Prof. Dr. Daniel Erni

Electrical Engineering and Information Technology
University of Duisburg-Essen

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  • A self-matched leaky-wave antenna for ultrahigh-field magnetic resonance imaging with low specific absorption rate
    Solomakha, G. and Svejda, J.T. and van Leeuwen, C. and Rennings, A. and Raaijmakers, A.J. and Glybovski, S. and Erni, D.
    Nature Communications 12 (2021)
    The technology of magnetic resonance imaging is developing towards higher magnetic fields to improve resolution and contrast. However, whole-body imaging at 7 T or even higher flux densities remains challenging due to wave interference, tissue inhomogeneities, and high RF power deposition. Nowadays, proper RF excitation of a human body in prostate and cardiac MRI is only possible to achieve by using phased arrays of antennas attached to the body (so-called surface coils). Due to safety concerns, the design of such coils aims at minimization of the local specific absorption rate (SAR), keeping the highest possible RF signal in the region of interest. Most previously demonstrated approaches were based on resonant structures such as e.g. dipoles, capacitively-loaded loops, TEM-line sections. In this study, we show that there is a better compromise between the transmit signal B1+ and the local SAR using non-resonant surface coils generating a low electric field in the proximity of their conductors. With this aim, we propose and experimentally demonstrate a leaky-wave antenna implemented as a periodically-slotted microstrip transmission line. Due to its non-resonant radiation, it induces only slightly over half the peak local SAR compared to a state-of-the-art dipole antenna but has the same transmit efficiency in prostate imaging at 7 T. Unlike other antennas for MRI, the leaky-wave antenna does not require to be tuned and matched when placed on a body, which makes it easy-to-use in prostate imaging at 7 T MRI. © 2021, The Author(s).
    view abstract10.1038/s41467-020-20708-w
  • Band-gap solitons in nonlinear photonic crystal waveguides and their application for functional all-optical logic gating
    Jandieri, V. and Khomeriki, R. and Onoprishvili, T. and Erni, D. and Chotorlishvili, L. and Werner, D.H. and Berakdar, J.
    Photonics 8 (2021)
    This review paper summarizes our previous findings regarding propagation characteristics of band-gap temporal solitons in photonic crystal waveguides with Kerr-type nonlinearity and a realization of functional and easily scalable all-optical NOT, AND and NAND logic gates. The proposed structure consists of a planar air-hole type photonic crystal in crystalline silicon as the nonlinear background material. A main advantage of proposing the gap-soliton as a signal carrier is that, by operating in the true time-domain, the temporal soliton maintains a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized paving the way to multiple-input ultrafast full-optical digital signal processing. In the suggested setup, due to the gap-soliton features, there is no need to amplify the output signal after each operation which can be directly used as a new input signal for another logical operation. The efficiency of the proposed logic gates as well as their scalability is validated using our original rigorous theoretical formalism confirmed by full-wave computational electromagnetics. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/photonics8070250
  • Beam divergence reduction of vortex waves with a tailored lens and a tailored reflector
    Hassan, M.H. and Sievert, B. and Svejda, J.T. and Ahmed, A.M. and Barowski, J. and Rennings, A. and Rolfes, I. and Sezgin, A. and Erni, D.
    IEEE Access 9 (2021)
    Reducing the strong beam divergence inherent to Orbital Angular Momentum waves (also known as OAM waves or vortex waves), a tailored lens and a tailored reflector are presented in this study. The generation of the OAM waves is accomplished by a Uniform Circular Patch Antenna Array (UCA) operating at 10 GHz. Here, the tailored lens and reflector are set up by two correspondingly designed shape functions rotated around the antenna's center axis in broadside direction (i.e. body of revolution approach). Initially, the tailored lens is introduced to be compared to the UCA in the presence and absence of the conventional lens separately. Upon the usage of the tailored lens, a gain improvement of 5.8 dB has been obtained in the simulation compared to a gain of 4.8 dB in the measurement. On the other hand, the tailored reflector is set under the same procedure to be compared also to the UCA with and without a conventional reflector. Both of the reflectors are simulated under idealized conditions with the aid of an OAM impressed field source used as an emitter for a meaningful comparison. The simulated gain has shown a better performance accomplished by the tailored reflector as the height r0 reaches a level less than 1.5 λ as well as the opening angle θ is less than 38° (given an UCA with an element separation distance d=λ/2). Furthermore, three different ground plane shapes with realistic UCA are applied for the simulation procedure where each of them is perturbing the radiation of the reflector. All of the lenses and the reflectors are manufactured and later measured in an anechoic chamber to undergo a comparison with the simulated results. This article demonstrates that the vortex waves need a tailored lens or a tailored reflector to decrease the beam divergence effectively especially when the radius of the UCA becomes increasingly large. © 2013 IEEE.
    view abstract10.1109/ACCESS.2021.3050043
  • Examination of the Liquid Volume Inside Metal Tanks Using Noncontact EMATs from Outside
    Rieger, K. and Erni, D. and Rueter, D.
    IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 68 (2021)
    This article discusses ultrasound echo examination of the liquid volume inside a metal enclosure using noncontact electromagnetic acoustic transducers (EMATs) from outside the tank. Because only longitudinal sound waves exist in liquids, a novel and powerful EMAT design (kA currents in small coils) with a specifically enhanced transduction of longitudinal ultrasound is presented. Different wall materials (aluminum, steel, and stainless steel) and working frequencies are considered. A particular challenge, in addition to the already weak EMAT signals across an air gap, is the highly reflective interface between the metal wall and liquid because the acoustic impedances of the two media differ considerably. With thicker metal walls (here, 3mm), only either low frequencies < 100 kHz or frequencies close to the corresponding thickness resonance of the wall (here, about 1 MHz) are more suitable. The higher frequencies are preferred, as they show advantages due to a directed beam profile, shorter wavelengths in the liquid, and an overall better pulse fidelity. The simultaneous operation of several and closely neighboring EMATs demonstrates the feasibility of more demanding detection tasks, ultimately leading toward a 3-D localization inside the liquid using a noncontact EMAT array with eight independent elements. © 1986-2012 IEEE.
    view abstract10.1109/TUFFC.2020.3022946
  • Generation of Vortex Waves Using Crossed 2λ-Dipole Antennas
    Hassan, M.H. and Sievert, B. and Rennings, A. and Erni, D.
    15th European Conference on Antennas and Propagation, EuCAP 2021 (2021)
    In this paper, a simple and novel method is presented to generate Orbital Angular Momentum (OAM) waves for the first OAM mode order with radial polarization. This approach consists of two crossed 2λ-dipole antennas (CDA) where each antenna is fed in the center with identical amplitude but with a 90° phase shift to radiate vortex waves. Further, in order to enhance the gain, the number of crossed dipoles can be increased. Therefore, the simulation of two and four CDA is performed with the full-wave simulator FEKO. Moreover, a two and four crossed 2λ-dipole phased array of two elements can increase the directivity of the radiation in the broadside direction. A reflector of 100 mm × 100 mm is also added for two and four crossed 2λ-dipole antennas to improve the gain in the same direction. © 2021 EurAAP.
    view abstract10.23919/EuCAP51087.2021.9411239
  • InP-Based THz Beam Steering Leaky-Wave Antenna
    Lu, P. and Haddad, T. and Sievert, B. and Khani, B. and Makhlouf, S. and Dülme, S. and Estévez, J.F. and Rennings, A. and Erni, D. and Pfeiffer, U. and Stöhr, A.
