics problems that consists of a combination of a finite element method (FEM) based flow solver with a ray tracing based tool for optic forces that are induced by a laser. We combined the open-source computational fluid dynamics (CFD) package FEATFLOW with the ray tracing software of the LAT-RUB to simulate optical trap configurations. We benchmark and analyze the solver first based on a configuration with a single spherical particle that is subjected to the laser forces of an optical trap. The setup is based on an experiment that is then compared to the results of our combined CFD-optics solver. As an extension of the standard procedure, we present a method with a time-stepping scheme that contains a macro step approach. The results show that this macro time-stepping scheme provides a significant acceleration while still maintaining good accuracy. A second configuration is analyzed that involves non-spherical geometries such as micro rotors. We proceed to compare simulation results of the final angular velocity of the micro rotor with experimental measurements. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
view abstract 10.1364/OSAC.399876
- Concept development for the generation of support structures in the laser metal deposition process
Marx, J. and Thiele, M. and Esen, C. and Ostendorf, A.
Procedia CIRP 94 (2020)For most additive manufacturing technologies, complex components require a support structure for thermal dissipation and printability. This paper shows for the laser metal deposition process, how weld spot pillars can be used as a support structure for surfaces with large overhangs produced on a three-axis system. A self-written software is used for generating machine and motion sequences, where layered construction of these structures is not mandatory. In the first step, parameters for generating different build strategies of single pillars were found. In a second step, different concepts of combining pillars are presented and compared. © 2020 The Authors. Published by Elsevier B.V.
view abstract 10.1016/j.procir.2020.09.054
- Corrosion behavior of NiTi alloy subjected to femtosecond laser shock peening without protective coating in air environment
Wang, H. and Jürgensen, J. and Decker, P. and Hu, Z. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
Applied Surface Science 501 (2020)Laser shock peening with femtosecond laser was used to improve the corrosion resistance of biomedical NiTi alloy without protective coating in the air environment. The energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) based analysis showed that the laser ablation could produce titanium oxide layer and femtosecond laser shock peening (FsLSP) can generate residual stress in the surface layer of NiTi alloy. The FsLSP improved the corrosion resistance of NiTi in 3.5% NaCl solution and Hank's solution and also prevented the formation of corrosion cracks and pits during corrosion testing. The reasons for the improvement of corrosion behavior may be the generation of residual stress and titanium oxide film during the laser surface treatment. © 2019 Elsevier B.V.
view abstract 10.1016/j.apsusc.2019.144338
- Deep-learning powered whispering gallery mode sensor based on multiplexed imaging at fixed frequency
Saetchnikov, A.V. and Tcherniavskaia, E.A. and Saetchnikov, V.A. and Ostendorf, A.
Opto-Electronic Advances 3 (2020)During the last decades the whispering gallery mode based sensors have become a prominent solution for label-free sensing of various physical and chemical parameters. At the same time, the widespread utilization of the approach is hindered by the restricted applicability of the known configurations for ambient variations quantification outside the laboratory conditions and their low affordability, where necessity on the spectrally-resolved data collection is among the main limiting factors. In this paper we demonstrate the first realization of an affordable whispering gallery mode sensor powered by deep learning and multi-resonator imaging at a fixed frequency. It has been shown that the approach enables refractive index unit (RIU) prediction with an absolute error at 3×10-6 level for dynamic range of the RIU variations from 0 to 2×10-3 with temporal resolution of several milliseconds and instrument-driven detection limit of 3×10-5. High sensing accuracy together with instrumental affordability and production simplicity places the reported detector among the most cost-effective realizations of the whispering gallery mode approach. The proposed solution is expected to have a great impact on the shift of the whole sensing paradigm away from the model-based and to the flexible self-learning solutions. © 2019 Institute of Optics and Electronics, Chinese Academy of Sciences.
view abstract 10.29026/oea.2020.200048
- Design and application of distributed microresonator-based systems for biochemical sensing
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11354 (2020)A design and application of distributed microresonator-based systems for biochemical sensing are discussed. Two different configurations of the distributed sensors based on integration of the spherical glass microresonators and fabrication of the toroidal microresonators via two-photon polymerization have been demonstrated. The performance of both configurations for biochemical sensing has been analysed. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2555391
- Development of a Method for Non-Invasive Measurement of Absolute Pressure in Partially Transparent Containers with Carbonated Beverages [Entwicklung einer Methode zur nichtinvasiven Messung des Absolutdrucks in teiltransparenten Gebinden mit karbonisierten Getränken]
Grafen, M. and Falkenstein, M. and Ostendorf, A. and Esen, C.
Chemie-Ingenieur-Technik 92 (2020)A non-invasive optical measurement technique for food analysis is presented, which allows for a reliable determination of the absolute pressure in beverage bottles with carbonated contents. The method uses a tunable laser diode with an emission wavelength around 2004 nm to record three to four absorption lines of CO2 and evaluates the pressure broadening of the lines proportional to the absolute pressure. With the developed measuring method, a standard deviation of repeated absolute pressure measurements of up to 5.5 bar of less than 50 mbar could finally be achieved in field measurements on sealed soft drink bottles made of PET. © 2020, Wiley-VCH Verlag. All rights reserved.
view abstract 10.1002/cite.202000198
- Directed Energy Deposition of PA12 carbon nanotube composite powder using a fiber laser
Kutlu, Y. and Wencke, Y.L. and Luinstra, G.A. and Esen, C. and Ostendorf, A.
Procedia CIRP 94 (2020)Directed Energy Deposition (DED) an AM-technology that is widely known for its applications with metals was adapted for use with thermoplastics. A drop-in approach was used based on a DED setup for metal powders consisting of a Yb fiber laser with a wavelength of 1075 nm and a powder feed from a coaxial powder nozzle. The possibility of manufacturing items with simple geometries from a polyamide 12 multi-walled carbon nanotube composite starting material is described as a proof of concept. Tensile properties of samples are reported for measurements perpendicular and parallel to the generating direction, showing still a low strength compared to Bulk PA12 or SLS sintered PA12 specimens. © 2020 The Authors. Published by Elsevier B.V.
view abstract 10.1016/j.procir.2020.09.025
- Femtosecond laser shock peening on the surface of NiTi shape memory alloy
Wang, H. and Gurevich, E.L. and Ostendorf, A.
Procedia CIRP 94 (2020)Laser shock peening with a femtosecond laser system was presented in this research work. The NiTi shape memory alloy was processed by the femtosecond laser shock peening (FsLSP) treatment without a protective layer in the air. Femtosecond laser shock peening is a new surface technology, which can induce an intense shock wave with low single laser pulse energy under atmospheric conditions. The surface topography, roughness, microhardness, and wear resistance were measured on the surface of NiTi alloy before and after femtosecond laser peening treatment. The results showed that the surface roughness and microhardness could be increased after femtosecond laser shock peening, which may be due to the laser ablation and micro-plastic deformation induced by the shock wave. The wear property of NiTi alloy was improved, which may be attributed to the FsLSPed surface texturing and enhancement of surface microhardness. © 2020 The Authors. Published by Elsevier B.V.
view abstract 10.1016/j.procir.2020.09.071
- Geometrical-optics analysis of whispering-gallery modes in the layer of a coated spherical resonator
Schweiger, G. and Weigel, T. and Ostendorf, A.
Physical Review A 102 (2020)The resonance properties of a layer on the surface of a spherical microresonator are investigated using geometrical optics. The resonance condition for these shell-guided modes is formulated. Shell-guided modes are located in the layer and are bounded by the inner and outer surfaces of the layer. It is shown that the resonance condition depends - similarly to the case of a homogeneous resonator - on the difference of the phase propagation of a ray crossing the layer and the corresponding phase propagation along the associated caustic. It is shown that the layer can support resonances only in a certain thickness range that depends, among other factors, on the resonance order and the wave number. A straightforward explanation is given for these limitations. Comparison with solutions from wave theory shows excellent agreement. A simple analytical solution is presented to calculate the sensitivity of the resonator to the refractive index and the thickness of the layer as well as to the refractive index of the environment. © 2020 American Physical Society.
view abstract 10.1103/PhysRevA.102.053506
- Laser metal deposition of refractory high-entropy alloys for high-throughput synthesis and structure-property characterization
Dobbelstein, H. and George, E.P. and Gurevich, E.L. and Kostka, A. and Ostendorf, A. and Laplanche, G.
International Journal of Extreme Manufacturing 3 (2020)Progress in materials development is often paced by the time required to produce and evaluate a large number of alloys with different chemical compositions. This applies especially to refractory high-entropy alloys (RHEAs), which are difficult to synthesize and process by conventional methods. To evaluate a possible way to accelerate the process, high-throughput laser metal deposition was used in this work to prepare a quinary RHEA, TiZrNbHfTa, as well as its quaternary and ternary subsystems by in-situ alloying of elemental powders. Compositionally graded variants of the quinary RHEA were also analyzed. Our results show that the influence of various parameters such as powder shape and purity, alloy composition, and especially the solidification range, on the processability, microstructure, porosity, and mechanical properties can be investigated rapidly. The strength of these alloys was mainly affected by the oxygen and nitrogen contents of the starting powders, while substitutional solid solution strengthening played a minor role. © 2020 The Author(s). Published by IOP Publishing Ltd
view abstract 10.1088/2631-7990/abcca8
- Laser shock peening on high-strength steel
Wang, H. and Keller, S. and Bai, Y. and Kashaev, N. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11546 (2020)High strength steel has been used in the aviation industry and automotive body structural applications to reduce its mass through a reduction in thickness. Therefore, it is very important to enhance its mechanical property, such as microhardness. In the present research, the high strength steel samples were treated by laser shock peening (LSP) with different laser pulse energy and laser pulse width. The microhardness and residual stress were measured to compare the difference between laser energy of 3 J with 10 ns and 5 J with 20 ns. The results in the study show that the surface LSP treatment can increase the microhardness and the compressive residual stress can be found when the samples were tested by hole drilling testing. © 2020 SPIE.
view abstract 10.1117/12.2574988
- Microhardness and microabrasion behaviour of NiTi shape memory alloy after femtosecond laser shock peening without coating in air
Wang, H. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11273 (2020)The experiment study presents the influence of femtosecond laser shock peening (FsLSP) without a protective layer in the air on the surface hardness and surface mechanical property of NiTi shape memory alloy. Femtosecond laser shock peening is a new possibility of direct laser ablation without any protective layer under atmospheric conditions, which can produce intense shock waves with low pulse energy in the air. The average surface roughness values of the NiTi alloy samples were measured, because the surface roughness may affect its friction resistance. The results showed that the surface roughness of NiTi increased after femtosecond laser shock peening treatment. In comparison with the initial state, the coefficient of friction decreased and surface microhardness increased after femtosecond laser shock peening treatment with different FsLSP parameters. This improvement of wear properties may be attributed to the enhancement of surface microhardness and surface titanium oxide layer induced by the shock wave and laser ablation during FsLSP treatment. © 2020 SPIE.
view abstract 10.1117/12.2543550
- Mode-locked diode laser-based two-photon polymerisation
Surkamp, N. and Zyla, G. and Gurevich, E.L. and Klehr, A. and Knigge, A. and Ostendorf, A. and Hofmann, M.R.
Electronics Letters 56 (2020)In this Letter, the authors present the construction of three-dimensional microstructures by two-photon polymerisation induced by ultrashort pulses of a mode-locked diode laser. The ultrafast light source is based on a diode laser with segmented metallisation to realise a waveguide integrated saturable absorber. It is subsequently amplified and compressed resulting in ultrashort laser pulses of 440 fs length and average output power of 160 mW at a fundamental repetition rate of 383.1 MHz. These pulses are coupled into a customised two-photon polymerisation setup. A series of suspended lines were fabricated between support cuboids for testing the process behaviour. A 3D structure with complex features was polymerised to demonstrate the high potential for mode-locked diode lasers in the field of direct laser writing. © The Institution of Engineering and Technology 2020.
view abstract 10.1049/el.2019.2385
- Mode-locked diode lasers as sources for two-photon polymerization
Surkamp, N. and Zyla, G. and Gurevich, E.L. and Esen, C. and Klehr, A. and Knigge, A. and Ostendorf, A. and Hofmann, M.R.
Proceedings of SPIE - The International Society for Optical Engineering 11349 (2020)In this study, we use a hybrid mode-locked external cavity diode laser with subsequent amplification and pulse compression. The system provides laser pulses of 440 fs width (assuming a sech pulse shape) and 160 mW average output power at a repetition rate of 383.1 MHz. The laser oscillator consists of a double quantum well laser diode with a gain segment of 1080 μm length and an absorber element of 80 μm lengths. The chip's back facet is covered with a high reflective coating, the front facet with an anti-reflective coating. The resonator itself is operated in a collimated geometry and folded by two dielectric mirrors. The used output coupler provides a transmission of 20 percent, which is coupled into a tapered amplifier. Two Faraday isolators are used to decouple the laser and the amplifier from any back reflections. Subsequently, the pulses are compressed using a single pass Martinez type pulse compressor. Experiments on Two-Photon Polymerization were conducted on a conventional setup consisting of a 2D galvo-scanner system with an attached microscope objective. The oil immersion objective (NA =1.4) focusses the light pulses through a cover glass into a droplet of the photosensitive material. Process monitoring can be achieved by observing the image on a camera placed behind a semi-transparent mirror in front of the galvo-scanner. Using this experimental setup, test structures that consist of free-hanging lines supported by cuboids were produced. In addition, a procedure for automated linewidth measurements is outlined and used for analyzation of the generated structures. This work shows that mode-locked diode lasers can be used for the fabrication of microstructures by Two-Photon Polymerization. Typically used Ti:Sapphire or fiber lasers can be replaced by mode-locked diode lasers for Two-Photon- Polymerization. This allows for much cheaper Two-Photon-Polymerization systems and therefore, may open this field for more application-based research groups. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2555890
- Structural colors with angle-insensitive optical properties generated by Morpho-inspired 2PP structures
Zyla, G. and Kovalev, A. and Gurevich, E.L. and Esen, C. and Liu, Y. and Lu, Y. and Gorb, S. and Ostendorf, A.
Applied Physics A: Materials Science and Processing 126 (2020)This paper describes how two-photon polymerization was used to generate biomimetic nanostructures with angle-insensitive coloration inspired by the blue butterflies of Morpho. Less angle dependence was achieved by engineering the structures with a certain degree of disorder, which delimited them from classical photonic crystals. Variations in the processing parameters enabled the color hue to be controlled. In this context, blue, green, yellow, and brown structures were demonstrated. Reflection spectra of the structures were simulated and studied experimentally in a broad range of incident angles. Additionally, a molding technique was performed as a potential scale-up strategy. The application of such biomimetic structures is discussed. © 2020, The Author(s).
view abstract 10.1007/s00339-020-03931-6
- Studies on ultra-short pulsed laser shock peening of stainless-steel in different confinement media
Elango, K. and Hoppius, J.S. and Kukreja, L.M. and Ostendorf, A. and Gurevich, E.L.
Surface and Coatings Technology 397 (2020)We investigate the role of liquid confinement media on ultra-short pulsed Laser Shock Peening (LSP). The LSP of stainless-steel 316 and 316 L was studied using Ti: Sapphire laser pulses of about 2 ps duration, maximum energy of about 1 mJ and pulse repetition rate of 5 kHz in different liquid confinement media of Ethanol, Deionized water and separate aqueous solutions of NaCl and Glycerol. It is found that the laser fluence and/or energy attenuating mechanisms like self-focusing, filamentation, plasma breakdown in the confinement media are less significant with ps laser pulses than those with sub-ps or fs pulse durations. It is shown that the resulting surface hardness of the peened steel as a function of laser fluence depends significantly on the confinement media and the relative increase in the hardness increases monotonically with the acoustic impedance of the liquid of the confinement medium used during LSP. © 2020 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2020.125988
- Surface modification of NiTi alloy by ultrashort pulsed laser shock peening
Wang, H. and Kalchev, Y. and Wang, H. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
Surface and Coatings Technology 394 (2020)This research paper presents the attempt at ultrashort pulsed laser shock peening with absence of absorptive layer and confining medium which could enhance surface microhardness and the abrasion property of NiTi shape memory alloy. The average roughness values of NiTi specimen were measured on the surface, because the roughness would affect the friction resistance. The microhardness and Young's modulus were investigated at different position of single laser spot by nanoindentation technique. The pin-on-plate sliding abrasion testing were performed with different load-force (0.5 N and 2 N) for different testing time. Results showed that ultrashort pulsed laser shock peening treatment would cause a significant improvement on friction coefficient and abrasion property, which was attributed to the change of surface modification, such as roughness, microhardness, microstructure and titanium oxide layer, but the ultrashort pulsed laser shock peening treatment did not enhance its tensile strength during present research. © 2020 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2020.125899
- Two-photon polymerization with diode lasers emitting ultrashort pulses with high repetition rate
Zyla, G. and Surkamp, N. and Gurevich, E.L. and Esen, C. and Klehr, A. and Knigge, A. and Hofmann, M.R. and Ostendorf, A.