    IEEE Transactions on Terahertz Science and Technology 11 (2021)
    For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are attractive in that regard since they do not require complex feeding control circuits and beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus, it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP)-based photonic or electronic THz sources and receivers. Therefore, in this article, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50-μm thin InP THz antenna chip has been established. By changing the operating frequency from 230 to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from-46° to 42°, i.e., the total scanning angle is 88°. The measured average realized gain and 3-dB beam width of a 1.5-mm wide InP LWA are ∼11 dBi and 10°. This article furthermore discusses the use of the fabricated LWA for THz interconnects. © 2011-2012 IEEE.
    view abstract10.1109/TTHZ.2020.3039460
  • A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks Ein einfaches und leistungsstarkes EMAT-Design für die kontaktlose Detektion von Inhomogenitäten in metallischen Flüssigkeitsbehältern
    Rieger, K. and Erni, D. and Rueter, D.
    Technisches Messen 87 (2020)
    A simple and powerful design of an electromagnetic acoustic transducer (EMAT) without bulky permanent magnets is presented. The EMAT is operated in a pulse echo modality and generates longitudinal ultrasound at about 1 MHz. Unlike shear waves, these longitudinal ultrasound pulses can propagate in liquids. The generally addressed application scenario is the examination of a liquid volume inside a metallic container or tank, e. g., the detection of inhomogeneities within the liquid. The herein proposed EMAT operates for virtually all metallic containers, i. e., it succeeds for container walls made of aluminum or ferromagnetic steel, and even for non-ferromagnetic (stainless) steel. Moreover, unlike piezo transducers, EMAT techniques allow for a noncontacting ultrasound transduction: the air gap between the EMAT sensor coil and the tank s metallic surface extends up to 2 mm. Even with this relatively large air gap, the biasing magnetic field approaches a flux density of 3.2 T at the surface, more than what is possible to achieve with the permanent magnets of conventional and bulkier EMATs. Strong fields improve the coupling efficiency of the principally low-efficiency EMAT mechanism, which is important for both ultrasound transmission and reception. For that superior field intensity, a unipolar current pulse of up to 3.6 kA is applied through the thin windings (0.5 mm) of the EMAT coil. This paper presents a novel solid-state EMAT circuitry for such strong currents and MHz pulsed voltages >1 kV. As a particularly delicate task, the powerful circuitry must also detect the rather weak echo signals in the V range. A very short recovery time is required after such a strong emission burst. The discussed circuitry consists of three unipolar high-current modules, which can each be independently launched. This allows for received echo signals that can be timed independently, e. g., objects deep inside the liquid tank can be specifically addressed. In general, this work concentrates on the novel circuitry in parallel connection, the general pulse-echo functionality and the magnetic fields. A detailed analysis and shaping of the ultrasonic fields through different EMAT coil geometries would exceed the scope of this contribution and is to be reported separately. © 2020 De Gruyter Oldenbourg. All rights reserved.
    view abstract10.1515/teme-2019-0124
  • A Liquid Crystal Based Dynamic Metasurface for Beam Steering and Computational Imaging
    Wang, P.-Y. and Rennings, A. and Erni, D.
    Asia-Pacific Microwave Conference Proceedings, APMC 2020-December (2020)
    In this paper, a dynamic metasurface capable of beam steering and computational imaging is proposed. Utilizing the anisotropic permittivity of the liquid crystal, scattering property of each metamaterial unit can be individually controlled. The reconfigurable metasurface provides diverse radiation patterns with specific addressing schemes. For verification, a 2D-steerable directive beam and a spatially diverse radiation pattern are both achieved in simulation based on the holographic algorithm and random addressing method, respectively. The design consideration of the metamaterial unit, configuration of the metasurface and the operation principle are also briefly presented. © 2020 IEEE.
    view abstract10.1109/APMC47863.2020.9331687
  • A Method of Side-lobe Suppression for Reactance Modulated Antennas
    Wang, P.-Y. and Meng, F.-Y. and Lyu, Y.-L. and Rennings, A. and Erni, D.
    14th European Conference on Antennas and Propagation, EuCAP 2020 (2020)
    This paper reveals the mechanism of the high side-lobe level (SLL) phenomenon in reactance modulated antennas (RMAs) and proposes a method to suppress the side-lobe for RMAs. The wave-guiding mode in RMAs is a surface wave with considerable EM field exposed to the free-space. We found that it is the exposure of EM power results in the slow-wave radiation phenomenon deteriorating the SLL, which is always neglected in conventional waveguides with local EM confined structures, such as rectangular waveguides and microstrip lines. To eliminate the slow-wave radiation, a 'complementary decoupling' method is proposed in this paper by introducing another RMA with inverse periodic variation to the original one. The two parallel antennas are excited with equal amplitude and reversed phase forming a complementary radiation pair. With the proposed method, the slow-wave radiation is eliminated and the SLL is improved. Meanwhile, all of the even radiation modes are also suppressed. Especially for the -2nd mode, which also carries considerable power and often appears in most RMAs. © 2020 EurAAP.
    view abstract10.23919/EuCAP48036.2020.9136067
  • A new RF-coil for UHF MRI based on a slotted microstrip line
    Solomakha, G. and Svejda, J.T. and Rennings, A. and Erni, D. and Glybovski, S.
    Journal of Physics: Conference Series 1461 (2020)
    We propose to use an antenna based on a microstrip line with a slotted ground plane, as a transcieve RF-coil for ultra-high field (UHF) human body MRI at 7 Tesla. In this work we show by numerical simulations, that while being loaded with a homogeneous dielectric phantom, this slotted line supports fast-wave propagation and radiation of power into the phantom. As a result, it produces high RF magnetic field at a deeply located region of interest, which is the same as one produced by the well-known fractionated dipole. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1461/1/012168
  • A simple superposition formulation to predict the underwater electric potential signature of naval vessels
    Thiel, C. and Broecheler, C. and Ludwar, F. and Rennings, A. and Doose, J. and Erni, D.