Optics Letters 45 (2020)In this Letter, we investigate the resolution of two-photon polymerization (2PP) with an amplified mode-locked external cavity diode laser with adjustable pulse length and a high repetition rate. The experimental results are analyzed with a newly developed 2PP model. Even with low pulse peak intensity, the produced structural dimensions are comparable to those generated by traditional 2PP laser sources. Thus, we show that a compact monolithic picosecond laser diode without amplification and with a repetition rate in the GHz regime can also be applied for 2PP. These results show the high application potential of compact mode-locked diode lasers for low-cost and compact 2PP systems. © 2020 Optical Society of America
view abstract 10.1364/OL.401738
- Application of high powered Laser Technology to alter hard rock properties towards lower strength materials for more efficient drilling, mining, and Geothermal Energy production
Jamali, S. and Wittig, V. and Börner, J. and Bracke, R. and Ostendorf, A.
Geomechanics for Energy and the Environment (2019)Resources such as minerals, geothermal energy, and unconventional hydrocarbons need to be excavated from deeper and harder geologic formations than in the past 100 years. Although, oil & gas drilling technologies have been continuously enhanced to enable more efficient and economic drilling processes, yet their performance suffers significantly in deep and hard formations. Problems mainly include low rate of penetrations, very high drill-bit wear-rates, short drill bit service life, and ultimately result in a high overall drilling cost. Laser Jet Drilling could potentially be such next fundamental drilling process for delivering more and alternative energy to the bit and breaking the rock more efficiently and thus, substantially improve drilling of deep hard rock formations. The application of high-power laser for rock breaking and drilling processes has been demonstrated in this study via Laser (thermally) induced rock softening. A comprehensive experimental approach has been chosen to study the LaserJet drilling process and its rock interaction. Multiple rock types including sandstones, granites, and metamorphic quartzite rocks were characterized to offer a comprehensive overview of possible effects of rock parameters on laser–rock interaction. The initial research results proved that various rock types including very hard ones could be transformed into a drillable state using a high-power laser through thermally induced rock softening. The rock strength, drilling strength and fracture toughness of the studied samples were obtained by means of scratch testing. The overall analysis of the results suggests that thermal rock softening allows to efficiently facilitate a combined laser mechanical assisted drilling system with a possible increase in the rate of penetration of the cutting tool with reduced drilling torque and weight on bit requirements and therefore having a lower energy consumption. Additionally, the drilling costs could be noticeably lessened by improving the drill bit life and reducing the overall drilling time. © 2019
view abstract 10.1016/j.gete.2019.01.001
- Biochemical kinetic parameters evaluation by resonator-based multiplexed sensor
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)Label-free sensing method based on optical resonance of whispering gallery modes is one of the promising techniques for biochemical sensing at the micro- and nanometer scale [1,2]. Despite the well-researched advantages of these sensors the following improvements are required for its widespread implementation as the real world-deployable systems: long-term stability, noise suppression, multiplexing, fabrication simplification and reusability . A developed chip with array of spherical resonators is one of the trade-off solutions by means of complexity, sensitivity and efficient mass production . Bulk refractive index sensing or absorption monitoring is commonly used to evaluate analyte properties. Here we report on quantification of the adsorption kinetic parameters by a multiplexed optical resonance sensor. © 2019 IEEE.
view abstract 10.1109/CLEOE-EQEC.2019.8873025
- Biomimetic structural coloration with tunable degree of angle-independence generated by two-photon polymerization
Zyla, G. and Kovalev, A. and Heisterkamp, S. and Esen, C. and Gurevich, E.L. and Gorb, S. and Ostendorf, A.
Optical Materials Express 9 (2019)A successful realization of photonic systems with characteristics of the Morpho butterfly coloration is reported using two-photon polymerization. Submicron structure features have been fabricated through the interference of the incident beam and the reflected beam in a thin polymer film. Furthermore, the influence of the lateral microstructure organization on the color formation has been studied in detail. The design of the polymerized structures was validated by scanning electron microscopy. The optical properties were analyzed using an angleresolved spectrometer. Tunable angle-independence, based on reflection intensity modulation, has been investigated by using photonic structures with less degree of symmetry. Finally, these findings were used to demonstrate the high potential of two-photon polymerization in the field of biomimetic research and for technical application, e.g. for sensing and anti-counterfeiting. © 2019 Optical Society of America.
view abstract 10.1364/OME.9.002630
- Digital holographic microscopy for sub-µm scale high aspect ratio structures in transparent materials
Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
Optics and Lasers in Engineering 121 (2019)This paper discusses a digital holographic microscopy approach for comprehensive and non-destructive inspection of spatially confined structures, which are characterized by the high aspect ratio of their lateral dimensions ( ∼ 1:30) and are fabricated out of transparent materials. Two-photon polymerization is chosen as a high-resolution lithographic technology for manufacturing of corresponding structures out of positive photoresin. The proposed holographic inspection method does not require any numerical methods for aberration correction and thus demonstrates rapid phase map evaluation ( ∼ 250 ms per cycle). Applicability and performance of the method has been successfully tested on a set of samples modelled to resemble step-index structures similar to the lab-on-chip basic elements, e.g. waveguides. In terms of lateral resolution a diffraction-limited operation of the proposed imaging system is ensured while vertical accuracy of 1.61 nm has been demonstrated. © 2019 Elsevier Ltd
view abstract 10.1016/j.optlaseng.2019.05.007
- Effects of femtosecond laser shock peening in distilled water on the surface characterizations of NiTi shape memory alloy
Wang, H. and Pöhl, F. and Yan, K. and Decker, P. and Gurevich, E.L. and Ostendorf, A.
Applied Surface Science 471 (2019)NiTi shape memory alloy was processed by femtosecond laser shock peening (FLSP) without protective coating in distilled water to modify its surface characterizations. The surface topography, microhardness, microstructure and scratch testing were studied before and after FLSP treatment. The experimental results show that FLSP with different laser scanning speeds and passes can change the surface roughness and microhardness due to laser ablation and high pressure of shock wave. The average microhardness value of the specimens FLSPed in distilled water increased. Grain refinement was observed in the surface layer of FLSPed NiTi alloy. The scratch testing revealed that FLSP process can decrease the frictional force and coefficient of friction, and it also demonstrated that the FLSP technique is beneficial to enhance the surface wear property of NiTi alloy. © 2018 Elsevier B.V.
view abstract 10.1016/j.apsusc.2018.12.087
- Femtosecond laser patterning of graphene electrodes for thin-film transistors
Kasischke, M. and Subaşı, E. and Bock, C. and Pham, D.-V. and Gurevich, E.L. and Kunze, U. and Ostendorf, A.
Applied Surface Science 478 (2019)The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappears, the mobility is enhanced by an order of magnitude and the subthreshold swing is reduced from S sub = 2.5 V/dec to S sub = 1.5 V/dec. © 2019 Elsevier B.V.
view abstract 10.1016/j.apsusc.2019.01.198
- Laser metal deposition of compositionally graded TiZrNbTa refractory high-entropy alloys using elemental powder blends
Dobbelstein, H. and Gurevich, E.L. and George, E.P. and Ostendorf, A. and Laplanche, G.
Additive Manufacturing 25 (2019)In the present study, laser metal deposition (LMD) was used to produce compositionally graded refractory high-entropy alloys (HEAs) for screening purposes by in-situ alloying of elemental powder blends. A compositional gradient from Ti25Zr50Nb0Ta25 to Ti25Zr0Nb50Ta25 is obtained by incrementally substituting Zr powder with Nb powder. A suitable strategy was developed to process the powder blend despite several challenges such as the high melting points of the refractory elements and the large differences in melting points among them. The influence of the LMD process on the final chemical composition was analyzed in detail and the LMD process was optimized to obtain a well-defined compositional gradient. Microstructures, textures, chemical compositions and mechanical properties were characterized using SEM, EBSD, EDX, and microhardness testing, respectively. Compositions between Ti25Zr0Nb50Ta25 and Ti25Zr25Nb25Ta25 were found to be single-phase bcc solid solutions with a coarse grain microstructure. Increasing the Zr to Nb ratio beyond the equiatomic composition results in finer and harder multiphase microstructures. The results shown in the present study clearly show for the first time that LMD is a suitable processing tool to screen HEAs over a range of chemical compositions. © 2018 The Authors
view abstract 10.1016/j.addma.2018.10.042
- Modelling of direct laser writing in multilayer absorbing medium
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 11057 (2019)A model that describes polymerization unit formation in multilayer absorbing medium for direct laser writing has been proposed. Properties of separate layers including surface roughness, illumination geometry, pulsed laser source and photosensitive material are parameters of the discussed model. A set of simulations has been carried out where the influence of the refractive indexes relation, layer thickness, roughness of the particular layers with respect to the structuring depth on the structure-model match and reproducibility has been analysed and discussed. © 2019 SPIE.
view abstract 10.1117/12.2525730
- Monitoring of photochemically induced changes in phase-modulating samples with digital holographic microscopy
Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
Applied Optics 58 (2019)This paper analyzes the performance of single-shot digital holographic microscopy for rapid characterization of static step-index structures in transparent polymer materials and for online monitoring of the photoinduced polymerization dynamics. The experiments are performed with a modified Mach–Zehnder transmission digital holographic microscope of high stability (phase accuracy of 0.69°) and of high magnification (of ≈90×). Use of near-infrared illumination allows both nondestructive examination of the manufactured samples and monitoring of optically induced processes in a photosensitive material concurrently with its excitation. The accuracy of the method for a precise sample’s topography evaluation is studied on an example of microchannel sets fabricated via two-photon polymerization and is supported by reference measurements with an atomic force microscope. The applicability of the approach for dynamic measurements is proved via online monitoring of the refractive index evolution in a photoresin layer illuminated with a focused laser beam at 405 nm. High correlation between the experimental results and a kinetics model for the photopolymerization process is achieved. © 2019 Optical Society of America
view abstract 10.1364/AO.58.000G41
- Near real-time digital holographic imaging on conventional central processing unit
Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
Proceedings of SPIE - The International Society for Optical Engineering 11056 (2019)In this paper we discuss a near real time digital holographic imaging algorithm achieving 4 fps operation speed on a common central processing unit. The hologram recording is performed in the off-axis geometry in the transmission mode. The algorithm follows a standard angular spectrum method routine and utilizes experimental calibration of the optical instrument for aberration correction. The main limiting factor is related to the size of the initial hologram and its Fourier transform (57% of the total execution duration). The performance of the approach is tested on different transparent and semi-transparent samples for reconstruction of sample topography and object in-depth allocation. © SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2526112
- Pump-probe microscopy of femtosecond laser ablation in air and liquids
Kanitz, A. and Förster, D.J. and Hoppius, J.S. and Weber, R. and Ostendorf, A. and Gurevich, E.L.
Applied Surface Science 475 (2019)The ablation process of femtosecond laser pulses of iron in air and different liquids was investigated for fluences of 0.5 J/cm2 and 2 J/cm2 by means of femtosecond pump-probe microscopy. Measurements of the relative change in reflectivity suggest that the surrounding liquid has a significant impact on the ablation process. During the heating phase of the metal within the first picoseconds after laser beam impact, the change in reflectivity in air and liquids is similar. Afterwards, the vapor and melt expulsion in air leads to a strong decrease in reflectivity, while the change in reflectivity in the liquids shows a more complex fluence and time-dependent behavior. This behavior is suggested to be triggered by the expansion of the molten surface and chemical reactions on the picosecond timescale. © 2018 Elsevier B.V.
view abstract 10.1016/j.apsusc.2018.12.184
- Reusable dispersed resonators-based biochemical sensor for parallel probing
Saetchnikov, A.V. and Tcherniavskaia, E.A. and Skakun, V.V. and Saetchnikov, V.A. and Ostendorf, A.
IEEE Sensors Journal 19 (2019)A biochemical arrayed sensor based on optical resonance technology and characterized by the dispersed positioning and properties of individual sensing units is discussed. Fabrication simplicity and reusability of the sensor as well as robustness of signal excitation and possibility for parallel probing are among the advantageous features for the implementation of optical resonance sensing for practical-oriented tasks. The arrayed sensor applicability, performance, and correspondence between the responses and analytical models are analyzed on the example of biochemical components' monitoring. The efficient regeneration of sensing properties realized by cleaning of the reacting surface allows to perform multiple sensing tasks in a single experimental run. The sensor interrogation approach based on the dispersion of the sensing units' spectral characteristics is discussed. © 2001-2012 IEEE.
view abstract 10.1109/JSEN.2019.2916861
- Review on experimental and theoretical investigations of the early stage, femtoseconds to microseconds processes during laser ablation in liquid-phase for the synthesis of colloidal nanoparticles
Kanitz, A. and Kalus, M.-R. and Gurevich, E.L. and Ostendorf, A. and Barcikowski, S. and Amans, D.
Plasma Sources Science and Technology 28 (2019)Laser ablation in liquid-phase (LAL) has been developed since the 1990s, but the interest in laser synthesis of colloids has emerged in the last decade due to a significant improvement in the production rate, proven comparative advantages in biomedical and catalysis applications, and recent commercialization. However, the method relies on highly transient phenomena, so that the fundamental understanding lacks behind the LAL synthesis refinement research. The complexity of the physics and chemistry involved has led to experimental and theoretical investigations that attempt to provide a basic description of the underlying processes but face the challenge of temporal and spatial resolution as well as non-equilibrium conditions. It appears that the processes occurring at the early time scales, ranging from femtoseconds to several microseconds are critical in the definition of the final product. The review is mainly dedicated to the comprehensive description of the processes occurring at early time scales, which include the description of laser-matter interaction for ultrashort and short laser pulses, plasma formation processes as well as comparison of the measured plasma parameters at these time scales, and subsequent description of the cavitation bubble dynamics. Furthermore, the plasma and cavitation bubble chemistry are addressed, and their impact on the nanoparticle formation is emphasized. © 2019 IOP Publishing Ltd.
view abstract 10.1088/1361-6595/ab3dbe
- Selective Delamination of Thin Films from Ceramic Surfaces upon Femtosecond Laser Ablation
Kiel, F. and Bulgakova, N.M. and Ostendorf, A. and Gurevich, E.L.
2019 Conference on Lasers and Electro-Optics, CLEO 2019 - Proceedings (2019)Experimental observation of selective delamination of YSZ ceramics upon femtosecond laser processing of its surface is reported. The delamination mechanism is identified as an interplay between beam defocusing by laser-generated free-electron plasma and Kerr nonlinearity. © 2019 The Author(s) 2019 OSA.
view abstract 10.23919/CLEO.2019.8750200
- Selective Delamination upon Femtosecond Laser Ablation of Ceramic Surfaces
Kiel, F. and Bulgakova, N.M. and Ostendorf, A. and Gurevich, E.L.
Physical Review Applied 11 (2019)We report on the experimental observation of selective delamination of semitransparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with a low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on both of its surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semitransparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials. © 2019 American Physical Society.
view abstract 10.1103/PhysRevApplied.11.024038
- Structural reproducibility enhancement of optical resonance arrays produced by two-photon polymerization
Saetchnikov, A. and Saetchnikov, V. and Tcherniavskaia, E. and Ostendorf, A.
2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)Two-photon polymerization (2PP) is a promising additive manufacturing technology for 3D objects fabrication with high optical quality by structuring them without any special requirements (e.g. masks, molds, vacuum) and with spatial resolution below 100 nm . However, just a 10-2-order refractive index difference between the substrate and photoresin may already cause the corruption of the structures that are few tens of micrometers high. Although tuning of the photoresin refractive index can benefit for reproducibility, the arising uncertainty in the substrate/photoresin interface localization leads then to corruption or even absence of the structure . © 2019 IEEE.
view abstract 10.1109/CLEOE-EQEC.2019.8872958
- Time and Mechanism of Nanoparticle Functionalization by Macromolecular Ligands during Pulsed Laser Ablation in Liquids
Letzel, A. and Reich, S. and Dos Santos Rolo, T. and Kanitz, A. and Hoppius, J. and Rack, A. and Olbinado, M.P. and Ostendorf, A. and Gökce, B. and Plech, A. and Barcikowski, S.
Langmuir 35 (2019)Laser ablation of gold in liquids with nanosecond laser pulses in aqueous solutions of inorganic electrolytes and macromolecular ligands for gold nanoparticle size quenching is probed inside the laser-induced cavitation bubble by in situ X-ray multicontrast imaging with a Hartmann mask (XHI). It is found that (i) the in situ size quenching power of sodium chloride (NaCl) in comparison to the ablation in pure water can be observed by the scattering contrast from XHI already inside the cavitation bubble, while (ii) for polyvinylpyrrolidone (PVP) as a macromolecular model ligand an in situ size quenching cannot be observed. Complementary ex situ characterization confirms the overall size quenching ability of both additive types NaCl and PVP. The macromolecular ligand as well as its monomer N-vinylpyrrolidone (NVP) are mainly effective for growth quenching of larger nanoparticles on later time scales, leading to the conclusion of an alternative interaction mechanism with ablated nanoparticles compared to the electrolyte NaCl, probably outside of the cavitation bubble, in the surrounding liquid phase. While monomer and polymer have similar effects on the particle properties, with the polymer being slightly more efficient, only the polymer is effective against hydrodynamic aggregation. © 2019 American Chemical Society.
view abstract 10.1021/acs.langmuir.8b01585
- Effect of a thin reflective film between substrate and photoresin on two-photon polymerization
Saetchnikov, A. and Saetchnikov, V. and Tcherniavskaia, E. and Ostendorf, A.