    Journal of Marine Science and Engineering 8 (2020)
    The underwater electric potential (UEP) signature is an electric signal, which can be exploited by navalmines to be utilized as a possible trigger indicator andmay cause severe damage to the vessel and the onboard crew. Hence, knowing the UEP signature as exactly as possible can help to evaluate a possible risk of the vessel being detected by naval mines or if the UEP signature is within a noncritical region. As the UEP signature differs for changes of the corrosion protection system, the UEP signature is usually unknown for new conditions. In this work, we present a simple mathematical formulation to predict the UEP signature based on themere use of a single reference UEP signature, and the corresponding currents, which are excited by the impressed current cathodic protection (ICCP) system. With this methodology, deviations below 10% between the maximum of the simulated UEP signature and the predicted UEP signature can be achieved, even in the presence of the nonlinear corrosion process. Furthermore, a corrosion protective coating of the propellers can significantly reduce the influence of the nonlinear corrosion process on the total UEP signature to improve the prediction accuracy of the superposition formulation as presented in this work. © 2020 by the authors.
    view abstract10.3390/jmse8020105
  • A Stochastic Large-Signal Model for Printed High-Frequency Rectifiers Used for Efficient Generation of Higher Harmonics
    Neumann, K. and Kuhnel, L. and Langer, F. and Rennings, A. and Benson, N. and Schmechel, R. and Erni, D.
    IEEE Transactions on Microwave Theory and Techniques 68 (2020)
    This article investigates the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering, yielding a Schottky diode when a corresponding top metallization is applied. The I - V characteristic and the voltage-dependent impedance of such diode is measured. By using the simulation software Advanced Design System (ADS), we develop a new scalable circuit model consisting of many different elementary diodes, which can explain the measured behavior. The elementary microcone diodes differ electrically in their barrier height, which is modeled as a stochastic process with a Gaussian distribution. A comparison between this model and a single diode model based on the thermionic field emission theory is conducted. We show that the distributed model outperforms the single-diode model in every regard and allows a prediction of the power levels of the harmonic frequency generation. Through more in-depth research, we find that a distributed barrier height leads to a smoother I - V curve, which in turn can lead to higher second and third harmonic power levels. By adjusting the barrier height distribution, the desired harmonics can be increased. © 1963-2012 IEEE.
    view abstract10.1109/TMTT.2020.2990561
  • Characterization of Dielectric Materials by Sparse Signal Processing with Iterative Dictionary Updates
    Thanthrige, U.S.K.P.M. and Barowski, J. and Rolfes, I. and Erni, D. and Kaiser, T. and Sezgin, A.
    IEEE Sensors Letters 4 (2020)
    Estimating parameters and properties of various materials without causing damage to the material under test (MUT) is important in many applications. Thus, in this letter, we address MUT's parameter estimation by wireless sensing. Here, the precision of the estimation depends on the accurate estimation of the properties of the reflected signal from the MUT (e.g., number of reflections, their amplitudes, and time delays). For a layered MUT, there are multiple reflections, and due to the limited bandwidth at the receiver, these reflections superimpose with each other. Since the number of reflections coming from the MUT is limited, we utilize sparse signal processing (SSP) to decompose the reflected signal. In SSP, a so called dictionary is required to obtain a sparse representation of the signal. Here, instead of a fixed dictionary, an iterative dictionary-update technique is proposed to improve the estimation of the reflected signal. To validate the proposed method, a vector network analyzer (VNA)-based measurement setup is used. It turns out that the estimated dielectric constants of the MUTs are in close agreement with those reported in literature. Further, the proposed approach outperforms the state-of-the-art model-based curve-fitting approach in thickness estimation. © 2017 IEEE.
    view abstract10.1109/LSENS.2020.3019924
  • Compact Metamaterial-based Coil Element for Combined 1H/23Na MRI at 7T
    Svejda, J.T. and Rennings, A. and Erni, D.
    Journal of Physics: Conference Series 1461 (2020)
    High static magnetic flux densities in the field of magnetic resonance imaging (MRI) allow X-nuclei-based imaging and spectroscopy as well as the utilization of electromagnetic metamaterials. A compact dual-tuned coil element based on a metamaterial transmission line which achieves a quarter-wave resonance at the two distinct operating frequencies of the hydrogen and sodium nuclei is presented. In addition, first combined 1 H/23Na MRI measurement results from a structured phantom are shown and discussed. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1461/1/012173
  • Equivalent Circuit Model Separating Dissipative and Radiative Losses for the Systematic Design of Efficient Microstrip-Based On-Chip Antennas
    Sievert, B. and Svejda, J.T. and Wittemeier, J. and Pohl, N. and Erni, D. and Rennings, A.
    IEEE Transactions on Microwave Theory and Techniques (2020)
    This article presents a comprehensive method to efficiently design capacitively enhanced resonant on-chip antennas using an equivalent circuit (EC) model instead of computationally demanding full-wave simulations. To systemize the design process by predicting the radiation efficiency, the input impedance, the current and voltage distributions, and the radiation pattern of the antenna based on an EC, a method to extract both dissipation and radiation mechanisms from full-wave simulation data is described and carried out. Based on this separation of loss mechanisms, an EC-based antenna optimization with respect to the radiation efficiency is conceivably possible. Additional to the EC, which enables this efficient antenna optimization and increases the physical insight in the radiation mechanism, an analytical estimation of key antenna parameters, as the resonant length, is presented. The results from the analytical calculations and the antenna parameters calculated using the EC model are compared with full-wave FDTD simulations and used to discuss the capabilities and limitations of the EC model. Finally, an on-chip antenna of the considered type operating at 290-300 GHz and manufactured with silicon-germanium technology is used to verify the full-wave antenna simulations and the presented approach in general. CCBY
    view abstract10.1109/TMTT.2020.3040453
  • Functional all-optical logic gates for true time-domain signal processing in nonlinear photonic crystal waveguides
    Jandieri, V. and Khomeriki, R. and Onoprishvili, T. and Werner, D.H. and Berakdar, J. and Erni, D.
    Optics Express 28 (2020)
    We present a conceptual study on the realization of functional and easily scalable all-optical NOT, AND and NAND logic gates using bandgap solitons in coupled photonic crystal waveguides. The underlying structure consists of a planar air-hole type photonic crystal with a hexagonal lattice of air holes in crystalline silicon (c-Si) as the nonlinear background material. The remaining logical operations can be performed using combinations of these three logic gates. A unique feature of the proposed working scheme is that it operates in the true time-domain, enabling temporal solitons to maintain a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized, paving the way to multiple-input all-optical logic gates for ultrafast full-optical digital signal processing. In the suggested setup, there is no need to amplify the output signal after each operation, which can be directly used as a new input signal for another logical operation. The feasibility and efficiency of the proposed logic gates as well as their scalability is demonstrated using our original rigorous theoretical formalism together with full-wave computational electromagnetics. © 2020 Optical Society of America.
    view abstract10.1364/OE.395015
  • InP-based THz Beam Steering Leaky-Wave Antenna
    Lu, P. and Haddad, T. and Sievert, B. and Khani, B. and Makhlouf, S. and Dulme, S. and Fernandez Estevez, J. and Rennings, A. and Erni, D. and Pfeiffer, U.R. and Stöhr, A.
    IEEE Transactions on Terahertz Science and Technology (2020)
    For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are quite attractive in that regard since they do not require complex feeding control circuits because beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP) based photonic or electronic THz sources and receivers. Therefore, in this paper, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50 m thin InP THz antenna chip has been established. By changing the operating frequency from 230 GHz to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from -46 to 42, i.e. the total scanning angle is 88. The measured average realized gain and 3 dB beam width of a 1.5 mm wide InP LWA are ~11 dBi and 10. The paper furthermore discusses the use of the fabricated LWA for THz interconnects. CCBY
    view abstract10.1109/TTHZ.2020.3039460
  • Investigating the influence of dielectric pads in 7T magnetic resonance imaging-Simulated and experimental assessment
    Garcia, M.M. and Chaim, K.T. and Otaduy, M.C.G. and Rennings, A. and Erni, D. and Vatanchi, M. and Zylka, W.
    Current Directions in Biomedical Engineering 6 (2020)
    Dipole radiofrequency (RF) elements have been successfully used to compose multi-channel RF coils for ultrahigh fields (UHF) magnetic resonance imaging (MRI). As magnetic components of RF fields (B1) can be very inhomogeneous at UHF (B0≥7T), dielectric pads with high dielectric constants were proposed to improve the B1 efficiency and homogeneity [1]. Dielectric pads can be used as a passive B1 shimmimg technique thanks to inducing a strong secondary magnetic field in their vicinity. The use of such dielectric pads affect not only the B1 field but also the electric field. This in turn affects the specific absorption rate (SAR) and consequently the temperature distribution inside the patient's body. To study these effects, a 29 cm-long transmission dipole RF coil element terminated by two meander was used for 7T MRI [2]. Using a cylindrical agarose-gel phantom, numerical and experimental results were analyzed with respect to homogeneity and amplitude of the magnetic and electric fields generated by the RF element in various configurations with and without dielectric pads. Calculated and measured B1 results were cross-checked and found to be in good agreement. When using dielectric pads B1 homogeneity and magnitude increase in regions where it was previously weak or insufficient. Calculations suggest that SAR distribution will change when using the pads. © 2020 by Walter de Gruyter Berlin/Boston.
    view abstract10.1515/cdbme-2020-3007
  • Limits of Effective Material Properties in the Context of an Electromagnetic Tissue Model
    Jerbic, K. and Neumann, K. and Svejda, J.T. and Sievert, B. and Rennings, A. and Erni, D.
    IEEE Access 8 (2020)
    Most calibration schemes for reflection-based tissue spectroscopy in the mm-wave/ THz-frequency range are based on homogenized, frequency-dependent tissue models where macroscopic material parameters have either been determined by measurement or calculated using effective material theory. However, as the resolution of measurement at these frequencies captures the underlying microstructure of the tissue, we will investigate the validity limits of such effective material models over a wide frequency range (10 MHz - 200 GHz). Embedded in a parameterizable virtual workbench, we implemented a numerical homogenization method using a hierarchical multiscale approach to capture both the dispersive and tensorial electromagnetic properties of the tissue, and determined at which frequency this homogenized model deviated from a full-wave electromagnetic reference model within the framework of a Monte-Carlo analysis. Simulations were carried out using a generic hypodermal tissue that emulated the morphology of the microstructure. Results showed that the validity limit occurred at surprisingly low frequencies and thus contradicted the traditional usage of homogenized tissue models. The reasons for this are explained in detail and thus it is shown how both the lower 'allowed' and upper 'forbidden' frequency ranges can be used for frequency-selective classification/identification of specific material and structural properties employing a supervised machine-learning approach. Using the implemented classifier, we developed a method to identify specific frequency bands in the forbidden frequency range to optimize the reliability of material classification. © 2013 IEEE.
    view abstract10.1109/ACCESS.2020.3045327
  • Modeling disorder in two-dimensional colloidal crystals based on electron microscope measurements
    Abdellatif, S.O. and Kirah, K. and Erni, D. and Marlow, F.
    Applied Optics 59 (2020)
    Self-assembled two-dimensional colloidal crystals (CCs) are critical components in many optical and optoelectronic devices. Such structures usually exhibit various types of disorder, which sometimes can be beneficial for the desired applications. However, disorder poses challenges to the modeling of two-dimensional structures. In this work, two-dimensional CCs employed in optoelectronic devices, especially dye-sensitized solar cells, are investigated. scanning electron microscope (SEM) images were used to quantify the disorder in the studied structures. As a basis for simulations, disordered model patterns were generated with properties extracted from the SEM images of prepared samples. Optical modeling was performed with a finite-difference time-domain simulator. The simulated transmission data are consistent with the experimentally measured spectra. © 2020 Optical Society of America
    view abstract10.1364/AO.408576
  • Numerical analysis of dielectric post-wall waveguides
    Archemashvili, E. and Yasumoto, K. and Jandieri, V. and Pistora, J. and Maeda, H. and Erni, D.
    2020 International Workshop on Antenna Technology, iWAT 2020 (2020)
    Functional post-wall waveguides that are formed by periodically distributed dielectric posts are numerically analyzed. In this preliminary study, the propagation constant and attenuation constant of the dielectric post-wall waveguide is rigorously calculated using our originally developed numerical method. The developed formulation is accurate and numerically very fast. Structural parameters of the dielectric post-wall waveguide are properly chosen in order to achieve the confinement of the field in the guiding region as strong as possible even in case of one-layered dielectric structure. The final goal of our investigations is to realize the functional dielectric post-wall waveguide-based filters, which are expected to have a practical application in THz region, where the metallic (PEC) rods are losing their properties. © 2020 IEEE.
    view abstract10.1109/iWAT48004.2020.1570608549
  • OAM Mode Order Conversion and Clutter Rejection with OAM-Coded RFID Tags
    Hassan, M.H. and Sievert, B. and Svejda, J.T. and Abbas, A.A. and Jimenez-Saez, A. and Ahmad, A.M. and Schubler, M. and Rennings, A. and Solbach, K. and Kaiser, T. and Jakoby, R. and Sezgin, A. and Erni, D.
    IEEE Access 8 (2020)
    In this article a uniform helically arranged dielectric resonator array can generate Orbital Angular Momentum waves (OAM) causing a conversion of OAM mode orders $m$ from an incoming mode ${m}_{in}$ to an outgoing mode ${m}_{out}$. The operating frequency is set as 10 GHz to facilitate the measuring process. This new approach provides additional OAM values per digit in the RFID technology according to the excited OAM modes ${m}_{n}\\\in \\\{\ldots-2-1012\ldots \}$ instead of the conventional binary values ${b}_{n}~\in ~\{0 1\}$. Thus more information content is revealed. Through the OAM concept a ${m}_{1} {m}_{2}~2$-digits OAM coded tag is obtained upon the employment of two uniform helically arranged cylindrical dielectric resonator arrays operating at two different frequencies ${f}_{1} = 10\\\text {GHz}$ and ${f}_{2} = 11\\\text {GHz}$. Each array has 8 DRs but with different radius yielding a reduction of the mutual coupling between the varied circular arrays. The interaction between the phase delayed radiation of each DR element in the array generates different vortex waves with corresponding OAM mode orders. In order to achieve the correct phase delay the elevation of each DR is specified by the desirable OAM mode order the number of elements and the propagation wavelength. At first the generation of OAM mode orders-1 2 and-3 is carried out. Then mode conversion from 0 to-1 +1 to-2-1 to 0 +2 to-3 and-2 to +1 are depicted. After that two simulated examples of 2-digits OAM coded tags with the code $\{-11\}$ and $\{-20\}$ are presented. A conversion of mode 0 to mode 1 has been simulated and also measured where an additional metal sheet is used to evaluate the distortion in the OAM modes. As a result this study demonstrates that the uniform helically arranged DR arrays can convert the incoming OAM mode order into another one where the clutter from broadside direction is rejected due to the Butler matrix (BM) which interferes the clutter destructively. © 2013 IEEE.