Additive Manufacturing 24 (2018)In this paper we present the results of a study on the impact of a thin reflective film between the substrate and photoresin on the two-photon polymerization procedure. We have proposed a model for the elementary polymerization volume (voxel) formation for the introduced case and carried out simulations to examine the influence of the refractive indexes relation, layer thickness, roughness, and polymerization depth on the polymerization performance. The experiments on fabrication of 2D and 2.5D structures have shown the benefit of the proposed configuration for the substrate/photoresin interface localization as well as for the distortion-free fabrication. © 2018 Elsevier B.V.
view abstract 10.1016/j.addma.2018.07.003
- Evaluation and benchmarking of an EC-QCL-based mid-infrared spectrometer for monitoring metabolic blood parameters in critical care units
Grafen, M. and Delbeck, S. and Busch, H. and Heise, H.M. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 10501 (2018)Mid-infrared spectroscopy hyphenated with micro-dialysis is an excellent method for monitoring metabolic blood parameters as it enables the concurrent, reagent-free and precise measurement of multiple clinically relevant substances such as glucose, lactate and urea in micro-dialysates of blood or interstitial fluid. For a marketable implementation, quantum cascade lasers (QCL) seem to represent a favourable technology due to their high degree of miniaturization and potentially low production costs. In this work, an external cavity (EC)-QCL-based spectrometer and two Fourier-transform infrared (FTIR) spectrometers were benchmarked with regard to the precision, accuracy and long-term stability needed for the monitoring of critically ill patients. For the tests, ternary aqueous solutions of glucose, lactate and mannitol (the latter for dialysis recovery determination) were measured in custom-made flow-through transmission cells of different pathlengths and analyzed by Partial Least Squares calibration models. It was revealed, that the wavenumber tuning speed of the QCL had a severe impact on the EC-mirror trajectory due to matching the digital-Analog-converter step frequency with the mechanical resonance frequency of the mirror actuation. By selecting an appropriate tuning speed, the mirror oscillations acted as a hardware smoothing filter for the significant intensity variations caused by mode hopping. Besides the tuning speed, the effects of averaging over multiple spectra and software smoothing parameters (Savitzky-Golay-filters and FT-smoothing) were investigated. The final settings led to a performance of the QCL-system, which was comparable with a research FTIR-spectrometer and even surpassed the performance of a small FTIR-mini-spectrometer. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2289625
- Femtosecond laser crystallization of amorphous titanium oxide thin films
Hoppius, J.S. and Bialuschewski, D. and Mathur, S. and Ostendorf, A. and Gurevich, E.L.
Applied Physics Letters 113 (2018)In this paper, we demonstrate experimentally that crystalline phases appear in amorphous titanium oxide upon processing with ultrafast laser pulses. Amorphous titanium thin films were produced by plasma-enhanced chemical vapor deposition and exposed to femtosecond laser pulses. Formation of a rutile phase was confirmed by X-ray diffraction, Raman measurements, and electron backscattering diffraction. A range of processing parameters for the crystallization is reported, and possible background mechanisms are discussed. © 2018 Author(s).
view abstract 10.1063/1.5027899
- Investigation of albumin-derived perfluorocarbon-based capsules by holographic optical trapping
Köhler, J. and Ruschke, J. and Ferenz, K.B. and Esen, C. and Kirsch, M. and Ostendorf, A.
Biomedical Optics Express 9 (2018)Albumin-derived perfluorocarbon-based capsules are promising as artificial oxygen carriers with high solubility. However, these capsules have to be studied further to allow initial human clinical tests. The aim of this paper is to provide and characterize a holographic optical tweezer to enable contactless trapping and moving of individual capsules in an environment that mimics physiological (in vivo) conditions most effectively in order to learn more about the artificial oxygen carrier behavior in blood plasma without recourse to animal experiments. Therefore, the motion behavior of capsules in a ring shaped or vortex beam is analyzed and optimized on account of determination of the optical forces in radial and axial direction. In addition, due to the customization and generation of dynamic phase holograms, the optical tweezer is used for first investigations on the aggregation behavior of the capsules and a statistical evaluation of the bonding in dependency of different capsule sizes is performed. The results show that the optical tweezer is sufficient for studying individual perfluorocarbon-based capsules and provide information about the interaction of these capsules for future use as artificial oxygen carriers. © 2018 Optical Society of America.
view abstract 10.1364/BOE.9.000743
- Investigation of multiple laser shock peening on the mechanical property and corrosion resistance of shipbuilding 5083Al alloy under a simulated seawater environment
Wang, H. and Huang, Y. and Zhang, W. and Ostendorf, A.
Applied Optics 57 (2018)To investigate the effect of laser shock peening (LSP) with different LSP impacts on the mechanical properties in artificial seawater and corrosion resistance of shipbuilding 5083Al alloy in 3.5% NaCl solution, wear property and electrochemical corrosion resistance tests were performed by a ball-on-disk sliding wear tester and electrochemical workstation. The wear mass losses of the samples treated by 1 and 3 LSP impacts are much lower, by 55.22% and 65.94%, respectively, than those of untreated specimens in artificial seawater. Compared with the untreated sample, the electrochemical corrosion rate of the treated samples decreased by 74.91% and 95.03% after being treated by 1 and 3 LSP impacts, respectively. The reasons for the enhancement of the wear properties and electrochemical corrosion behavior were caused by the increased residual stress and microhardness after the LSP treatment. © 2018 Optical Society of America.
view abstract 10.1364/AO.57.006300
- Laser metal deposition of a refractory TiZrNbHfTa high-entropy alloy
Dobbelstein, H. and Gurevich, E.L. and George, E.P. and Ostendorf, A. and Laplanche, G.
Additive Manufacturing 24 (2018)Refractory elements have high melting points and are difficult to melt and cast. In this study it is successfully demonstrated for the first time that laser metal deposition can be used to produce TiZrNbHfTa high-entropy alloy from a blend of elemental powders by in-situ alloying. Columnar specimens with a height of 10 mm and a diameter of 3 mm were deposited with a pulsed Nd:YAG laser. The built-up specimen has near-equiatomic composition, nearly uniform grain size, equiaxed grain shape, is bcc single phase and exhibits a high hardness of 509 HV0.2. © 2018 The Authors
view abstract 10.1016/j.addma.2018.10.008
- Laser metal deposition of lattice structures by columnar built-up
Sharma, M. and Dobbelstein, H. and Thiele, M. and Ostendorf, A.
Procedia CIRP 74 (2018)Independent of the geometrical complexity of a laser metal deposition component, the fundamental geometrical element is given by basic weld seams most of the times. These weld seams are deposited on a surface area and therefore not suitable to generate lattice structures. In this study, it is successfully demonstrated that basic spot welds can be used to achieve a vertical columnar built-up of pillars. Tilting of these pillars was used to merge them or to form forks. In that way, laser metal deposition of a lattice structure was implemented. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
view abstract 10.1016/j.procir.2018.08.098
- Mapping of the detecting units of the resonator-based multiplexed sensor
Saetchnikov, A. and Tcherniavskaia, E. and Saetchnikov, V. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 10678 (2018)An approach for localization of the randomly distributed spherical whispering gallery mode resonators on the multiplexed chip has been proposed. The method consists of several steps: chip image enhancement, sensing unit edge detection and light out-coupling area/sensing unit matching. The proposed approach has been successfully tested for detection of the bovine serum albumin protein solution. © 2018. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2309660
- On femtosecond laser shock peening of stainless steel AISI 316
Hoppius, J.S. and Kukreja, L.M. and Knyazeva, M. and Pöhl, F. and Walther, F. and Ostendorf, A. and Gurevich, E.L.
Applied Surface Science 435 (2018)In this paper we report on the competition in metal surface hardening between the femtosecond shock peening on the one hand, and formation of laser-induced periodic surface structures (LIPSS) and surface oxidation on the other hand. Peening of the stainless steel AISI 316 due to shock loading induced by femtosecond laser ablation was successfully demonstrated. However, for some range of processing parameters, surface erosion due to LIPSS and oxidation seems to dominate over the peening effect. Strategies to increase the peening efficiency are discussed. © 2017 Elsevier B.V.
view abstract 10.1016/j.apsusc.2017.11.145
- Printing structural colors via direct laser writing
Zyla, G. and Kovalev, A. and Gurevich, E.L. and Esen, C. and Ostendorf, A. and Gorb, S.
Proceedings of SPIE - The International Society for Optical Engineering 10544 (2018)Most common colors in our world as we see them, for example, in crystals, pigments, metals and salt solutions are the result from light scattering properties of electrons in atoms and molecules. Nevertheless, colors can also result from light interference effects, which are of great importance in the life of organisms. The structural colors of living organisms, e.g., the wings of some birds, insects and butteries, are often more intense and almost angle-independent. Understanding this specific color formation is of great interest for biology and for engineered materials with a broad range of biomimetic real world applications due to forgoing of toxic dyes and pigments. Therefore, the generation of artificial color formation with lithographic methods offers many advantages not available in coated multilayer systems because it avoids multiple complex fabrication steps. In the present work, we report an effortless fabrication method to generate structural coloration based on microand nano-structures using 3d-laser writing technique. The uniform micro- A nd nano-structures were produced in a thin polymer film with an refractive index of 1.51. The single structures are aligned in an array to create a blue color field. The identification of the influence of the structures on the artificial color formation was performed using scanning electron microscopy. The optical properties of the blue color was analyzed via an angle-resolved spectrometer. © 2018 SPIE.
view abstract 10.1117/12.2289820
- Simultaneous nanopatterning and reduction of graphene oxide by femtosecond laser pulses
Kasischke, M. and Maragkaki, S. and Volz, S. and Ostendorf, A. and Gurevich, E.L.
Applied Surface Science 445 (2018)This paper presents a novel one-step method for the periodical nanopatterning and reduction of graphene oxide (GO). Self-organized periodic structures of reduced graphene oxide (rGO) appear on GO surfaces upon processing with a femtosecond laser at fluences slightly higher than the fluence needed for reduction of the GO. This indicates that the periodic pattern is formed either simultaneously with or due to the reduction of the GO. The laser-induced reduction of GO was identified by sheet resistance measurements, Raman and X-ray photoelectron spectroscopy. This fast and simple method to both reduce and periodically structure GO offers a variety of possible applications in printed and flexible electronics. © 2018 Elsevier B.V.
view abstract 10.1016/j.apsusc.2018.03.086
- Tailored β-Ketoiminato Complexes of Iron: Synthesis, Characterization, and Evaluation towards Solution-Based Deposition of Iron Oxide Thin Films
Sadlo, A. and Beer, S.M.J. and Rahman, S. and Grafen, M. and Rogalla, D. and Winter, M. and Ostendorf, A. and Devi, A.
European Journal of Inorganic Chemistry 2018 (2018)The synthesis and characterization of five new and closely related homoleptic iron(II) β-ketoiminate complexes is reported. Molecular structures of compounds 1, 2, and 5 were determined by single-crystal XRD, which revealed monomeric four- and sixfold coordination, depending on the functionalized side chain. The stepwise elimination of the ligand from the complex observed by thermogravimetric analysis and the stability in solution are encouraging features for solution-based processing of hematite thin films. As a representative example, compound 1 was successfully employed in a straightforward spin-coating process. The fabricated iron oxide films were characterized in terms of their structure and phase by XRD and Raman spectroscopy, morphology by SEM, and composition by Rutherford backscattering spectrometry accompanied by nuclear reaction analysis, which revealed the formation of crystalline and stoichiometric α-Fe2O3 films. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/ejic.201800094
- Temporal temperature evolution in laser micro-spot welding of copper considering temperature-dependent material parameters
Mattern, M. and Weigel, T. and Ostendorf, A.
Materials Research Express 5 (2018)One of the main obstacles to the industrial application of laser micro-spot welding of copper is the poor process repeatability at common laser wavelengths. The inhomogeneities in the oxide layer in combination with the high reflectivity of copper itself are often stated as the main reason for the poor repeatability of welding results. However, the typical local variations of the reflectivity alone are not sufficient to describe the instability of the welding process completely. Through numerical FEM simulations, the temperature dependence of the absorptivity could be identified as the main reason for the narrowing of the process window. The effect of the temperature-dependent absorptivity is enhanced by the temperature-dependence of the heat conductivity and slightly reduced by the enthalpy of fusion. The results of the simulation show a good agreement with experimental data. © 2018 IOP Publishing Ltd.
view abstract 10.1088/2053-1591/aacc3a
- Time-resolved measurement of elemental carbon in urban environment: Comparison of Raman backscattering and aethalometer results
Grafen, M. and Schweiger, G. and Esen, C. and Ostendorf, A.
Journal of Aerosol Science 117 (2018)Time-resolved quantitative and qualitative measurement of the particulate matter (PM) - composition is an ongoing challenge for scientists and instrument developers. Raman spectroscopy is especially useful for PM analysis, however, its application has been restricted mainly to qualitative laboratory investigation. A low particle load on the filter and the resulting high elastic backscattering make Raman spectroscopic quantitative monitoring a demanding task with regard to optical design and the mathematical tools needed for data evaluation. In this paper, we present environmental measurements taken over several weeks with the fully automatized Raman Soot Spectrometer (RaSoS). The hourly recorded Raman spectra are characterized using factor analytic methods and a linear model for quantitative determination of the apparent elemental carbon (EC) filter loading is derived. Its predictions are compared with the measurements of an aethalometer which utilizes light absorption to measure the black carbon (BC) - concentration. Raman spectra taken a year earlier at the same location yield plausible predictions using the same model. The EC-scores of the RaSoS generally agree well with the BC measurements of the aethalometer (σ=0.82–0.92). However, for very high concentrations of BC and for several short time periods, there are deviations which may be connected to varying PM-composition. © 2017 Elsevier Ltd
view abstract 10.1016/j.jaerosci.2017.12.003
- Development and characterization of a microsnap-fit for optical assembly
Köhler, J. and Kutlu, Y. and Ksouri, S.I. and Esen, C. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 10120 (2017)Snap-fits are classified as interlocking connections and commonly used to assemble two or more components in a fast and cost efficient way. The mechanism is simply based on mechanical flexibility. Therefore, the applications cover a broad field ranging from automotive engineering to mobile phone design. By scaling and transferring the snap-fit mechanism into micrometer scale, advantages can also be utilized to assemble complex microsystems. In this paper, a microsnap-fit based on a cantilever design is developed and investigated by means of optical techniques only. Two-photon polymerization as micro-stereolithography is utilized to manufacture the microcomponents and the mechanical flexibility is analyzed by optical forces in a holographic optical tweezer setup. The locking mechanism is theoretically and experimentally characterized, e.g, the flexibility of the polymer with regard to the design is studied. It can be demonstrated that assembling as well as disassembling of microcomponents is achievable. These findings provide fast and easy assembling of complex microsystems in the fields of microrobotics, -sensors, and -mechanics. © 2017 SPIE.
view abstract 10.1117/12.2254951
- Development of electrically conductive microstructures based on polymer/CNT nanocomposites via two-photon polymerization
Staudinger, U. and Zyla, G. and Krause, B. and Janke, A. and Fischer, D. and Esen, C. and Voit, B. and Ostendorf, A.
Microelectronic Engineering 179 (2017)Femtosecond laser-induced two-photon polymerization (2PP) of carbon nanofiller doped polymers was utilized to produce electrically conductive microstructures, which are expected to be applicable as microelectronic components or micro-electromechanical systems in sensors. The nanocomposites were processed by compounding an inorganic-organic hybrid material with two different types (short and long) of single walled carbon nanotubes (SWCNTs). Different SWCNT contents were dispersed in the polymer by sonication to adjust the electrical conductivity of the nanocomposites. Low surface resistivity values of ~ 4.6 × 105 Ω/sq. could be measured for coated reference films with a thickness of 30 μm having an exceptionally low SWCNT content of 0.01 wt% of the long type of SWCNTs. In contrast, a higher minimum resistivity of 1.5 × 106 Ω/sq. was exhibited for composites with a higher content, 2 wt%, of short SWCNTs. The structural quality of the microstructures processed by 2PP was mainly influenced by the dispersion quality of the SWCNTs. To characterize the electrical conductivity, conductive atomic force microscopy was applied for the first time. In microstructures with 0.05 wt% of the long type of SWCNTs, a contact current could be detected over a wide range of the measured area visualizing the electrical conductive CNT network, which has not been reported before. © 2017
view abstract 10.1016/j.mee.2017.04.024
- Generation of bioinspired structural colors via two-photon polymerization
Zyla, G. and Kovalev, A. and Grafen, M. and Gurevich, E.L. and Esen, C. and Ostendorf, A. and Gorb, S.
Scientific Reports 7 (2017)Colors of crystals, pigments, metals, salt solutions and bioluminescence occur in nature due to the optical properties of electrons in atoms and molecules. However, colors can also result from interference effects on nanostructures. In contrast to artificial coloration, which are caused by well-defined regular structures, the structural colors of living organisms are often more intense and almost angle-independent. In this paper, we report the successful manufacturing of a lamellar nanostructure that mimics the ridge shape of the Morpho butterfly using a 3d-direct laser writing technique. The viewing angle dependency of the color was analyzed via a spectrometer and the structure was visualized using a scanning electron microscope. The generated nano-and micro-structures and their optical properties were comparable to those observed in the Morpho butterfly. © 2017 The Author(s).
view abstract 10.1038/s41598-017-17914-w
- Graphene oxide reduction induced by femtosecond laser irradiation
Kasischke, M. and Maragkaki, S. and Volz, S. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 10356 (2017)A promising fabrication method for graphene is the reduction of graphene oxide (GO), this can be achieved photochemically by laser irradiation. In this study, we examine the results of latter method by a femtosecond fiber laser (1030 nm, 280 fs). The chemical properties of the irradiated areas were analyzed by Raman and X-ray photoelectron spectroscopy (XPS) and electrical properties were evaluated using sheet resistance measurements. We found that, within a wide range of fluences (8.5 mJ/cm2to 57.8 mJ/cm2) at high overlapping rates (>99.45 %), photochemical oxygen reduction can be achieved. However, hybridization transition of sp3 to sp2 graphene-like structures only takes place at upper fluences of the mentioned range. © 2017 SPIE.
view abstract 10.1117/12.2274976
- Impact of liquid environment on femtosecond laser ablation
Kanitz, A. and Hoppius, J.S. and Fiebrandt, M. and Awakowicz, P. and Esen, C. and Ostendorf, A. and Gurevich, E.L.