    view abstract10.1109/ACCESS.2020.3043053
  • Passive Orbital Angular Momentum RFID Tag based on Dielectric Resonator Arrays
    Haj Hassan, M. and Alhaj Abbas, A. and Jimenez-Saez, A. and Mostafa Ahmad, A. and Sievert, B. and Schussler, M. and Rennings, A. and Solbach, K. and Kaiser, T. and Jakoby, R. and Sezgin, A. and Erni, D.
    2020 3rd International Workshop on Mobile Terahertz Systems, IWMTS 2020 (2020)
    In this paper, a novel approach is proposed to increase the information content of RFID tags using uniform helically arranged spherical dielectric resonator arrays. The proposed tag design is scalable to operating frequency in the mm-wave/THz range and provides a larger information content due to new additional values per digits according to the excited OAM modes (mn=-2,-1,0,1,2,) instead of the binary values (bn=0 and 1). This new values can be called OAM values. By means of the orbital angular momentum (OAM) concept three uniform helically arranged spherical dielectric resonator arrays operating at three different frequencies f1=10GHz,\ f2=10.5 GHz, and f3=11GHz are established to provide a m1m2m3 3-digits OAM coded tag. Each array consists of 8 dielectric resonators (DRs) with three different radii in order to enhance the isolation between the different arrays. The OAM mode orders are generated by the interaction of phase delayed radiation from each DR element in the array. Each DR element is elevated in order to achieve the right phase shift, which is related to the mode order, the number of elements, and the propagation wavelength. Two examples of 3-digits OAM coded tags with the code -110 and -210 have been simulated and compared with a b1b2b3 3-digits conventional coded tag with the code 111 in order to compare the two approaches. © 2020 IEEE.
    view abstract10.1109/IWMTS49292.2020.9166434
  • Spherical mm-Wave/THz Antenna Measurement System
    Sievert, B. and Svejda, J.T. and Erni, D. and Rennings, A.
    IEEE Access 8 (2020)
    This paper presents an automatic characterization system for mm-wave antennas based on a spherical positioning system. It features network-analysis based far-field- and $S$ -parameter measurement of probe- and waveguide-fed antennas between 220 GHz and 330 GHz, expandable down to 75 GHz. In either case, the antenna under test (AUT) is fixed in the center of a spherical coordinate system and fed by an appropriate feeding structure, whereas the receiving antenna is moved along the surface of a hemisphere. Since the movement of the receiving antenna is inherently limited to constant radii, the measurement of amplitude and phase far-field-pattern is possible in principle. Additionally to the measurement results of an open-ended waveguide as AUT, this paper describes two methods for the self-characterization of possible systematic and stochastic measurement uncertainties. On one hand, repetitive measurements along a constant trace are carried out to obtain information about the stochastic uncertainty of far-field measurements. On the other hand, a synthetic aperture radar (SAR)-approach is used to characterize possible unwanted reflections within the measurement setup. Finally, the insight obtained from both antenna measurements and self-characterization is concluded into performance parameters of the presented measurement approach. © 2013 IEEE.
    view abstract10.1109/ACCESS.2020.2993698
  • Subharmonic Injection Locking for Phase and Frequency Control of RTD-Based THz Oscillator
    Arzi, K. and Suzuki, S. and Rennings, A. and Erni, D. and Weimann, N. and Asada, M. and Prost, W.
    IEEE Transactions on Terahertz Science and Technology 10 (2020)
    Phase and frequency control of resonant tunneling diode (RTD) based terahertz oscillators are major challenges in realizing coherent signal sources for arrayed applications, such as spatial power combining, beam steering, or multi-in multi-out systems. In this letter, we demonstrate frequency locking and control of an RTD oscillating at f0 ∼ 550 GHz, via radiative injection of a weak sinusoidal subharmonic signal at f0/2. Precise frequency control, within the locking range of around 2 GHz, is demonstrated. A peak output power enhancement of 14 dB in the whole locking range, compared to the free running oscillator, is achieved. Furthermore, occurrence of phase locking is identified by the spectral linewidth reduction, quantifiable in the full-width at half-maximum parameter. A signal linewidth of 490 Hz was achieved in locked operation. © 2011-2012 IEEE.
    view abstract10.1109/TTHZ.2019.2959411
  • Three-dimensional magnetic induction tomography: Improved performance for the center regions inside a low conductive and voluminous body
    Klein, M. and Erni, D. and Rueter, D.
    Sensors (Switzerland) 20 (2020)
    Magnetic induction tomography (MIT) is a contactless technique that is used to image the distribution of passive electromagnetic properties inside a voluminous body. However, the central area sensitivity (CAS) of this method is critically weak and blurred for a low conductive volume. This article analyzes this challenging issue, which inhibits even faint imaging of the central interior region of a body, and it suggests a remedy. The problem is expounded via two-dimensional (2D) and three-dimensional (3D) eddy current simulations with different transmitter geometries. On this basis, it is shown that a spatially undulating exciter coil can significantly improve the CAS by >20 dB. Consequently, the central region inside a low conductive voluminous object becomes clearly detectable above the noise floor, a fact which is also confirmed by practical measurements. The improved sensitivity map of the new arrangement is compared with maps of more typical circular MIT geometries. In conclusion, 3D MIT reconstructions are presented, and for the same incidence of noise, their performance is much better with the suggested improvement than that with a circular setup. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/s20051306
  • Toward Mobile Integrated Electronic Systems at THz Frequencies
    Hillger, P. and van Delden, M. and Thanthrige, U.S.M. and Ahmed, A.M. and Wittemeier, J. and Arzi, K. and Andree, M. and Sievert, B. and Prost, W. and Rennings, A. and Erni, D. and Musch, T. and Weimann, N. and Sezgin, A. and Pohl, N. and Pfeiffer, U.R.
    Journal of Infrared, Millimeter, and Terahertz Waves (2020)
    This paper discusses advances related to the integration of future mobile electronic THz systems. Without claiming to provide a comprehensive review of this surging research area, the authors gathered research on selected topics that are expected to be of relevance for the future exploration of components for practical mobile THz imaging and sensing applications. First, a brief technology review of integrated mobile THz components is given. Advances in III-V technology, silicon technology, and resonant-tunneling diodes (RTD) are discussed. Based on an RTD source and a SiGe-HBT direct detector, low-cost and compact computed tomography is presented for volumetric continuous-wave imaging at around 300 GHz. Moreover, aspects of system integration of mobile THz MIMO radars are discussed. Thereby, a novel phase-locked loop concept utilizing a high-stability yttrium-iron-garnet-tuned oscillator to synthesize ultra-stable reference mmWave signals is shown, and an adaptive self-interference cancellation algorithm for THz MIMO in the digital domain based on Kalman filter theory is proposed. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s10762-020-00699-x
  • Transparency and Diffused Light Efficiency of Dye-Sensitized Solar Cells: Tuning and a New Figure of Merit
    Abdellatif, S.O. and Josten, S. and Khalil, A.S.G. and Erni, D. and Marlow, F.
    IEEE Journal of Photovoltaics 10 (2020)
    Tunability is considered one of the main advantages of dye-sensitized solar cells (DSSCs) over conventional Si-based solar cells. In DSSCs, the thickness of the active layer can tune the transparency of the cell. This, however, creates a tradeoff between the transparency and the cell's efficiency. DSSCs with tailored transparency would be capable of being utilized in photovoltaic window applications, where Si cells are barely suitable. In this article, a new figure of merit is introduced to evaluate the performance of the DSSCs, named 'TED efficiency.' The proposed TED parameter (i.e., the transparency, conversion efficiency, and diffused light efficiency) not only is based on the cell conversion efficiency but also considers the optical transparency as well as the cell performance under diffused light. TED efficiency measurements were performed on three different types of DSSCs: standard (DSSC-A), simple semitransparent (DSSC-B), and scattering-enhanced DSSCs (DSSC-C). A 22.5%-Transparent DSSC has been fabricated by reducing the thickness of the porous TiO2 layer. To optimize the TED efficiency of the semitransparent DSSC, an opaline SiO2 layer is used. This layer enhances the forward scattering, acts as an UV protecting layer, and colorizes the semitransparent cell in a decorative manner. The TED efficiency for the scattering-enhanced DSSC showed a significant improvement to the standard DSSC as well as to the semitransparent cell. © 2011-2012 IEEE.
    view abstract10.1109/JPHOTOV.2020.2965399
  • Using an artificial neural network to evaluate the hull condition of naval vessels Bewertung des Rumpfzustands von Wasserfahrzeugen unter Verwendung künstlicher neuronaler Netze
    Thiel, C. and Neumann, K. and Broecheler, C. and Ludwar, F. and Rennings, A. and Doose, J. and Erni, D.
    Technisches Messen 87 (2020)
    The evaluation of the hull condition of naval vessels is a crucial part for corrosion protection systems due to the direct linkage between the electrochemical process at the hull/water interface leading to corrosion and the overall coating of the hull to prevent the corrosion process. In the case of the latter, the condition is unknown while the vessel is on a mission and either has to be evaluated by divers (in open water) or on dry docks which is a time consuming process, respectively. In our work, we present a methodology to localize coating damages without the need of divers or dry docks using an artificial neural network (ANN) combined with the information provided by the onboard impressed current cathodic protection (ICCP) system to predict said damages in a specific sector of a generic ship model. Using only the ICCP currents as highly aggregated input variables for the ANN, approximately 86 % of randomly sized and positioned coating damages are correctly predicted. © 2020 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/teme-2019-0123
  • Above water electric potential signatures of submerged naval vessels
    Schaefer, D. and Thiel, C. and Doose, J. and Rennings, A. and Erni, D.
    Journal of Marine Science and Engineering 7 (2019)
    In this paper, we investigate the fundamental linkage between underwater electric potential (UEP) signatures and their related electric fields above the waterline, which are introduced as above water electric potential (AEP) signatures. As a first step, the field distribution for an underwater point source excitation (fundamental solution) is derived analytically, using an adjusted method of images. Subsequently a numerical approach is introduced, whereby the calculation of the stationary current density distribution and electrostatic fields are coupled within an FEM simulation. Simulation results are presented for the aforementioned point source, as well as for a submarine model, where the latter includes considering non-linear polarization curves to model the electrochemical behavior at the metal-seawater interface. Finally, the relevance of AEP signatures in the context of anti-submarine warfare (ASW) is discussed. Our results show that AEP signatures inevitably occur along with UEP signatures, and could therefore in principal be used to detect submerged submarines via airborne sensors. However, an estimation of the expectable signal-to-noise-ratio (SNR) suggests that AEP signatures are difficult to exploit and therefore entail a much lower risk compared to other signatures. © 2018 by the authors.
    view abstract10.3390/JMSE7020053
  • Analysis of Scattering by Plasmonic Gratings of Circular Nanorods Using Lattice Sums Technique
    Jandieri, V. and Yasumoto, K. and Pistora, J. and Erni, D.
    Sensors (Basel, Switzerland) 19 (2019)
    A self-contained formulation for analyzing electromagnetic scattering by a significant class of planar gratings composed of plasmonic nanorods, which were infinite length along their axes, is presented. The procedure for the lattice sums technique was implemented in a cylindrical harmonic expansion method based on the generalized reflection matrix approach for full-wave scattering analysis of plasmonic gratings. The method provided a high computational efficiency and can be considered as one of the best-suited numerical tools for the optimization of plasmonic sensors and plasmonic guiding devices both having a planar geometry. Although the proposed formalism can be applied to analyze a wide class of plasmonic gratings, three configurations were studied in the manuscript. Firstly, a multilayered grating of silver nanocylinders formed analogously to photonic crystals was considered. In the region far from the resonances of a single plasmonic nanocylinder, the structure showed similar properties compared to conventional photonic crystals. When one or a few nanorods were periodically removed from the original crystal, thus forming a crystal with defects, a new band was formed in the spectral responses because of the resonant tunneling through the defect layers. The rigorous formulation of plasmonic gratings with defects was proposed for the first time. Finally, a plasmonic planar grating of metal-coated dielectric nanorods coupled to the dielectric slab was investigated from the viewpoint of design of a refractive index sensor. Dual-absorption bands attributable to the excitation of the localized surface plasmons were studied, and the near field distributions were given in both absorption bands associated with the resonances on the upper and inner surfaces of a single metal-coated nanocylinder. Resonance in the second absorption band was sensitive to the refractive index of the background medium and could be useful for the design of refractive index sensors. Also analyzed was a phase-matching condition between the evanescent space-harmonics of the plasmonic grating and the guided modes inside the slab, leading to a strong coupling.
    view abstract10.3390/s19183923
  • Analysis of stochastic Schottky barrier variations within printed high frequency rectifiers for harmonics generation
    Neumann, K. and Kuehnel, L. and Langer, F. and Rennings, A. and Benson, N. and Schmechel, R. and Erni, D.