Applied Physics A: Materials Science and Processing 123 (2017)The ablation rate by femtosecond laser processing of iron in different liquids is investigated for fluences up to 5 J/cm2. The resulting fluence dependency is modeled by an approach derived from the two-temperature model. In our experiments, the liquid environment strongly affects the effective penetration depth, e.g, the ablation rate in water is almost ten times higher than in toluene. This effect is discussed and introduced phenomenologically into the model. Additional reflectivity measurements and plasma imaging provide improved insight into the ablation process. © 2017, Springer-Verlag GmbH Germany.
view abstract 10.1007/s00339-017-1280-z
- Influence of solvent mixture on the ablation rate of iron using femtosecond laser pulses
Kanitz, A. and Hoppius, J.S. and Ostendorf, A. and Gurevich, E.L.
Proceedings of SPIE - The International Society for Optical Engineering 10092 (2017)Ultrashort pulse laser ablation has become an important tool for material processing. Adding liquids to the process can be beneficial for a reduced debris and heat affected zone width. Another application is the production of ligand-free nanoparticles. By measuring the ablation rate of iron for femtosecond pulsed laser ablation in different solvents and solvent-mixtures, the influence of the solvent properties on the ablation process is studied. The ablation efficiency is quantified by measuring the ablation rate in dependency of the fluence from 0.05 J/cm2 up to 5 J/cm2 in water-ethanol and water-acetone mixtures which are varied in 25 % steps. The ablation rate is significantly influenced by the solvent-mixtures. © 2017 SPIE.
view abstract 10.1117/12.2254444
- Microresonator array: A particular optical sensor
Weigel, T. and Schweiger, G. and Esen, C. and Ostendorf, A.
Technisches Messen 84 (2017)Spherical microresonators are high sensitivity sensors for the measurement of important physical quantities e.g. temperature or pressure. Measuring methods based on single optical resonators need expensive and delicate laser systems or spectral devices. The aim of this paper is to present a novel multi-purpose sensing technology based on whispering gallery modes in spherical microparticle arrays. Examples for different applications are given to prove the flexibility and usability of the method.
view abstract 10.1515/teme-2016-0063
- Nanostructured Fe2O3 Processing via Water-Assisted ALD and Low-Temperature CVD from a Versatile Iron Ketoiminate Precursor
Peeters, D. and Sadlo, A. and Lowjaga, K. and Mendoza Reyes, O. and Wang, L. and Mai, L. and Gebhard, M. and Rogalla, D. and Becker, H.-W. and Giner, I. and Grundmeier, G. and Mitoraj, D. and Grafen, M. and Ostendorf, A. and Beranek, R. and Devi, A.
Advanced Materials Interfaces (2017)Vapor phase deposited iron oxide nanostructures are promising for fabrication of solid state chemical sensors, photoelectrodes for solar water splitting, batteries, and logic devices. The deposition of iron oxide via chemical vapor deposition (CVD) or atomic layer deposition (ALD) under mild conditions necessitates a precursor that comprises good volatility, stability, and reactivity. Here, a versatile iron precursor, namely [bis(N-isopropylketoiminate) iron(II)], which possesses ideal characteristics both for low-temperature CVD and water-assisted ALD processes, is reported. The films are thoroughly investigated toward phase, composition, and morphology. As-deposited ALD grown Fe2O3 layers are amorphous, while the CVD process in the presence of oxygen leads to polycrystalline hematite layers. The nanostructured iron oxide grown via CVD consists of nanoplatelets that are appealing for photoelectrochemical applications. Preliminary tests of the photoelectrocatalytic activity of CVD-grown Fe2O3 layers show photocurrent densities up to 0.3 mA cm-2 at 1.2 V versus reversible hydrogen electrode (RHE) and 1.2 mA cm-2 at 1.6 V versus RHE under simulated sunlight (1 sun). Surface modification by cobalt oxyhydroxide (Co-Pi) co-catalyst is found to have a highly beneficial effect on photocurrent, leading to maximum monochromatic quantum efficiencies of 10% at 400 nm and 4% at 500 nm at 1.5 V versus RHE. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/admi.201700155
- Optical assembly of microsnap-fits fabricated by two-photon polymerization
Köhler, J. and Kutlu, Y. and Zyla, G. and Ksouri, S.I. and Esen, C. and Gurevich, E.L. and Ostendorf, A.
Optical Engineering 56 (2017)To respond to current demands of nano- and microtechnologies, e.g., miniaturization and integration, different bottom-up strategies have been developed. These strategies are based on picking, placing, and assembly of multiple components to produce microsystems with desired features. This paper covers the fabrication of arbitrary-shaped microcomponents by two-photon polymerization and the trapping, moving, and aligning of these structures by the use of a holographic optical tweezer. The main focus is on the assembly technique based on a cantilever microsnap-fit. More precisely, mechanical properties are characterized by optical forces and a suitable geometry of the snap-fit is designed. As a result of these investigations, a fast and simple assembly technique is developed. Furthermore, disassembly is provided by an optimized design. These findings suggest that the microsnap-fit is suitable for the assembly of miniaturized systems and could broaden the application opportunities of bottom-up strategies. © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE).
view abstract 10.1117/1.OE.56.10.105105
- Optical screw-wrench for microassembly
Kohler, J. and Ksouri, S. I. and Esen, C. and Ostendorf, A.
Microsystems & Nanoengineering 3 (2017)For future micro- and nanotechnologies, the manufacturing of miniaturized, functionalized, and integrated devices is indispensable. In this paper, an assembly technique based on a bottom-up strategy that enables the manufacturing of complex microsystems using only optical methods is presented. A screw connection is transferred to the micrometer range and used to assemble screw- and nut-shaped microcomponents. Micro-stereolithography is performed by means of two-photon polymerization, and microstructures are fabricated and subsequently trapped, moved, and screwed together using optical forces in a holographic optical tweezer set-up. The design and construction of interlocking microcomponents and the verification of a stable and releasable joint form the main focus of this paper. The assembly technique is also applied to a microfluidic system to enable the pumping or intermixing of fluids on a microfluidic chip. This strategy not only enables the assembly of microcomponents but also the combination of different materials and features to form complex hybrid microsystems.
view abstract 10.1038/micronano.2016.83
- Optimized expression-based microdissection of formalin-fixed lung cancer tissue
Grafen, M. and Hofmann, T.R. and Scheel, A.H. and Beck, J. and Emmert, A. and Küffer, S. and Danner, B.C. and Schütz, E. and Büttner, R. and Ostendorf, A. and Ströbel, P. and Bohnenberger, H.
Laboratory Investigation 97 (2017)Analysis of specific DNA alterations in precision medicine of tumors is crucially important for molecular targeted treatments. Lung cancer is a prototypic example and one of the leading causes of cancer-related deaths worldwide. One major technical problem of detecting DNA alterations in tissue samples is cellular heterogeneity, that is, mixture of tumor and normal cells. Microdissection is an important tool to enrich tumor cells from heterogeneous tissue samples. However, conventional laser capture microdissection has several disadvantages like user-dependent selection of regions of interest (ROI), high costs for dissection systems and long processing times. ROI selection in expression-based microdissection (xMD) directly relies on cancer cell-specific immunostaining. Whole-slide irradiation leads to localized energy absorption at the sites of most intensive staining and melting of a membrane covering the slide, so that tumor cells can be isolated by removing the complete membrane. In this study, we optimized xMD of lung cancer tissue by enhancing staining intensity of tumor cell-specific immunostaining and processing of the stained samples. This optimized procedure did not alter DNA quality and resulted in enrichment of mutated EGFR DNA from lung adenocarcinoma specimens after xMD. We here also introduce a quality control protocol based on digital whole-slide scanning and image analysis before and after xMD to quantify selectivity and efficiency of the procedure. In summary, this study provides a workflow for xMD, adapted and tested for lung cancer tissue that can be used for lung tumor cell dissection before diagnostic or investigatory analyses. © 2017 USCAP, Inc All rights reserved.
view abstract 10.1038/labinvest.2017.31
- Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach
Peeters, D. and Taffa, D.H. and Kerrigan, M.M. and Ney, A. and Jöns, N. and Rogalla, D. and Cwik, S. and Becker, H.-W. and Grafen, M. and Ostendorf, A. and Winter, C.H. and Chakraborty, S. and Wark, M. and Devi, A.
ACS Sustainable Chemistry and Engineering 5 (2017)Owing to its narrow band gap and promising magnetic and photocatalytic properties, thin films of zinc ferrite (ZFO, ZnFe2O4) are appealing for fabrication of devices in magnetic recording media and photoelectrochemical cells. Herein we report for the first time the fabrication of photactive zinc ferrites via a solvent free, conventional CVD approach, and the resulting ZFO layers show promise as a photocatalyst in PEC water-splitting. For large scale applications, chemical vapor deposition (CVD) routes are appealing for thin film deposition; however, very little is known about ZFO synthesis following CVD processes. The challenge in precisely controlling the composition for multicomponent material systems, such as ZFO, via conventional thermal CVD is an issue that is caused mainly by the mismatch in thermal properties of the precursors. The approach of using two different classes of precursors for zinc and iron with a close match in thermal windows led to the formation of polycrystalline spinel type ZFO. Under the optimized process conditions, it was possible to fabricate solely ZFO in the desired phase. This work demonstrates the potential of employing CVD to obtain photoactive ternary material systems in the right composition. For the first time, the application of CVD grown ZFO films for photoelectrochemical applications is being demonstrated, showing a direct band gap of 2.3 eV and exhibiting activity for visible light driven photoelectrochemical water splitting. © 2017 American Chemical Society.
view abstract 10.1021/acssuschemeng.6b02233
- Simultaneous real-Time application and direct comparison of optical resonance sensing and fluorescence tagging techniques for biochemical component detection
Saetchnikov, A.V. and Saetchnikov, V.A. and Tcherniavskaia, E.A. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 10333 (2017)New approach to perform a real-Time biochemical component detection based on simultaneous analysis of spectral changes of whispering gallery modes (WGM) and fluorescence markers used for biochemical components tagging. Microcavity array sensor was chosen as detection unit. Experimental data on detection of bovine serum albumin protein solution using both techniques simultaneously is represented. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2272785
- Synthesis and evaluation of new copper ketoiminate precursors for a facile and additive-free solution-based approach to nanoscale copper oxide thin films
Karle, Sarah and Rogalla, Detlef and Ludwig, Arne and Becker, Hans-Werner and Wieck, Andreas Dirk and Grafen, Markus and Ostendorf, Andreas and Devi, Anjana
Dalton Transactions 46 (2017)Novel copper ketoiminate compounds were synthesized and for the first time applied for additive-free solution-based deposition of nanoscale copper oxide thin films. The two closely related compounds, namely the bis[4-(2-ethoxyethyl-imino)-3-pentanonato] copper, [Cu(EEKI)(2)], and bis[4-(3-methoxypropylimino)- 3-pentanonato] copper, [Cu(MPKI)(2)], were characterized by means of elemental and thermogravimetric analysis (TGA), as well as electron impact mass spectrometry (EI-MS). The advantages of these compounds are that they are liquid and possess excellent solubility in common organic solvents in addition to an optimum reactivity towards ambient moisture that enables a facile solution-based approach to nanoscale copper oxide thin films. Moreover, no additives or aging is needed to stabilize the solution processing of the copper oxide layers. [Cu(MPKI)(2)] was tested in detail for the deposition of copper oxide thin films by spin coating. Upon one-step annealing, high-quality, uniform, crystalline copper oxide thin films were deposited on Si, SiO2, as well as on quartz substrates. Structural, morphological and compositional characteristics of the copper oxide nanostructures were investigated in detail by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and a combined analysis using Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). It was possible to control the copper oxide phases (CuO and Cu2O) by systematic tuning of the post-deposition annealing conditions. The functional properties in terms of optical band gap were investigated using UV/Vis spectroscopy, while the transport properties, such as resistivity, mobility and carrier concentration were analyzed employing Hall measurements, which confirmed the p-type conductivity of the copper oxide layers.
view abstract 10.1039/c6dt04399b
- Synthesis of Magnetic Nanoparticles by Ultrashort Pulsed Laser Ablation of Iron in Different Liquids
Kanitz, A. and Hoppius, J.S. and del Mar Sanz, M. and Maicas, M. and Ostendorf, A. and Gurevich, E.L.
ChemPhysChem 18 (2017)Magnetic nanoparticles were generated by ultrashort pulsed laser ablation of an iron target in water, methanol, ethanol, acetone and toluene. The relationship between ablation rate, liquid properties and the physical and chemical properties of the nanoparticles was studied. Composition, morphology and magnetic properties were investigated by TEM, XPS and vibrating-sample (VSM) and SQUID magnetometry. The properties of the generated nanoparticle ensembles reflected the influence of the liquid environment on the particle formation process. For example, the composition was strongly dependent on the carbon to oxygen ratio within the molecules of the liquid. In contrast to short pulsed laser ablation in liquids, the nanoparticles generated by ultrashort pulses had a higher level of polycrystallinity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/cphc.201601252
- Wavelength dependence of picosecond laser-induced periodic surface structures on copper
Maragkaki, S. and Derrien, T.J.-Y. and Levy, Y. and Bulgakova, N.M. and Ostendorf, A. and Gurevich, E.L.
Applied Surface Science 417 (2017)The physical mechanisms of the laser-induced periodic surface structures (LIPSS) formation are studied in this paper for single-pulse irradiation regimes. The change in the LIPSS period with wavelength of incident laser radiation is investigated experimentally, using a picosecond laser system, which provides 7-ps pulses in near-IR, visible, and UV spectral ranges. The experimental results are compared with predictions made under the assumption that the surface-scattered waves are involved in the LIPSS formation. Considerable disagreement suggests that hydrodynamic mechanisms can be responsible for the observed pattern periodicity. © 2017 Elsevier B.V.
view abstract 10.1016/j.apsusc.2017.02.068
- Characterization of a multi-module tunable EC-QCL system for mid-infrared biofluid spectroscopy for hospital use and personalized diabetes technology
Grafen, M. and Nalpantidis, K. and Ostendorf, A. and Ihrig, D. and Heise, H.M.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9715 (2016)Blood glucose monitoring systems are important point-of-care devices for the hospital and personalised diabetes technology. FTIR-spectrometers have been successfully employed for the development of continuous bed-side monitoring systems in combination with micro-dialysis. For implementation in miniaturised portable systems, external-cavity quantum cascade lasers (EC-QCL) are suited. An ultra-broadly tunable pulsed EC-QCL system, covering a spectral range from 1920 to 780 cm-1, has been characterised with regard to the spectral emission profiles and wavenumber scale accuracy. The measurement of glucose in aqueous solution is presented and problems with signal linearity using Peltier-cooled MCT-detectors are discussed. The use of larger optical sample pathlengths for attenuating the laser power in transmission measurements has recently been suggested and implemented, but implications for broad mid-infrared measurements have now been investigated. The utilization of discrete wavenumber variables as an alternative for sweep-tune measurements has also been studied and sparse multivariate calibration models intended for clinical chemistry applications are described for glucose and lactate. © 2016 SPIE.
view abstract 10.1117/12.2213635
- Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number
Köhler, J. and Friedrich, J. and Ostendorf, A. and Gurevich, E.L.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 93 (2016)We theoretically and experimentally investigate the flow field that emerges from a rodlike microrotor rotating about its center in a nonaxisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo et al. [Phys. Rev. Lett. 96, 134502 (2006)PRLTAO0031-900710.1103/PhysRevLett.96.134502]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices. © 2016 American Physical Society.
view abstract 10.1103/PhysRevE.93.023108
- Direct metal deposition of refractory high entropy alloy MoNbTaW
Dobbelstein, H. and Thiele, M. and Gurevich, E.L. and George, E.P. and Ostendorf, A.
Physics Procedia 83 (2016)Alloying of refractory high entropy alloys (HEAs) such as MoNbTaW is usually done by vacuum arc melting (VAM) or powder metallurgy (PM) due to the high melting points of the elements. Machining to produce the final shape of parts is often needed after the PM process. Casting processes, which are often used for aerospace components (turbine blades, vanes), are not possible. Direct metal deposition (DMD) is an additive manufacturing technique used for the refurbishment of superalloy components, but generating these components from the bottom up is also of current research interest. MoNbTaW possesses high yield strength at high temperatures and could be an alternative to state-of-the-art materials. In this study, DMD of an equimolar mixture of elemental powders was performed with a pulsed Nd:YAG laser. Single wall structures were built, deposition strategies developed and the microstructure of MoNbTaW was analyzed by back scattered electrons (BSE) and energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope. DMD enables the generation of composition gradients by using dynamic powder mixing instead of pre-alloyed powders. However, the simultaneous handling of several elemental or pre-alloyed powders brings new challenges to the deposition process. The influence of thermal properties, melting point and vapor pressure on the deposition process and chemical composition will be discussed. © 2016 The Authors.
view abstract 10.1016/j.phpro.2016.08.065
- Influence of the liquid on femtosecond laser ablation of iron
Kanitz, A. and Hoppius, J.S. and Gurevich, E.L. and Ostendorf, A.
Physics Procedia 83 (2016)Ultrashort pulse laser ablation has become a very important industrial method for highly precise material removal ranging from sensitive thin film processing to drilling and cutting of metals. Over the last decade, a new method to produce pure nanoparticles emerged from this technique: Pulsed Laser Ablation in Liquids (PLAL). By this method, the ablation of material by a laser beam is used to generate a metal vapor within the liquid in order to obtain nanoparticles from its recondensation process. It is well known that the liquid significantly alters the ablation properties of the substrate, in our case iron. For example, the ablation rate and crater morphology differ depending on the used liquid. We present our studies on the efficiency and quality of ablated grooves in water, methanol, acetone, ethanol and toluene. The produced grooves are investigated by means of white-light interferometry, EDX and SEM. © 2016 The Authors.
view abstract 10.1016/j.phpro.2016.08.022
- Influence of the sensitivity of an optical resonator with a surface layer by its properties
Schweiger, G. and Weigel, T. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9727 (2016)In the last years, optical resonators have emerged as a promising tool for highly sensitive measurements. Especially for label-free measurements of biological substances, the resonators have to be functionalized by additional surface layers. Since the properties of the resonator, like the refractive index of the core and the layer as well as the layer thickness or the core radius can deeply in fluence the sensitivity. For this reason, a geometrical optics based theory is used to investigate the dependence of the resonance wavelength on the resonator properties. © 2016 SPIE.
view abstract 10.1117/12.2208542
- Investigation on femto-second laser irradiation assisted shock peening of medium carbon (0.4% C) steel
Majumdar, J.D. and Gurevich, E.L. and Kumari, R. and Ostendorf, A.
Applied Surface Science 364 (2016)In the present study, the effect of femtosecond laser irradiation on the peening behavior of 0.4% C steel has been evaluated. Laser irradiation has been conducted with a 100 μJ and 300 fs laser with multiple pulses under varied energy. Followed by laser irradiation, a detailed characterization of the processed zone was undertaken by scanning electron microscopy, and X-ray diffraction technique. Finally, the residual stress distribution, microhardness and wear resistance properties of the processed zone were also evaluated. Laser processing leads to shock peening associated with plasma formation and its expansion, formation of martensite and ferrito-pearlitic phase in the microstructure. Due to laser processing, there is introduction of residual stress on the surface which varies from high tensile (140 MPa) to compressive (-335 MPa) as compared to 152 MPa of the substrate. There is a significant increase in microhardness to 350-500 VHN as compared to 250 VHN of substrate. The fretting wear behavior against hardened steel ball shows a significant reduction in wear depth due to laser processing. Finally, a conclusion of the mechanism of wear has been established. © 2015 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.apsusc.2015.12.058
- Long-Term functionalization of optical resonance sensor spots
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9884 (2016)New approach to increase density of sensing units for higher precision as well as the selectivity of biological components under investigation in microcavity evanescent wave optical sensor systems is proposed. Long-Term functionalization results of array sensor cells by different agents are represented. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
view abstract 10.1117/12.2227496
- Optical screw-wrench for interlocking 2PP-microstructures
Köhler, J. and Zyla, G. and Ksouri, S.I. and Esen, C. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9764 (2016)Two-photon polymerization (2PP) has emerged as a powerful platform for processing three-dimensional microstructures with high resolution. Furthermore, by adding nanoparticles of different materials to the photopolymer the microstructures can be functionalized, e.g. magnetic or electric properties can be adjusted. However, to combine different functions within one microstructure or to manufacture complex microsystems, assembling techniques for multiple 2PP written building blocks are required. In this paper a qualitative approach for assembling microstructures utilizing optical forces is presented. Therefore, screw and nut shaped microstructures are produced by 2PP-technique and screwed together using a holographic optical tweezer (HOT). The interlocking structures are trapped and rotated into each other to cause connection. In this paper the used parameters and possible designs of the interlocking connection are discussed. These findings provide not only the assembling of building blocks to complex microstructures, rather different functionalized 2PP-microstructures can be combined by simply screwing them together with the use of optical forces. © 2016 SPIE.
view abstract 10.1117/12.2209325
- Performance testing of a mid-infrared spectroscopic system for clinical chemistry applications utilising an ultra-broadband tunable EC-QCL radiation source
Grafen, M. and Nalpantidis, K. and Ihrig, D. and Heise, H.M. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9715 (2016)Mid-infrared (MIR) spectroscopy is a valuable analytical method for patient monitoring within point-of-care diagnostics. For implementation, quantum cascade lasers (QCL) appear to be most suited regarding miniaturization, complexity and eventually also costs. External cavity (EC)-QCLs offer broad tuning ranges and recently, ultra-broadly tunable systems covering spectral ranges around the mid-infrared fingerprint region became commercially available. Using such a system, transmission spectra from the wavenumber interval of 780 to 1920 cm-1, using a thermoelectrically cooled MCT-detector, were recorded while switching the aqueous glucose concentrations between 0, 50 and 100 mg/dL. In order to optimize the system performance, a multi-parameter study was carried out, varying laser pulse width, duty cycle, sweep speed and the optical sample pathlength for scoring the absorbance noise. Exploratory factor analysis with pattern recognition tools (PCA, LDA) was used for the raw data, providing more than 10 significantly contributing factors. With the glucose signal causing 20 % of the total variance, further factors include short-term drift possibly related to thermal effects, long-term drift due to varying atmospheric water vapour in the lab, as well as wavenumber shifts and drifts of the single tuners. For performance testing, the noise equivalent concentration was estimated based on cross-validated Partial-Least Squares (PLS) predictions and the a-posteriori obtained scores of the factor analysis. Based on the optimized parameters, a noise equivalent glucose concentration of 1.5 mg/dL was achieved. © 2016 SPIE.
view abstract 10.1117/12.2213538
- Recognition of pharmaceuticals with compact mini-Raman-spectrometer and automized pattern recognition algorithms
Jähme, H. and Di Florio, G. and Conti Nibali, V. and Esen, C. and Ostendorf, A. and Grafen, M. and Henke, E. and Soetebier, J. and Brenner, C. and Havenith, M. and Hofmann, M.R.
Proceedings of SPIE - The International Society for Optical Engineering 9899 (2016)Robust classification of pharmaceuticals in an industrial process is an important step for validation of the final product. Especially for pharmaceuticals with similar visual appearance a quality control is only possible if a reliable algorithm based on easily obtainable spectroscopic data is available. We used Principal Component Analysis (PCA) and Support Vector Machines (SVM) on Raman spectroscopy data from a compact Raman system to classify several look-alike pharmaceuticals. This paper describes the data gathering and analysis process to robustly discriminate 19 different pharmaceuticals with similar visual appearance. With the described process we successfully identified all given pharmaceuticals which had a significant amount of active ingredients. Thus automatic validation of these pharmaceuticals in a process can be used to prevent wrong administration of look-alike drugs in an industrial setting, e.g. patient individual blistering. © 2016 SPIE.
view abstract 10.1117/12.2228070
- Throughput optimization for laser micro structuring
Hoppius, J.S. and Kanitz, A. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9736 (2016)Laser pulses in the picosecond and femtosecond regime enable nearly non-thermal material processing where heat effects like molten pools and thermal tensions are often significantly reduced. However, a residual amount of laser energy transforms into heat. As a consequence cumulative multiple shot processing leads to heat accumulation and subsequently lower manufacturing accuracy. To increase the processing throughput without losing quality, it is important to optimize the laser pulse properties and the ablation strategy to further reduce thermal effects. Due to a low heat capacity in small structures, it is necessary to consider the substrate dimensions while performing micro- and nanoprocessing. In contrast to bulk material ablation, the heat dissipation is confined by the small heat capacity of microstructures. Especially for complex structures, it is time-consuming to find efficient processing parameters manually. For this reason, an in-situ evaluation system based on electrical resistivity measurements for on-line control of the ablation process was developed to optimize the laser parameters. In the work presented, the efficiency of 35 femtosecond pulsed laser ablation was evaluated on copper structures in the micrometer range. Furthermore, these results have been compared and evaluated with surface profiles measured by white-light interferometry. © 2016 SPIE.
view abstract 10.1117/12.2216849
- Array sensor: Plasmonic improved optical resonance methods and instrument for biomedical diagnostics
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9540 (2015)Microcavity array sensor has been developed for biomedical objects identification. Experimental data on detection and identification of variety of biochemical agents, such as proteins, microelements, antibiotic of different generation etc. in both single and multi-component solutions analyzed on the light scattering of whispering gallery mode optical resonance are represented. © 2015 SPIE.
view abstract 10.1117/12.2183675
- Drying of iron chloride solutions: Laser heating of levitated single particles
Schiemann, M. and Baer, S. and Esen, C. and Ostendorf, A.
Chemical Engineering and Technology 38 (2015)Iron chloride solutions are a waste product from the steel industry, which has to be recovered by the so-called spray roasting process. As this process is a complex sequence of different steps, the drying process of the droplets was separated to get deeper insight into the particle formation process from aqueous iron chloride solutions. Experiments were carried out on single droplets in an acoustic levitator. A CO<inf>2</inf> laser was used as heat source for the drying process. Particles with different shapes were generated by various concentrations of FeCl<inf>2</inf> and laser power. The characteristic time scales and particle size evolution are compared with literature data. To get deeper insight into the particle formation process from aqueous iron chloride solutions, experiments were performed with single droplets in an acoustic levitator. A CO<inf>2</inf> laser served as heat source for drying. Varying FeCl<inf>2</inf> concentrations and laser power allowed for generating particles with different shapes. Time scales and particle size evolution were compared to literature data. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/ceat.201400594
- Effect of process parameters on the formation of laser-induced nanoparticles during material processing with continuous solid-state lasers
Scholz, T. and Dickmann, K. and Ostendorf, A. and Uphoff, H. and Michalewicz, M.
Journal of Laser Applications 27 (2015)During laser material processing with high laser beam intensities, a laser-induced vapor formation can occur. Due to the shockwave behavior of the vapor plume and the associated rapid cooling, a significant particle formation can be initiated by nucleation. The laser radiation interacts with the particles which can result in a dynamic change of the intensity distribution on the surface. Especially in the field of laser remote processing, the attenuation of laser radiation by nanoparticles can influence the process stability and reduce the processing quality. The presented work is focused on the particle formation at a height of 10 mm above the material surface during the laser welding of stainless steel with a fiber laser. The laser beam intensity on the surface was varied between 1.3 and 5.1 MW/cm2. Transmission electron microscopy images of the nanoparticles and high speed images of the vapor propagation in the ambient atmosphere were analyzed. The attenuation of a probe beam in the vapor plume was evaluated in dependence on the wavelength. The results indicate a linear connection between the laser beam power and the particle formation rate. © 2015 Laser Institute of America.
view abstract 10.2351/1.4916081
- Graphene-intercalated Fe2O3/TiO2 heterojunctions for efficient photoelectrolysis of water
Kaouk, A. and Ruoko, T.-P. and Gönüllü, Y. and Kaunisto, K. and Mettenbörger, A. and Gurevich, E. and Lemmetyinen, H. and Ostendorf, A. and Mathur, S.
RSC Advances 5 (2015)Interfacial modification of α-Fe2O3/TiO2 multilayer photoanodes by intercalating few-layer graphene (FLG) was found to improve water splitting efficiency due to superior transport properties, when compared to individual iron and titanium oxides and heterojunctions thereof. Both metal oxides and graphene sheets were grown by plasma-enhanced chemical vapor deposition. Compared to the onset potential achieved for α-Fe2O3 films (1 V vs. RHE), the α-Fe2O3/TiO2 bilayer structure yielded a better onset potential (0.3 V vs. RHE). Heterojunctioned bilayers exhibited a higher photocurrent density (0.32 mA cm-2 at 1.23 V vs. RHE) than the single α-Fe2O3 layer (0.22 mA cm-2 at 1.23 V vs. RHE), indicating more efficient light harvesting and higher concentration of photogenerated charge carriers. For more efficient charge transport at the interface, a few layer graphene sheet was intercalated into the α-Fe2O3/TiO2 interface, which substantially increased the photocurrent density to 0.85 mA cm-2 (1.23 V vs. RHE) and shifted the onset potential (0.25 V vs. RHE). Ultrafast transient absorption spectroscopy studies indicated that the incorporation of FLG between the α-Fe2O3 and TiO2 layers resulted in reduced recombination in the α-Fe2O3 layer. The results showed that graphene intercalation improved the charge separation and the photocurrent density of the FTO/α-Fe2O3/FLG/TiO2 system. © The Royal Society of Chemistry.
view abstract 10.1039/c5ra18330h
- Mid-infrared spectroscopic characterisation of an ultra-broadband tunable EC-QCL system intended for biomedical applications
Vahlsing, T. and Moser, H. and Grafen, M. and Nalpantidis, K. and Brandstetter, M. and Heise, H.M. and Lendl, B. and Leonhardt, S. and Ihrig, D. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9537 (2015)Mid-infrared spectroscopy has been successfully applied for reagent-free clinical chemistry applications. Our aim is to design a portable bed-side system for ICU patient monitoring, based on mid-infrared absorption spectra of continuously sampled body-fluids. Robust and miniature bed-side systems can be achieved with tunable external cavity quantum cascade lasers (EC-QCL). Previously, single EC-QCL modules covering a wavenumber interval up to 250 cm-1 have been utilized. However, for broader applicability in biomedical research an extended interval around the mid-infrared fingerprint region should be accessible, which is possible with at least three or four EC-QCL modules. For such purpose, a tunable ultra-broadband system (1920 - 780 cm-1, Block Engineering) has been studied with regard to its transient emission characteristics in ns time resolution during different laser pulse widths using a VERTEX 80v FTIR spectrometer with step-scan option. Furthermore, laser emission line profiles of all four incorporated EC-QCL modules have been analysed at high spectral resolution (0.08 cm-1) and beam profiles with few deviations from the TEM 00 spatial mode have been manifested. Emission line reproducibility has been tested for various wavenumbers in step tune mode. The overall accuracy of manufacturer default wavenumber setting has been found between ± 3 cm-1 compared to the FTIR spectrometer scale. With regard to an application in clinical chemistry, theoretically achievable concentration accuracies for different blood substrates based on blood plasma and dialysate spectra previously recorded by FTIRspectrometers have been estimated taking into account the now accessible extended wavenumber interval. © 2015 SPIE.
view abstract 10.1117/12.2183952
- Multivariate Characterization of a Continuous Soot Monitoring System Based on Raman Spectroscopy
Grafen, M. and Nalpantidis, K. and Platte, F. and Monz, C. and Ostendorf, A.
Aerosol Science and Technology 49 (2015)The demand for precise and continuous monitoring of air quality has increased. An important descriptor of air quality is the concentration of problematic carbonaceous particles responsible for diseases and climate change. The specific measurement of carbonaceous components in the air is still a topic in research and development. Here, we introduce an integrated and continuous soot monitoring system based on Raman spectroscopy. In comparison to the often utilized light absorption, Raman spectroscopy is capable of determining the graphitic microstructure found in carbonaceous particles. We present first measurements taken in a controlled environment contaminated with varying concentrations of diesel soot. The Raman bands of soot turn out to be tightly mixed up with signals from secondary physical factors. In order to evaluate the data, multivariate methods are applied. After determination of the latent variables using principal component analysis (PCA), the system is further rotated using a linear discriminant analysis (LDA)-criterion and a subsequent nonlinear iterative partial least squares (NIPALS)-like step. One of the variables obtained by this methodology can be shown to exclusively describe the optical filter loading while the orthogonal factor space allows for conclusions on the secondary factors.Copyright 2015 American Association for Aerosol Research © 2015 Copyright © American Association for Aerosol Research.
view abstract 10.1080/02786826.2015.1089352
- Tutorial: Laser in material nanoprocessing
König, K. and Ostendorf, A.