    IMWS-AMP 2019 - 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (2019)
    In this paper we investigate the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering yielding a Schottky diode when a corresponding top metallization is applied. The elementary microcone diodes differ electrically in their barrier height, which is modelled as a gaussian distribution. The circuit simulation software Advanced Design System (ADS) is used to analyze the rectification abilities of the overall structure. The results show that a distributed barrier height leads to a smoother IV-characteristic, which can also be interpreted as an aggregated diode with a smaller turn on voltage. A Fourier analysis of the rectified time-domain signal shows an amplification of the frequency components up to the third harmonic in comparison to a non distributed single diode. © 2019 IEEE.
    view abstract10.1109/IMWS-AMP.2019.8880082
  • Broadband detection capability of a triple barrier resonant tunneling diode
    Arzi, K. and Clochiatti, S. and Mutlu, E. and Kowaljow, A. and Sievert, B. and Erni, D. and Weimann, N. and Prost, W.
    2019 2nd International Workshop on Mobile Terahertz Systems, IWMTS 2019 (2019)
    The monolithic on-chip integration and design of a high current density InP-based Triple Barrier Resonant Tunneling Diode within a bow-tie antenna structure for detection application is presented. The asymmetrical current-voltage characteristics of the Triple Barrier Resonant Tunneling Diode and its small capacitance provide a powerful candidate for THz signal detection. The integration into a planar broadband antenna structure such as a bow-tie design enables realization of a broadband detector from single GHz up to THz frequencies. In this work, experimental data are demonstrated in the frequency range of 75 to 110 GHz and from 220 to 330 GHz. © 2019 IEEE
    view abstract10.1109/IWMTS.2019.8823724
  • Coating damage localization of naval vessels using artificial neural networks
    Thiel, C. and Neumann, K. and Ludwar, F. and Rennings, A. and Doose, J. and Erni, D.
    Ocean Engineering 192 (2019)
    For the localization of coating damages of naval vessels numerical simulation using the FEM software COMSOL Multiphysics were carried out to calculate the corresponding underwater electric potential (UEP) signature. Therefore, we defined said damages at random hull surface positions and used the information provided by the impressed current cathodic protection (ICCP) system, more exactly the cathodic current itself, and the calculated UEP signatures as input parameters to train an artificial neural network (ANN) for predicting the coating damage location. With this deep learning approach, more than 90% of all coating damages are predicted correctly, considering a generic ship model with 50m length, whose hull is divided into 12 different sectors. Even the mere use of ICCP currents as highly aggregated input parameters for the ANN lead to a satisfactory prediction rate over 80% within the predefined sectors, thus providing quite accurate results using minimal amount of data. © 2019 Elsevier Ltd
    view abstract10.1016/j.oceaneng.2019.106560
  • CT extended hounsfield unit range in radiotherapy treatment planning for patients with implantable medical devices
    Ese, Z. and Qamhiyeh, S. and Kreutner, J. and Schaefers, G. and Erni, D. and Zylka, W.
    IFMBE Proceedings 68 (2019)
    Radiotherapy (RT) treatment planning is based on computed tomography (CT) images and traditionally uses the conventional Hounsfield unit (CHU) range. This HU range is suited for human tissue but inappropriate for metallic materials. To guarantee safety of patient carrying implants precise HU quantification is beneficial for accurate dose calculations in planning software. Some modern CT systems offer an extended HU range (EHU). This study focuses the suitability of these two HU ranges for the quantification of metallic components of active implantable medical devices (AIMD). CT acquisitions of various metallic and non-metallic materials aligned in a water phantom were investigated. From our acquisitions we calculated that materials with mass-density ρ, &gt;, 3.0, g/cm3 cannot be represented in the CHU range. For these materials the EHU range could be used for accurate HU quantification. Since the EHU range does not effect the HU values for materials ρ, &lt;, 3.0, g/cm3, it can be used as a standard for RT treatment planning for patient with and without implants. © Springer Nature Singapore Pte Ltd. 2019.
    view abstract10.1007/978-981-10-9023-3_111
  • Design and numerical demonstration of a 2D millimeter-wave beam-scanning reflectarray based on liquid crystals and a static driving technique
    Li, J.-X. and Jin, T. and Erni, D. and Meng, F.-Y. and Wu, Q. and Li, W.-N.
    Journal of Physics D: Applied Physics 52 (2019)
    In this paper, the design method for a two-dimensional (2D) beam-scanning reflectarray based on liquid crystals (LCs) and static driving technique in the millimeter-wave band is proposed. The reflectarray is composed of multi-resonant elements which obtain the phase shift by applying the driving voltage on the LC. The LC elements of the reflectarray are controlled by static driving method. In order to reduce the negative impact of the densely distributed LC driving lines on the performance of the reflectarray, a novel LC driving strategy is presented to further reduce the number of driving lines. Subarray partition is employed, and more importantly, the ground plane of the reflectarray is divided into several segments so that about one third part of the driving lines can be transferred from the patch layer to the ground plane layer. A 32 × 30 reflectarray is designed and taken for an example to demonstrate the proposed method. Full wave numerical simulations are conducted, and the results show large beam scanning ranges of the reflectarray over 50° in the plane containing the offset feed and over 40° in the plane perpendicular to the feed plane. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab16bc
  • Digital signal processing in coupled photonic crystal waveguides and its application to an all-optical AND logic gate
    Jandieri, V. and Onoprishvili, T. and Khomeriki, R. and Erni, D. and Pistora, J.
    Optical and Quantum Electronics 51 (2019)
    The realization of all-optical AND logic gates for pulsed signal operation based on the photonic bandgap transmission phenomenon is proposed. We are using realistic planar air-hole type coupled photonic crystal waveguides (C-PCWs) with Kerr-type nonlinear background medium. The novelty of our analysis is that the proposed AND logic gate operates with the temporal solitons, which maintain a stable envelope propagating in the nonlinear C-PCWs, enabling true ultrafast full-optical digital signal processing in the time-domain. The bandgap transmission takes place when the operating frequency is chosen at the very edge of the dispersion curve of one of the supermodes in the C-PCWs. In this regard, our original fast and accurate method is used to efficiently calculate the supermodes of the C-PCW system. The underlying semi-analytical full-wave modal analysis is based on the evaluation of the lattice sums for complex wavenumbers using the transition-matrix method in combination with the generalized reflection-matrix approach. As a proof of concept successful pulse operation of the all-optical AND logic gate is demonstrated in the framework of extensive full-wave finite-difference time-domain electromagnetics analysis. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstract10.1007/s11082-019-1833-9
  • Dynamic-range enhancement of heterodyne THz imaging by the use of a soft paraffin-wax substrate lens on the detector
    Yuan, H. and Lisauskas, A. and Zhang, M. and Rennings, A. and Erni, D. and Roskos, H.G.