Optically Induced Nanostructures: Biomedical and Technical Applications (2015)
view abstract 10.1515/9783110354324-004
- Biochemical component identification by light scattering techniques in whispering gallery mode optical resonance based sensor
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8952 (2014)Experimental data on detection and identification of variety of biochemical agents, such as proteins (albumin, interferon, C reactive protein), microelements (Na+, Ca+), antibiotic of different generations, in both single and multi component solutions under varied in wide range concentration are represented. Analysis has been performed on the light scattering parameters of whispering gallery mode (WGM) optical resonance based sensor with dielectric microspheres from glass and PMMA as sensitive elements fixed by spin - coating techniques in adhesive layer on the surface of substrate or directly on the coupling element. Sensitive layer was integrated into developed fluidic cell with a digital syringe. Light from tuneable laser strict focusing on and scattered by the single microsphere was detected by a CMOS camera. The image was filtered for noise reduction and integrated on two coordinates for evaluation of integrated energy of a measured signal. As the entrance data following signal parameters were used: relative (to a free spectral range) spectral shift of frequency of WGM optical resonance in microsphere and relative efficiency of WGM excitation obtained within a free spectral range which depended on both type and concentration of investigated agents. Multiplexing on parameters and components has been realized using spatial and spectral parameters of scattered by microsphere light with developed data processing. Biochemical component classification and identification of agents under investigation has been performed by network analysis techniques based on probabilistic network and multilayer perceptron. Developed approach is demonstrated to be applicable both for single agent and for multi component biochemical analysis. © 2014 SPIE.
view abstract 10.1117/12.2039026
- Biochemical component identification by plasmonic improved whispering gallery mode optical resonance based sensor
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9126 (2014)Experimental data on detection and identification of variety of biochemical agents, such as proteins, microelements, antibiotic of different generation etc. in both single and multi component solutions under varied in wide range concentration analyzed on the light scattering parameters of whispering gallery mode optical resonance based sensor are represented. Multiplexing on parameters and components has been realized using developed fluidic sensor cell with fixed in adhesive layer dielectric microspheres and data processing. Biochemical component identification has been performed by developed network analysis techniques. Developed approach is demonstrated to be applicable both for single agent and for multi component biochemical analysis. Novel technique based on optical resonance on microring structures, plasmon resonance and identification tools has been developed. To improve a sensitivity of microring structures microspheres fixed by adhesive had been treated previously by gold nanoparticle solution. Another technique used thin film gold layers deposited on the substrate below adhesive. Both biomolecule and nanoparticle injections caused considerable changes of optical resonance spectra. Plasmonic gold layers under optimized thickness also improve parameters of optical resonance spectra. Biochemical component identification has been also performed by developed network analysis techniques both for single and for multi component solution. So advantages of plasmon enhancing optical microcavity resonance with multiparameter identification tools is used for development of a new platform for ultra sensitive label-free biomedical sensor. © 2014 SPIE.
view abstract 10.1117/12.2051486
- Comparison of in situ and ex situ methods for synthesis of two-photon polymerization polymer nanocomposites
Guo, Q. and Ghadiri, R. and Weigel, T. and Aumann, A. and Gurevich, E.L. and Esen, C. and Medenbach, O. and Cheng, W. and Chichkov, B. and Ostendorf, A.
Polymers 6 (2014)This article reports about nanocomposites, which refractive index is tuned by adding TiO2 nanoparticles. We compare in situ/ex situ preparation of nanocomposites. Preparation procedure is described, properties of nanocomposites are compared, and especially we examine the applicability of two-photon polymerization (2PP) of synthesized nanocomposites. All prepared samples exhibit suitable optical transparency at specific laser wavelengths. Three-dimensional structures were generated by means of two-photon polymerization effect induced by a femtosecond laser. © 2014 by the authors; licensee MDPI.
view abstract 10.3390/polym6072037
- Dynamical behavior of laser-induced nanoparticles during remote processing
Scholz, T. and Dickmann, K. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8963 (2014)Laser remote processing is used in a wide field of industrial applications. Among other things, it is characterized by flexible beam guidance in combination with high processing velocities. But in most cases process gas support in the interaction zone is omitted. Consequently, interaction mechanism between the vapor plume and the incident laser radiation can dynamically affect the process stability. Referring to remote welding with high brilliant laser sources having a wavelength around 1 μm, the interaction between the incident laser radiation and formed particles plays an important role. The presented work shows results of the investigation of the laser-induced particle formation during the laser welding of stainless steel with a 2 kW fiber laser under remote conditions. It is therefore concentrated on the dynamical behavior of the laser-induced particle formation and the dependence of the particle formation on the laser beam power. TEM images of formed particles were analyzed. In addition, the radiation of a LED was directed through the vapor plume. On the one hand, the dynamic of the attenuation was considered. On the other hand, the Rayleigh approximation was used in order to evaluate the detected signals. © 2014 SPIE.
view abstract 10.1117/12.2035197
- Ex-situ preparation of high-conductive polymer/SWNTs nanocomposites for structure fabrication
Guo, Q. and Ghadiri, R. and Weigel, T. and Aumann, A. and Gurevich, E.L. and Esen, C. and Li, Y. and Cheng, W. and Chichkov, B. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9277 (2014)This paper reports ex-situ preparation of conductive polymer/single-walled carbon nanotubes (SWNTs) nanocomposites by adding high conductive SWNTs to the polymer matrix. Sonication methods were used to disperse the SWNTs in the polymer. The conductivity of the nanocomposites is tuned by increasing the concentration of SWNTs. Furthermore, we present two-photon polymerization (2PP) method to fabricate structures on the basis of conductive photosensitive composites. The conductive structures were successfully generated by means of 2PP effect induced by a femtosecond laser. © 2014 SPIE.
view abstract 10.1117/12.2071870
- Generation of microfluidic flow using an optically assembled and magnetically driven microrotor
Köhler, J. and Ghadiri, R. and Ksouri, S.I. and Guo, Q. and Gurevich, E.L. and Ostendorf, A.
Journal of Physics D: Applied Physics 47 (2014)The key components in microfluidic systems are micropumps, valves and mixers. Depending on the chosen technology, the realization of these microsystems often requires rotational and translational control of subcomponents. The manufacturing of such active components as well as the driving principle are still challenging tasks. A promising all-optical approach could be the combination of laser direct writing and actuation based on optical forces. However, when higher actuation velocities are required, optical driving might be too slow. Hence, a novel approach based on optical assembling of microfluidic structures and subsequent magnetic actuation is proposed. By applying the optical assembly of microspherical building blocks as the manufacturing method and magnetic actuation, a microrotor was successfully fabricated and tested within a microfluidic channel. The resulting fluid flow was characterized by introducing an optically levitated measuring probe particle. Finally, a freely moving tracer particle visualizes the generated flow. The tracer particle analysis shows average velocities of 0.4-0.5 μm s-1 achieved with the presented technology. © 2014 IOP Publishing Ltd Printed.
view abstract 10.1088/0022-3727/47/50/505501
- Generation of NiTi nanoparticles by femtosecond laser ablation in liquid
Chakif, M. and Essaidi, A. and Gurevich, E. and Ostendorf, A. and Prymak, O. and Epple, M.
Journal of Materials Engineering and Performance 23 (2014)NiTi was investigated as a model system for a binary alloy where the properties strongly depend on the relative proportion of the two elements and on the grain size. The NiTi nanoparticles were generated by laser ablation in water. For the analysis of the particle size distribution, we used transmission electron microscopy and dynamic light scattering. Here, we found a broad particle size distribution (10-200 nm). Furthermore, the temperature-resolved x-ray powder diffraction and differential scanning calorimetry (DSC) were used to evaluate the phase transition behavior of the generated NiTi nanoparticles. Here, we found an interesting effect. During the heating by DSC, an austenite phase transition and a weak martensite phase transition in the NiTi nanoparticles appeared. Moreover, the phase transformation temperature was about 40 K lower than that of the bulk target. © 2014 ASM International.
view abstract 10.1007/s11665-014-1007-7
- Holographic optical tweezers: Microassembling of shape-complementary 2PP building blocks
Ksouri, S.I. and Mattern, M. and Köhler, J. and Aumann, A. and Zyla, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9164 (2014)Based on an ongoing trend in miniaturization and due to the increased complexity in MEMS-technology new methods of assembly need to be developed. Recent developments show that particularly optical forces are suitable to meet the requirements. The unique advantages of optical tweezers (OT) are attractive due to their contactless and precise manipulation forces. Spherical as well as non-spherical shaped pre-forms can already be assembled arbitrarily by using appropriate beam profiles generated by a spatial light modulator (SLM), resulting in a so called holographic optical tweezer (HOT) setup. For the fabrication of shape-complementary pre-forms, a two-photon-polymerization (2PP) process is implemented. The purpose of the process combination of 2PP and HOT is the development of an optical microprocessing platform for assembling arbitrary building blocks. Here, the optimization of the 2PP and HOT processes is described in order to allow the fabrication and 3D assembling of interlocking components. Results include the analysis of the dependence of low and high qualities of 2PP microstructures and their manufacturing accuracy for further HOT assembling processes. Besides, the applied detachable interlocking connections of the 2PP building blocks are visualized by an application example. In the long-term a full optical assembly method without applying any mechanical forces can thus be realized. © 2014 SPIE.
view abstract 10.1117/12.2061829
- Impact of process parameters on the laser-induced nanoparticle formation during keyhole welding under remote conditions
Scholz, T. and Dickmann, K. and Ostendorf, A.
Physics Procedia 56 (2014)The interaction between the vapor plume and the incident laser radiation affects remote laser welding. Relating to laser systems with an emitted wavelength around 1 μm, a significant loss mechanism can be traced back to the extinction by laser-induced particle formation. Due to the tight coupling between the particle formation and the evaporation rate inside the keyhole, the particle formation shows a strong dependence on the keyhole geometry and thus on process parameters (e.g. feed rate and laser beam power). In order to verify the relationship between particle formation and process parameters, the beam of a broadband LED was guided through the vapor plume during the welding processes with a fiber laser. The attenuated probe beam was analyzed in dependence on the wavelength. In addition, the propagation of the vapor plume was investigated by using high speed imaging. © 2014 The Authors. Published by Elsevier B.V.
view abstract 10.1016/j.phpro.2014.08.151
- Impact of solvent mixture on iron nanoparticles generated by laser ablation
Chakif, M. and Prymak, O. and Slota, M. and Heintze, E. and Gurevich, E.L. and Esen, C. and Bogani, L. and Epple, M. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8955 (2014)The present work reveals the structural and magnetic properties of iron oxide (FexOy) nanoparticles (NPs) prepared by femtosecond laser ablation. The FexOy-NPs were produced in solutions consisting of different ratios of water and acetone. Laser ablation in water yields agglomerates and that in acetone yields chain structures whereas that in water/acetone show a mixture of both. We observe significant fabrication dependent properties such as different crystallinities and magnetic behaviors. The structural characterization shows a change from iron (Fe) to a Fe xOy state of the NPs which depends on the solution composition. Furthermore, transmission electron microscopy measurements exhibit a broad particle size distribution in all samples but with significant differences in the mean sizes. Using magnetic measurements we show that nanoparticles fabricated in pure acetone have lower coercive fields which come along with a smaller mean particle size and therefore increasing superparamagnetic behavior. © 2014 SPIE.
view abstract 10.1117/12.2037682
- Influence of multiple particles in optical tweezers on the trapping efficiency
Weigel, T. and Ghadiri, R. and Esen, C. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8999 (2014)Since the early times of Arthur Ashkins groundbreaking experiments on optical tweezers, a great number of theoretical works was dedicated to this subject. Most of them treated the optical trapping of single spherical or elliptical particles. In the last years optical tweezers have become more and more a tool for assembling three dimensional structures using single microspheres as building blocks. Since all structures and particles inside the light beams influence the properties of the traps, we investigated theoretically the influence of additional single particles and particle arrays on the properties of optical traps. For this reason a geometrical optics based model is used with the inherent flexibility to be applied for various shapes and particle numbers.
view abstract 10.1117/12.2037322
- Micro-patterning of self-assembled organic monolayers by using tunable ultrafast laser pulses
Maragkaki, S. and Aumann, A. and Schulz, F. and Schröter, A. and Schöps, B. and Franzka, S. and Hartmann, N. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer (SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers, which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of 480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated glass substrates with a film thickness of 1-2 nm and 40 nm respectively. © 2014 SPIE.
view abstract 10.1117/12.2037716
- Optical tweezers as manufacturing and characterization tool in microfluidics
Köhler, J. and Ghadiri, R. and Ksouri, S.I. and Gurevich, E.L. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9164 (2014)Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields. © 2014 SPIE.
view abstract 10.1117/12.2063080
- Phase equilibrium measurements of acoustically levitated squalane-CO 2 mixtures by Raman spectroscopy
Baer, S. and Esen, C. and Ostendorf, A.
Journal of Raman Spectroscopy 45 (2014)This work describes the phase equilibrium measurements of acoustically levitated binary mixtures with concentration measurements using Raman spectroscopy without sample extraction of the autoclave. The levitator design is implemented in a Single-Droplet Optical Cell for levitation processes under varying atmospheres. The advantages of acoustic levitation of small droplets under increased temperatures and pressure combined with spectroscopic applications like Raman spectroscopy enable novel experiments possibly relevant to the fields of chemical engineering. To the author's knowledge, this is the first use of Raman spectroscopy for phase equilibria investigations on acoustically levitated droplets under high pressure and temperature. The results show very good agreements with literature data. Copyright © 2014 John Wiley & Sons, Ltd. This work describes the phase equilibrium measurements of acoustically levitated binary mixtures with concentration measurements using Raman spectroscopy without sample extraction of the autoclave. The levitator design is implemented in a Single-Droplet Optical Cell for levitation processes under varying atmospheres. The advantages of acoustic levitation of small droplets under increased temperatures and pressure combined with spectroscopic applications like Raman spectroscopy enable novel experiments possibly relevant to the fields of chemical engineering. Copyright © 2014 John Wiley & Sons, Ltd.
view abstract 10.1002/jrs.4511
- Plasmonic improvement of microcavity biomedical sensor spectroscopic characteristics
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A. and Ghadiri, R.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8957 (2014)New opportunity to improve a sensetivity of a label-free biomolecule detection in sensing systems based on microcavity evanescent wave optical sensors has been recently found and is being under intensive development. Novel technique based on combination of optical resonance on microring structures with plasmon resonance. Recently developed tools based on neural network data processing can realize real-Time identification of biological agents. So combining advantages of plasmon enhancing optical microcavity resonance with identification tools can give a new platform for ulta sensitive label-free biomedical sensor. Our developed technique used standard glass and polymer microspheres as sensetive elements. They are fixed in the solution flow by adhesive layer on the surface being in the field of evanescence wave. Sensitive layer have been treated by gold nanoparticel (GN) solution. Another technique used thin film gold layers deposited on the substrate below adhesive. The light from a tuneable diode laser is coupled into the microsphere through a prism and was sharply focussed on the single microsphere. Images were recorded by CMOS camera. Normalized by free spectral range resonance shift of whispering gallery mode (WGM) and a relative efficiency of their excitation were used as input data for biomolecule classification. Both biomolecules and NP injection was obtained caused WGM spectra modification. But after NP treatment spectral shift and intensity of WGM resonances in biomolecule solutions increased. WGM resonances in microspheres fixed on substrate with gold layer with optimized layer thickness in biomolecule solutions also had higher intensity and spectra modification then without gold layer. © 2014 SPIE.
view abstract 10.1117/12.2039049
- Resolution and aspect ratio in two-photon lithography of positive photoresist
Aumann, A. and Isabelle Ksouri, S. and Guo, Q. and Sure, C. and Gurevich, E.L. and Ostendorf, A.
Journal of Laser Applications 26 (2014)In this work, the authors report on investigations of two-photon lithography of positive photoresist. The dependency of the pattern linewidth on variation in the processing parameters, like the laser patterning velocity or power of the femtosecond laser oscillator, is presented. The influence of the scan velocity between 0.38 and 1.90 mm/s on the resolution is discussed for a layer thickness of 3.5 μm. By using a commercial positive photoresist, an aspect ratio of 5 has been realized for grid structures and the qualities of the produced structures are discussed. © 2014 Laser Institute of America.
view abstract 10.2351/1.4857275
- Selective Ablation Of Thin Films By Pulsed Laser
Ostendorf, A. and Gurevich, E.L. and Shizhou, X.
Springer Series in Materials Science 195 (2014)Laser direct patterning of thin films with minimal substrate damage is receiving attention in many industrial applications, e.g., photovoltaic or flat displays. Substantial progress has been made in understanding of the laser-matter interactions and reveals that laser-induced thermal effects are significantly critical in most of laser ablation processes. The thermal penetration depth, determined by the optical absorption and subsequently the thermal diffusion length, are heavily dependent on the applied laser pulse duration. The ratios between the film thickness, the thermal and the optical penetration depths separate the ablation to be film-like or bulk-like behavior of the thin-film ablation. © Springer International Publishing Switzerland 2014.
view abstract 10.1007/978-3-319-05987-7_9
- Selective laser patterning in organic solar cells
Abreu Fernandes, S. and Maragkaki, S. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 9180 (2014)Selective laser patterning for integrative serious connection has been industrially established in inorganic thin film solar cells based on glass substrates since a few years. In organic solar cells (OSC) the used materials significantly differ in terms of their patterning behavior. Due to their processability by wet chemical methods inverted architectures are often preferred in organic solar cells which allow the patterning by ultrashort laser pulses in substrate and superstrate configuration. Starting with an introduction of the ablation mechanisms taking place in OSC thin films, an overview of the current state-of-the-art in laser patterning of organic solar cells is presented. Besides progress in research also current achievements in industrial applications are illustrated. © 2014 SPIE.
view abstract 10.1117/12.2061248
- Spherical optical microresonator array as a multi-purpose measuring device
Weigel, T. and Dobbelstein, H. and Esen, C. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8960 (2014)Optical resonances of spherical microresonators are of great interest measurements with high sensitivity. Usually the quantity to be measured is determined by the shift of the resonances of a single particle. Unfortunately, for this purpose, an expensive low-bandwidth tunable laser system with high accuracy is needed. When using an array of microresonators with slightly different size, each particle has a different resonance behavior. A change of the quantity to be measured leads to a change of the intensity distribution over the entire array. Therefore, using a microresonator array it is sufficient to measure the intensity distribution over all particles at a fixed wavelength. © 2014 SPIE.
view abstract 10.1117/12.2037312
- Tailored beam shaping for laser spot joining of highly conductive thin foils
Funck, K. and Nett, R. and Ostendorf, A.
Physics Procedia 56 (2014)Laser spot joining of thin metallic foils in the order of 100 micrometer and below has a number of interesting applications in electronic industry. However, high thermal conductivity and thermal expansion of common materials largely prohibit spot joints with a sufficiently large contact area needed to satisfy mechanical and electrical requirements. Of the numerous possibilities to positively influence the process of such joints we investigate using a pulsed Nd:YAG laser to generate spot joints of thin foils in combination with a beam shaping optic to tailor the temperature profile during laser spot joining of thin foils. This allows for increased contact area, stabilized process behavior and offers the potential for joining ultra thin foils far below 100 μm. Different configurations are examined, results are presented and discussed, mainly in terms of their general impact on the micro joining process. © 2014 The Authors. Published by Elsevier B.V.
view abstract 10.1016/j.phpro.2014.08.082
- A modular assembling platform for manufacturing of microsystems by optical tweezers
Ksouri, S.I. and Aumann, A. and Ghadiri, R. and Prüfer, M. and Baer, S. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8810 (2013)Due to the increased complexity in terms of materials and geometries for microsystems new assembling techniques are required. Assembling techniques from the semiconductor industry are often very specific and cannot fulfill all specifications in more complex microsystems. Therefore, holographic optical tweezers are applied to manipulate structures in micrometer range with highest flexibility and precision. As is well known non-spherical assemblies can be trapped and controlled by laser light and assembled with an additional light modulator application, where the incident laser beam is rearranged into flexible light patterns in order to generate multiple spots. The complementary building blocks are generated by a two-photon-polymerization process. The possibilities of manufacturing arbitrary microstructures and the potential of optical tweezers lead to the idea of combining manufacturing techniques with manipulation processes to microrobotic processes. This work presents the manipulation of generated complex microstructures with optical tools as well as a storage solution for 2PP assemblies. A sample holder has been developed for the manual feeding of 2PP building blocks. Furthermore, a modular assembling platform has been constructed for an -all-in-one' 2PP manufacturing process as a dedicated storage system. The long-term objective is the automation process of feeding and storage of several different 2PP micro-assemblies to realize an automated assembly process. © 2013 SPIE.
view abstract 10.1117/12.2025273
- Diagnostics of biomedical agents by whispering gallery mode optical resonance based sensor
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Proceedings of the International Conference on Advanced Optoelectronics and Lasers, CAOL (2013)Experimental data on optical resonance spectra of whispering gallery modes of dielectric microspheres in antibiotic solutions under varied in wide range concentration are represented. Optical resonance was demonstrated could be detected at a laser power of less than 1 microwatt. Several antibiotics of different generations: Amoxicillin, Azithromycin, Cephazolin, Chloramphenicol, Levofloxacin, Lincomicin Benzylpenicillin, Riphampicon both in de-ionized water and physiological solution had been used for measurements. Both spectral shift and the structure of resonance spectra were of specific interest in this investigation. Drag identification has been performed by developed multilayer perceptron network. The network topology was designed included: a number of the hidden layers of multilayered perceptron, a number of neurons in each of layers, a method of training of a neural network, activation functions of layers, type and size of a deviation of the received values from required values. For a network training the method of the back propagation error in various modifications has been used. Input vectors correspond to 6 classes of biological substances under investigation. The result of classification was considered as positive when each of the region, representing a certain substance in a space: relative spectral shift of an optical resonance maxima - relative efficiency of excitation of WGM, was singly connected. © 2013 IEEE.
view abstract 10.1109/CAOL.2013.6657657
- Drag detection and identification by whispering gallery mode optical resonance based sensor
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8801 (2013)Experimental data on optical resonance spectra of whispering gallery modes of dielectric microspheres in antibiotic solutions under varied in wide range concentration are represented. Optical resonance was demonstrated could be detected at a laser power of less than 1 microwatt. Several antibiotics of different generations: Amoxicillin, Azithromycin, Cephazolin, Chloramphenicol, Levofloxacin, Lincomicin Benzylpenicillin, Riphampicon both in deionized water and physiological solution had been used for measurements. Both spectral shift and the structure of resonance spectra were of specific interest in this investigation. Drag identification has been performed by developed multilayer perceptron network. The network topology was designed included: a number of the hidden layers of multilayered perceptron, a number of neurons in each of layers, a method of training of a neural network, activation functions of layers, type and size of a deviation of the received values from required values. For a network training the method of the back propagation error in various modifications has been used. Input vectors correspond to 6 classes of biological substances under investigation. The result of classification was considered as positive when each of the region, representing a certain substance in a space: relative spectral shift of an optical resonance maxima - relative efficiency of excitation of WGM, was singly connected. It was demonstrated that the approach described in the paper can be a promising platform for the development of sensitive, lab-on-chip type sensors that can be used as an express diagnostic tools for different drugs and instrumentation for proteomics, genomics, drug discovery, and membrane studies.
view abstract 10.1117/12.2028487
- Femtosecond laser ablation of ITO/ZnO for thin film solar cells
Abreu Fernandes, S. and Schoeps, B. and Kowalick, K. and Nett, R. and Esen, C. and Pickshaus, M. and Ostendorf, A.
Physics Procedia 41 (2013)Femtosecond laser ablation of an Indium tin oxide/Zinc oxide (ITO/ZnO-Nanoparticles) multilayer coated on polyethylene terephthalate (PET) substrate has been investigated. Single pulse ablation thresholds were determined for front side and back side irradiation. The ablation behavior of ITO/ZnO on PET is studied and laser scribing using singlepass and multi-pass patterning is performed and analyzed regarding the usability to serial connect organic solar cells. © 2013 The Authors.
view abstract 10.1016/j.phpro.2013.03.151
- Femtosecond-laser processing of nitrobiphenylthiol self-assembled monolayers
Schröter, A. and Franzka, S. and Koch, J. and Chichkov, B.N. and Ostendorf, A. and Hartmann, N.
Applied Surface Science 278 (2013)Single-pulse femtosecond laser patterning of nitrobiphenylthiol monolayers on Au-coated Si substrates at λ = 800 nm, τ < 30 fs and ambient conditions has been investigated. After laser processing wet etching experiments are performed. Laser irradiation reduces the chemical resistance of the coating. In particular, the monolayer acts as a positive-tone resist. Burr-free pattern transfer is feasible at laser pulse fluences between 1 and 2.7 J/cm 2. Minimum structure sizes at a 1/e laser spot diameter of about 1 μm are close to 300 nm, i.e. sub-wavelength processing is demonstrated. Noteworthy, however, no indications for negative-tone resist properties of processed monolayers are evident, that is, cross-linking of the biphenyl moieties, if at all, is marginal. Also, complementary labeling experiments provide no evidence for chemical transformation of the nitro end groups into amine functionalities. Perspectives of resonant fs-laser processing in exploiting the particular prospects of nitrobiphenylthiol monolayers as negative-tone resists and chemically patternable platforms are discussed. © 2013 Elsevier B.V.
view abstract 10.1016/j.apsusc.2013.01.170
- Fluid pumping cell of photonic - Plasmonic microcavity sensor for biomedical application
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Saetchnikov, A.V. and Schweiger, G. and Ostendorf, A.
Proceedings of the International Conference on Advanced Optoelectronics and Lasers, CAOL (2013)Fluid pumping cell for plasmonic - photonic microcavity sensor for label-free biomolecule detection and identification has been developed and tested with drug and gold nanoparticle solutions including additional gold layer. Resonant spectra parameters have being analyzed. © 2013 IEEE.
view abstract 10.1109/CAOL.2013.6657577
- Investigation of the formation of nanoparticles during laser remote welding
Scholz, T. and Dickmann, K. and Ostendorf, A.
Physics Procedia 41 (2013)New developments and characteristics of high brilliant laser sources have led to new applications in the field of laser remote processing. Due to high particle formation rates within the vapor plume, a significant influence of the interaction between laser radiation and nanoparticles on the process may occur. The presented work shows results of the investigation of the dynamical formation of nanoparticles within the vapor plume during the welding of stainless steel with a 2 kW Multi-Mode fiber laser under laser remote conditions. The particle size distribution is measured by the evaluation of TEM-images, whereas, the plasma temperature and particle density are analyzed in dependence of the irradiation time. © 2013 The Authors.
view abstract 10.1016/j.phpro.2013.03.055
- Optical micro-assembling of non-spherical particles
Ksouri, S. I. and Aumann, A. and Ghadiri, R. and Ostendorf, A.
Complex Light and Optical Forces Vii 8637 (2013)Holographic optical tweezers have been developed for the manipulation of polymeric microparticles or biological cells with almost circular shape. As is well known, spherical particles can be trapped and controlled by optical tweezers and assembled with an additional light modulator application. Complementary building blocks, which are used in the following experiments, are generated by a two-photon-polymerization process in micrometer range and are not equipped with spherical trapping points. The possibilities of manufacturing arbitrary building blocks within the 2PP process and the potential of HOTs lead to the idea of combining manufacturing techniques with manipulation processes in a bottom-up operation. In this work we present an experimental setup with an integrated fiber laser for holographic optical trapping of non-spherical building blocks. Furthermore experimental requirements which permit trapping will be illustrated.
view abstract 10.1117/12.2002315
- Optical tweezers in microassembly
Ostendorf, A. and Ghadiri, R. and Ksouri, S.I.
Proceedings of SPIE - The International Society for Optical Engineering 8607 (2013)Integrated hybrid MEMS require new micromanipulation devices in assembly processes. Although absolute forces are restricted optical tweezers are promising tools with unique advantages. Recent developments in beam shaping allow the control of a large number of different particles. Optical manipulation can also be used to assemble tiny structures by a generative process. Any type of particle, primarily coated with high-affinity biomolecules, can be applied as building blocks to form complex structures. By moving the particle into the requested orientation by holographic optical tweezers complex parts become possible. Also, shape-complimentary preforms can be fabricated with 2-photon-polymerization (2PP) and utilized to assemble the desired structure. Finally, microvalves and motors in lab-on-a-chip systems can be optically fabricated and also driven by optical forces. © 2013 Copyright SPIE.
view abstract 10.1117/12.2006127
- Optically generated sub-100 nm structures for biomedical and technical applications
König, K. and Ostendorf, A.
Physics Procedia 41 (2013)Near infrared femtosecond laser - material interactions such as multiphoton ionization and plasma formation provide the possibility to perform 3D nanoprocessing in a variety of organic and non-organic materials. Sub-100 nm structures that are more than one order smaller than the laser wavelength and therefore far below Abbes diffraction limit can be generated on the surface and inside the bulk. Compared to conventional extreme ultraviolet lithography, multiphoton technology provides the chance to produce 3D nanofeatures much simpler, less expensive, more flexible, and even inside transparent bulk material. Rapid prototyping and low power nanosurgery are two examples of applications of this novel nonlinear nanotechnology tool. This paper provides an overview of projects within Germany regarding to multiphoton generation of sub-100 nm structures. © 2013 The Authors.
view abstract 10.1016/j.phpro.2013.03.044
- Size control of gold nanoparticles during laser ablation in liquids with different functional molecules
Essaidi, A. and Chakif, M. and Schöps, B. and Aumman, A. and Xiao, S. and Esen, C. and Ostendorf, A.
Journal of Laser Micro Nanoengineering 8 (2013)Over the last decade many papers dedicated to the generation of nanoparticles in liquids using la-ser ablation have been published. They have shown that the nanoparticles size distribution is de-pendent of the laser fluence and shifts towards smaller sizes when the surfactant concentration is in-creased. In this paper an alternative approach is presented that permits the control of the size distri-bution of gold nanoparticles during laser ablation in aqueous solutions containing different surfactant molecules with different shapes and size.
view abstract 10.2961/jlmn.2013.02.0003
- A light-driven turbine-like micro-rotor and study on its light-to-mechanical power conversion efficiency
Lin, X.-F. and Hu, G.-Q. and Chen, Q.-D. and Niu, L.-G. and Li, Q.-S. and Ostendorf, A. and Sun, H.-B.
Applied Physics Letters 101 (2012)A light driven micro-rotor is a useful telecontrolled device free of mechanical contact for power supply. However, low efficiency in converting light to mechanical power detracts from its advantages because it incurs a high power consumption that might result in unwanted effects. For a systematic study on conversion efficiency, we designed a turbine-like micro-rotor and made a quantitative analysis by computational fluid dynamics and semiclassical optics. Much larger in size than those ever reported, our rotor could rotate at over 500 r/min. Denoted by average angular momentum transfer, its conversion efficiency was experimentally determined as high as 34.55 /photon. © 2012 American Institute of Physics.
view abstract 10.1063/1.4751464
- Classification of antibiotics by neural network analysis of optical resonance data of whispering gallery modes in dielectric microspheres
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8424 (2012)A novel emerging technique for the label-free analysis of nanoparticles and biomolecules in liquid fluids using optical micro cavity resonance of whispering-gallery-type modes is being developed. A scheme based on polymer microspheres fixed by adhesive on the evanescence wave coupling element has been used. We demonstrated that the only spectral shift can't be used for identification of biological agents by developed approach. So neural network classifier for biological agents and micro/nano particles classification has been developed. The developed technique is the following. While tuning the laser wavelength images were recorded as avi-file. All sequences were broken into single frames and the location of the resonance was allocated in each frame. The image was filtered for noise reduction and integrated over two coordinates for evaluation of integrated energy of a measured signal. As input data normalized resonance shift of whispering-gallery modes and the relative efficiency of whispering-gallery modes excitation were used. Other parameters such as polarization of excited light, "center of gravity" of a resonance spectra etc. are also tested as input data for probabilistic neural network. After network designing and training we estimated the accuracy of classification. The classification of antibiotics such as penicillin and cephasolin have been performed with the accuracy of not less 97 %. Developed techniques can be used for lab-on-chip sensor based diagnostic tools as for identification of different biological molecules, e.g. proteins, oligonucleotides, oligosaccharides, lipids, small molecules, viral particles, cells and for dynamics of a delivery of medicines to bodies. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
view abstract 10.1117/12.920397
- Development of a single-axis ultrasonic levitator and the study of the radial particle oscillations
Baer, S. and Andrade, M.A.B. and Esen, C. and Adamowski, J.C. and Ostendorf, A.
AIP Conference Proceedings 1433 (2012)This work describes the development and analysis of a new single-axis acoustic levitator, which consists of a 38 kHz Langevin-type piezoelectric transducer with a concave radiating surface and a concave reflector. The new levitator design allows to significantly reducing the electric power necessary to levitate particles and to stabilize the levitated sample in both radial and axial directions. In this investigation the lateral oscillations of a levitated particle were measured with a single point Laser Doppler Vibrometer (LDV) and an image evaluation technique. The lateral oscillations were measured for different values of particle diameter, particle density and applied electrical power. © 2012 American Institute of Physics.
view abstract 10.1063/1.3703133
- Experimental analysis of the particle oscillations in acoustic levitation
Andrade, M.A.B. and Buiochi, F. and Baer, S. and Esen, C. and Ostendorf, A. and Adamowski, J.C.
IEEE International Ultrasonics Symposium, IUS (2012)The objective of this paper is to investigate the spontaneous particle oscillations that occur in a single-axis acoustic levitator. A high speed camera was used to record the particle oscillations of spheres and a tracking algorithm was implemented in Matlab to obtain the sphere position as a function of time. A FFT analysis was applied to obtain the frequencies of the particle oscillations in both axial and radial directions. The frequencies of the particle oscillations were compared with the theoretical frequencies, which were obtained by a theoretical model that assumes that the oscillations of the levitated sphere can be described by a spring-mass system. © 2012 IEEE.
view abstract 10.1109/ULTSYM.2012.0502
- Incubation effect and its influence on laser patterning of ITO thin film
Xiao, S. and Gurevich, E.L. and Ostendorf, A.
Applied Physics A: Materials Science and Processing 107 (2012)Results are presented on the surface damage thresholds of ITO thin films induced by single-and multipulse laser irradiation at a pulse duration of 10 ps and a wavelength of 1064 nm. For multi-pulse ablation the incubation effect results in a reduction of the damage threshold, especially apparent at low pulse numbers and very small film thicknesses. The incubation effect attributes to the accumulation of defect sites and/or the storage of thermal stressstrain energy induced by the incident laser pulses. An incubation coefficient of S = 0.82 has been obtained which is independent on the film thickness in the range of 10-100 nm. In practical applications, the incubation effect determines the laser patterning structure of ITO films while increasing the pulse overlapping rate. The width of the patterned line can be predicted by the proposed model involving the laser fluence, the overlapping rate and the incubation coefficient. © 2012 Springer-Verlag.
view abstract 10.1007/s00339-012-6820-y
- Laser direct writing of high refractive index polymer/TiO 2 nanocomposites
Guo, Q. and Ghadiri, R. and Xiao, S. and Esen, C. and Medenbach, O. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8243 (2012)This work reports the preparation of polymer/TiO 2 nanocomposite by adding TiO 2 nanoparticles to the polymer matrices. TiO 2 nanoparticles can be effectively dispersed into the polymer. The refractive index of the nanocomposites can be tuned by increasing the concentration of TiO 2 nanoparticles. The prepared samples exhibit excellent optical transparency in the Vis-NIR region, i.e. at two-photon polymerization (TPP) processing wavelength, and can be used to write threedimensional structures by means of TPP. Structures with high refractive index have been produced with the novel ultrahigh resolution technology based on TPP processing of polymer/TiO 2 nanocomposites. © 2012 SPIE.
view abstract 10.1117/12.906688
- Laser direct writing of nanocompounds
Ostendorf, A. and Chakif, M. and Guo, Q.
Materials Research Society Symposium Proceedings 1365 (2012)Laser direct polymerization has been proven as a powerful tool to generate microstructures. Often photosensitive polymer materials are used because they can be tuned by photoactive molecules to be susceptible to a specific wavelength of light to initiate the polymerization process. One of the main drawbacks of this technique is the lack of functional polymers, e.g. conductive, magnetic, mechanical, optical or bioactive materials. Nanocomposites (nanocompounds), i.e. polymers with inorganic nanomaterials incorporated in the matrix offer a huge variety of new functionalities. A new approach will be presented how functional nanocomposite polymers can be generated and used for laser direct writing techniques. This can open the door for completely new MEMS and MOEMS devices comprising active and passive subcomponents. © 2011 Materials Research Society.
view abstract 10.1557/opl.2011.1221
- Microassembly of complex and three-dimensional microstructures using holographic optical tweezers
Ghadiri, R. and Weigel, T. and Esen, C. and Ostendorf, A.
Journal of Micromechanics and Microengineering 22 (2012)In this paper we investigate a flexible method for the fabrication of complex microstructures using binding microparticles. Utilizing optical forces, micro-objects are caught, positioned and used as building blocks to form defined structures, analogous to assembling processes in the macroscopic world. Durable linkage between the individual particles is realized using biomolecules with high affinities applied as particle coatings. Planar structures can be assembled employing optical manipulation as well as three-dimensional patterns by stacking the generated layers. Even the properties of the generated structures can be locally designed as desired by using building blocks from diverse materials exhibiting different properties. This method benefits from its simplicity and the potential extensibility of the fabricated structure at any time of the experiment. © 2012 IOP Publishing Ltd.
view abstract 10.1088/0960-1317/22/6/065016
- Process limitations in microassembling using holographic optical tweezers
Ghadiri, R. and Guo, Q. and Yeoh, I. and Esen, C. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8244 (2012)Microassembling with holographic optical tweezers (HOT) is a flexible manufacturing technology for the precise fabrication of complex microstructures. In contrast to classical direct writing techniques, here, microparticles are transported within a fluid to appropriate positions, where they are finally bound. Therefore, optical forces act against the inner friction of the fluid. This effect limits the microassembling process in the meaning of process speed. In this work we investigate these limitations depending on the applied laser power and particle size. Additionally, different to conventional optical tweezers, HOTs use spatial light modulators (SLM) to control the laser beam and the object's position. This is performed at discrete step sizes caused by successively imaging respective kinoforms on the SLM at specific refresh rates. An optimization of the step size and the applied update rate are crucial to reach maximum velocities in particle movement. Therefore, the performance of dynamic particle manipulation is investigated in individual experiments. Stable manipulation velocities of up to 114 μm/s have been reported in our work using 6 μm polystyrene particles and an applied laser power of 445 mW. © 2012 SPIE.
view abstract 10.1117/12.906679
- Selective Ablation of Thin Films by Ultrashort Laser Pulses
Xiao, S. Z. and Schops, B. and Ostendorf, A.
Laser Assisted Net Shape Engineering 7 (lane 2012) 39 (2012)Laser ablation of bulk solid could obtain superior machining accuracy by utilizing ultrashort pulses due to the smaller laser-induced thermal diffusion length, which minimizes the heat affected zone. In selective laser ablation of thin films, the precise control of the heat affected zone in the vertical direction becomes critical in order to avoid the damage of the substrate. For this application an effective thermal penetration depth can be defined determined by not only the optical penetration depth, but also the lattice thermal diffusion length in short pulse ablation or the hotelectron penetration depth in case of ultrashort laser ablation. The ablation characteristics, such as the threshold fluence and the multi-pulse incubation effect, are strongly dependent on the film thickness when it is in the range of the effective thermal penetration depth. (C) 2012 by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH
view abstract 10.1016/j.phpro.2012.10.078
- Using laser microfabrication to write conductive polymer/swnts nanocomposites
Guo, Q. and Xiao, S. and Aumann, A. and Jaeger, M. and Chakif, M. and Ghadiri, R. and Esen, C. and Ma, M. and Ostendorf, A.
Journal of Laser Micro Nanoengineering 7 (2012)We present a novel laser microfabrication method to generate structures on the basis of a new class of functional photosensitive composites. In particular, the focus lies on the development of conductive composites by incorporating SWNTs into the matrix of polymers thus forming highly conductive nanocomposites. Conductive microstructures have been produced with the ultra-high resolution technology based on laser direct writing (e.g. single-, two-photon polymerization) using polymer/SWNTs nanocomposites. This technology opens new prospects for the realization of novel MEMS and MOEMS with increased functionality, integration, and higher level of miniaturization.
view abstract 10.2961/jlmn.2012.01.0008
- Whispering gallery mode pressure sensing
Weigel, T. and Esen, C. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8439 (2012)Optical resonances of microresonators, also known as whispering gallery modes, are attracting considerable interest as highly sensitive measuring devices with a variety of applications. Such resonators can be used for pressure, force or strain measurement. Droplets, embedded in an appropriate substrate, form perfect spheres due to their surface tension and can be used as optical resonators with high quality factors. The resonance frequencies of these droplets depend sensitively on their size and shape. Pressure changes affect the droplet shape. Therefore, pressure change can be measured with high sensitivity. In the work presented here, ethanol droplets embedded in a silicone matrix are considered. The shift of the resonance frequencies of microdroplets embedded in silicone as function of the applied pressure is investigated. © 2012 SPIE.
view abstract 10.1117/12.921759
- Analysis of the particle stability in a new designed ultrasonic levitation device
Baer, S. and Andrade, M.A.B. and Esen, C. and Adamowski, J.C. and Schweiger, G. and Ostendorf, A.
Review of Scientific Instruments 82 (2011)The use of acoustic levitation in the fields of analytical chemistry and in the containerless processing of materials requires a good stability of the levitated particle. However, spontaneous oscillations and rotation of the levitated particle have been reported in literature, which can reduce the applicability of the acoustic levitation technique. Aiming to reduce the particle oscillations, this paper presents the analysis of the particle stability in a new acoustic levitator device. The new acoustic levitator consists of a piezoelectric transducer with a concave radiating surface and a concave reflector. The analysis is conducted by determining numerically the axial and lateral forces that act on the levitated object and by measuring the oscillations of a sphere particle by a laser Doppler vibrometer. It is shown that the new levitator design allows to increase the lateral forces and reduce significantly the lateral oscillations of the levitated object. © 2011 American Institute of Physics.
view abstract 10.1063/1.3652976
- Laser selective patterning of ITO on flexible PET for organic photovoltaics
Xiao, S. and Gröger, B. and Fernandes, S.A. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 7921 (2011)Flexible organic photovoltaics have gained increasing interests during the last decades. Toward increasing the efficiency and decreasing the cost per Watt, they are on their way to the market. The approach of laser patterning technology has been expected to motivate the industrialization of organic photovoltaics. In this paper high repetition picosecond laser radiation fabricated trenches of ITO on flexible PET (Polyethylene terephthalate) substrate are presented. In order to obtain clean removal ITO layer without damaging PET substrate, 1064nm, 532nm and 355nm wavelengths with different laser fluencies and scanning strategies are applied and optimized. The results reveal the different principles for ablation of ITO layer with different wavelengths. The ITO layer is successfully and selectively removed by 1064nm laser radiation with 0.63J/cm2 fluence and 4m/s scanning speed. © 2011 SPIE.
view abstract 10.1117/12.873289
- Microfabrication by optical tweezers
Ghadiri, R. and Weigel, T. and Esen, C. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 7921 (2011)A new method to fabricate microstructures built by polymer microparticles using a bottom-up technique is presented. The microstructures find broad application in micro-fluidics technology, photonics and tissue-engineering. The handling of the particles is realized by a holographic optical tweezers setup, ensuring the precise allocation of the particles to the desired structure. A biochemical technique ensures that the structure remains stable independent of the laser source. We show that with this method complex two-dimensional durable structures can be assembled and cannot be separated by optical forces. The structures are extendable during the entire fabrication process and can be linked to further particles and structures as desired. © 2011 SPIE.
view abstract 10.1117/12.887264
- Picosecond laser direct patterning of poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS) thin films
Xiao, S. and Abreu Fernandes, S. and Esen, C. and Ostendorf, A.
Journal of Laser Micro Nanoengineering 6 (2011)Laser selective micro patterning of thin film of Poly (3,4-ethylenedioxythiophene):Poly (styrene sulfonate) (PEDOT:PSS) blend is investigated. Picosecond pulsed laser with 355 nm and 1064 nm wavelengths are used to study the ablation behavior of PEDOT:PSS thin film coated on glass. We present and discuss ablation thresholds for different film thicknesses as well as ablation lines with different overlapping rates. The results observed by SEM and white-light interference microscopy reveal that PEDOT:PSS film on glass substrates can be selectively patterned by optimized laser parameters.
view abstract 10.2961/jlmn.2011.03.0015
- Pollen characterization and identification by elastically scattered light
Surbek, M. and Esen, C. and Schweiger, G. and Ostendorf, A.
Journal of Biophotonics 4 (2011)The authors recorded the elastic light-scattering pattern of pollen over a large spatial angle range to investigate the potential light scattering for pollen identification. The scattering from elm, hazel, birch, chestnut, willow, sunflower, ragweed and pine was measured. The scattering patterns show distinct differences that can be used for the classification of pollen with simple algorithms. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/jbio.200900088
- Selective patterning of ITO on flexible PET Substrate by 1064nm picosecond Laser
Xiao, S. and Fernandes, S.A. and Ostendorf, A.
Physics Procedia 12 (2011)Pulsed picosecond laser ablation of indium tin oxide (ITO) coated on flexible polyethylene terephthalate (PET) substrate has been investigated. Pulses with 355 nm, 532 nm and 1064 nm wavelengths are performed for comparison. Laser irradiation from the front side and the back side are analyzed. The single pulse ablation threshold is found at 0.56 J/cm2 from the study of the spot sizes varied according to pulse fluence. The overlap rate in multi pulses ablation of ITO layer is discussed during the scribing of lines. Optical microscopy, SEM, surface stylus and EDX analysis reveal that the ITO layer can be completely removed with little damage of the PET substrate using optimized parameters. © 2011 Published by Elsevier Ltd.
view abstract 10.1016/j.phpro.2011.03.114
- Surface texturing by laser cladding
Grueninger, A. and Hustedt, M. and Herzog, D. and Huse, M. and Kracht, D. and Haferkamp, H. and Ostendorf, A.
Journal of Laser Applications 23 (2011)Surface texturing is an established method for altering the structure of a surface. All known methods deal with material removal. For applications where only small walls remain on the surface, this means that most of the surface must be treated. In this paper, a new approach is presented in which texturing and laser cladding are combined in a single-step buildup process. Since only small selected areas are built up, instead of removing large areas, optimization of the processing time is expected. A galvanometer scanner and a lateral flat-bed powder nozzle are used. First results showed proof of the principle. Cladding thicknesses of more than 100 μm can be generated for small structures as well as for large areas. © 2011 Laser Institute of America.
view abstract 10.2351/1.3573371
- Biocompatibility of nanoactuators: Stem cell growth on laser-generated nickel-titanium shape memory alloy nanoparticles
Barcikowski, S. and Hahn, A. and Guggenheim, M. and Reimers, K. and Ostendorf, A.
Journal of Nanoparticle Research 12 (2010)Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.
view abstract 10.1007/s11051-009-9834-4
- High resolution spectroscopy with a microparticle array sensor
Weigel, T. and Nett, R. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 7726 (2010)In recent years, optical microresonators have been extensively investigated for possible applications in many different areas of research. In optical communications such resonators can be used for switching, filtering or multiplexing devices. Due to its high quality factors, spherical microresonators are of great interest for optical sensing. Here we will discuss the use of a microparticle array as a spectral sensing device. Especially the accuracy in wavelength determination for broad light sources are in the focus of this work. Beside this, results for two light sources with different wavelengths are given. © 2010 Copyright SPIE - The International Society for Optical Engineering.
view abstract 10.1117/12.854360
- Micro- and nano-parts generated by laser-based solid freeform fabrication
Ostendorf, A. and Neumeister, A. and Dudziak, S. and Passinger, S. and Stampfl, J.
Advances in Laser Materials Processing: Technology, Research and Application (2010)Additive manufacturing techniques (AMTs) directly benefit from their fast and flexible way of realizing a small or medium number of parts in a cost-effective way. By using automated manufacturing processes, 3-D parts with a high degree of geometrical complexity can directly be built from the user-defined CAD data. The complete part is assembled in a layer-based production fashion from the bottom of the structure to its top. The rapidly expanding market of micro system technologies is constantly raising also the requirements for fast and flexible production of functional micro parts. Stereolithography and laser sintering offer a huge potential for further miniaturization by systematically adapting the right parameters and tailoring the materials. From micro components to functional micro systems with detail features on the micron level, optimized AMTs enable fabrication on either a polymer or metal basis. © 2010 Woodhead Publishing Limited All rights reserved.
view abstract 10.1533/9781845699819.7.695
- Optical micro resonance based sensor schemes for detection and identification of nano particles and biological agents in situ
Saetchnikov, V.A. and Tcherniavskaia, E.A. and Schweiger, G. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 7712 (2010)A novel emerging technique for the label-free analysis of nano particles including biomolecules using optical micro cavity resonance is being developed. Various schemes based on a mechanically fixed microspheres as well as microspheres melted by laser on the tip of a standard single mode fiber have been investigated to make further development for microbial application. Water solutions of ethanol, HCl, glucose, vitamin C and biotin have been used to test refractive index changes by monitoring the magnitude of the whispering gallery modes spectral shift. Particular efforts were made for effective fixing of the micro spheres in the water flow, an optimal geometry for micro resonance observation and material of microsphere the most appropriate for microbial application. Optical resonance in free micro spheres from PMMA fixed in micro channels produced by photolithography has been observed under the laser power of less then 1 microwatt. Resonance shifts of C reactive protein water solutions as well as albumin solutions in pure water and with HCl modelling blood have been investigated. Introducing controlled amount of glass gel nano particles into sensor microsphere surrounding were accompanied by both correlative resonance shift (400 nm in diameter) and total reconstruct of resonance spectra (57 nm in diameter). Developed schemes have been demonstrated to be a promising technology platform for sensitive, lab-on-chip type sensor of diagnostic tools for different biological molecules, e.g. proteins, oligonucleotides, oligosaccharides, lipids, small molecules, viral particles, cells as well as in different experimental contexts e.g. proteomics, genomics, drug discovery, and membrane studies. © 2010 SPIE.
view abstract 10.1117/12.853691
- Optically based manufacturing with polymer particles
Ghadiri, R. and Surbek, M. and Esen, C. and Ostendorf, A.
Physics Procedia 5 (2010)We present a new single-laser optical trapping technique for the exact manipulation and durable assembly of transparent polymer microparticles. This technique comprises the trapping of microparticles and the assembly by using a laser-driven thermal process for the joining of the particles. The thermal energy necessary for the systematic joining is applied partly by global heating of the processing chamber and by absorption of the electromagnetic radiation of the laser tweezer. The main advantage of this contact free joining technology is to use the same laser for the optical trapping, positioning and the durable assembly. The generated joints are stable and cannot be broken up with optical forces. In summary, a new micromanufacturing process based on an optical machining process is reported with promising applications in the MEMS and photonics area.
view abstract 10.1016/j.phpro.2010.08.121
- Raman spectroscopy on single levitated particles
Esen, C. and Šurbek, M. and Baer, S. and Ostendorf, A.
Chemie-Ingenieur-Technik 82 (2010)In this paper some applications of the Raman spectroscopy on levitated single particles are presented. The levitation of the particles take place by the three mostly used techniques, the ultrasonic levitator, electro dynamical balance, and optical levitation. Raman spectrosopy is used for identification of substances, for investigation of fluid phase equilibria of mixtures or for observation of chemical reactions. Additionally information about the form and morphology of the particles could be generated from the Raman spectra. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/cite.201000136
- Real time monitoring of micro and nano particles, blood phantoms in situ by optical micro resonance methods
Saetchnikov, V.A. and Tcheriavskaia, E.A. and Schweiger, G. and Ostendorf, A.
Conference Proceedings - 5th International Conference on Advanced Optoelectronics and Lasers, CAOL' 2010 (2010)Methods and instrumentation based on resonance frequency dependence of dielectric micro resonators on the surrounding medium is being developed as a real-time one-way disposable sensor for a number of parameters of nano particles and modeling blood in situ. © 2010 IEEE.
view abstract 10.1109/CAOL.2010.5634195
- Temperature sensing by using whispering gallery modes with hollow core fibers
Özel, B. and Nett, R. and Weigel, T. and Schweiger, G. and Ostendorf, A.
Measurement Science and Technology 21 (2010)We describe temperature sensing by hollow core fibers using whispering gallery modes of a spherical microresonator. Light from a tunable laser was coupled into the input end of the hollow core fiber. Optical resonances were excited in a microsphere inserted in the modified output end. Part of the light was coupled back from the resonator into the hollow core fiber and transported back to the input end. This light was recorded via a beam splitter by a diode. The sensing principle is based on the shift of the optical resonances by changing the temperature of the resonator. This shift is monitored and leads to the temperature of the resonator and surrounding respectively. © 2010 IOP Publishing Ltd.
view abstract 10.1088/0957-0233/21/9/094015