    2019 Photonics and Electromagnetics Research Symposium - Fall, PIERS - Fall 2019 - Proceedings (2019)
    Imaging with terahertz (THz) radiation gains increasing interest, because it reaches better spatial resolution than imaging with microwaves, while it maintains the advantages of a non-ionizing character and the transparency of many materials. Coherent detection, which provides both intensity and phase information, plays a key role in many emerging THz imaging applications. However, in conventional heterodyne detection, usually a semi-transparent beam-combiner is employed to couple the object wave and the reference wave onto the same beam path in front of the detector chip. This results in a bulky detector system and considerable loss. In the measurement systems considered in our work, the waves are coupled to the detector element, a THz-sensitive field-effect transistor (TeraFET), through the detector chip's substrate to which a silicon substrate lens is attached. Coupling the reference radiation onto the TeraFET from the front side would allow to avoid the use of the beam-combiner. However, front-side coupling is inefficient because of the absence of a silicon substrate lens whose implementation to the chip's front side is prohibited by the presence of the detector's bonding wires. In this contribution, we introduce a novel lens-formation approach using paraffin wax with a low melting temperature. Dripping the liquid wax onto the detector allows to embed the bonding wires within the lens. The surface tension of the molten wax leads to a close-to-super-hemispherical shape of the droplet which is maintained upon solidification. Care has to be taken to correctly center the lens on the detector. Imaging results with a TeraFET without and with an integrated wax lens (of a diameter of 8 mm) in a Fourier imaging experiment show that the paraffin wax lens improves the coupling efficiency of the 300-GHz reference radiation by at least 6 dB. The dynamic range and the useful focal-plane area of the raw data are enhanced by the wax lens, with the consequence that the resolution of the reconstructed image is improved considerably. © 2019 IEEE.
    view abstract10.1109/PIERS-Fall48861.2019.9021735
  • Effect of hepatic vein on gold nanoparticle-mediated-hyperthermia in liver cancer
    Jalali, M. and Mertin, P. and Rennings, A. and Erni, D.
    Proceedings of SPIE - The International Society for Optical Engineering 11207 (2019)
    Gold nanoparticle-mediated hyperthermia is a non-invasive, target-based cancer treatment with significantly reduced side effects compared to conventional treatments. In this work a simulation model for gold nanoparticlemediated hyperthermia is set up and used to investigate the case of a liver tumor located in the vicinity of a hepatic vein. Gold nanorods with optimized size and aspect ratio are embedded within the liver, and the temperature raise under CW laser illumination is calculated, while taking into account the convective heat transfer through blood perfusion. For this purpose, an analytical model based on the Navier-Stokes equation is used. Results show that due to the heat drain in the blood stream, an effective temperature raise is not achievable when the tumor is located in the vicinity of the hepatic vein. Additionally it is shown that even in the case of a 90% occluded vein, the temperature raise with such nanoparticle arrangement is still not enough for tumor ablation. © 2019 SPIE.
    view abstract10.1117/12.2526978
  • Effective coupling of a plasmonic nanorod to a microsphere resonator
    Jalali, M. and Erni, D.
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
    Coupled photonic-plasmonic systems have already proven successful as ultra-sensitive sensors with the potential of single virus detection [1]. Such systems are e.g. composed of a plasmonic nanoparticle in vicinity of an optical resonator usually in the form of a microsphere, a mirroring or a microdisk. These systems are proven highly sensitive due to their large quality factor and are thus utilized as bio, pressure, gas or temperature sensors [2]. When a plasmonic nanoparticle is positioned near such resonator, coupling of the plasmonic and photonic modes can result in an ultra-sensitive biosensor [3], since nanoparticle presence enhance ambient refractive index (RI) variation sensitivity. © 2019 IEEE.
    view abstract10.1109/CLEOE-EQEC.2019.8871574
  • Experimental and numerical investigations of a small animal coil for ultra-high field magnetic resonance imaging (7T)
    Garcia, M.M. and Oliveira, T.R. and Papoti, D. and Chaim, K.T. and Otaduy, M.C.G. and Erni, D. and Zylka, W.
    Current Directions in Biomedical Engineering 5 (2019)
    The purpose of this work was to develop and investigate a radiofrequency (RF) coil to perform image studies on small animals using the 7T magnetic resonance imaging (MRI) system, installed in the imaging platform in the autopsy room (Portuguese acronym PISA), at the University of Sao Paulo, Brazil, which is the unique 7T MRI scanner installed in South America. Due to a high demand to create new specific coils for this 7T system, it is necessary to carefully assess the distribution of electromagnetic (EM) fields generated by the coils and evaluate the patient/object safety during MRI procedures. To achieve this goal 3D numerical methods were used to design and analyse a 8-rungs transmit/receive linearly driven birdcage coil for small animals. Calculated magnetic field (B1) distributions generated by the coil were crosschecked with measured results, indicating good confidence in the simulated results. Electric field results were post-processed and predictions of local specific absorption rate (SAR) values were achieved for a spherical phantom filled with muscle-like tissue, indicating that the sample would not suffer any unsafe deposition of energy. Post mortem abdomen images obtained from a rat presented good image quality and no artifacts related to field non-homogeneity were observed. © 2019 by Walter de Gruyter Berlin/Boston.
    view abstract10.1515/cdbme-2019-0132
  • Functionalizing plasmonic nanoparticles through adding a shell to improve electronic properties of c-Si thin-film solar cells
    Jalali, M. and Jalali, T. and Nadgaran, H. and Erni, D.
    Journal of the Optical Society of America B: Optical Physics 36 (2019)
    Embedding plasmonic nanoparticles (p-NPs) inside the solar cell’s active layer is capable of enhancing active layer optical absorption; however, such inclusion has some detrimental effects on the electronic properties of the solar cells. In addition, p-NPs are highly catalytic, their presence enables other non-radiative decay channels besides generation of electron–hole pairs, and the electrons usually get absorbed by these p-NPs. This results in the unfavorable fact that the potential enhancement in the carrier generation rate and the generated current is no longer in line with the enhancement in the optical absorption. In this paper, we propose to functionalize p-NPs by adding a dielectric or semiconductor shell, to passivate the p-NP without deteriorating scattering and/or plasmonic effects. Ag@SiO 2 and Ag@TiO 2 core–shell p-NPs have been intensely studied using extensive computational electromagnetic simulations to model the spectral response of the active layer’s optical absorption as well as electronic properties as a function of both shell composition and thickness. It is shown that a 5 nm TiO 2 shell is apt to optically passivate the p-NP without any reduction in optical absorption, while improving the short circuit current density (J sc ) of the thin-film solar cell by 33.3%. © 2018 Optical Society of America.
    view abstract10.1364/JOSAB.36.000101
  • Maximizing Information Extraction of Extended Radar Targets Through MIMO Beamforming
    Ahmed, A.M. and Alameer, A. and Erni, D. and Sezgin, A.
    IEEE Geoscience and Remote Sensing Letters 16 (2019)
    We jointly design an information-theoretic transmit and receive radar beamformers for spatially near multiple extended targets. We maximize the mutual information (MI) between the received signals and the targets signatures that allows the extraction of the unknown features, which may include shape, dimensions, and material. However, high interference caused by spatially near targets might obstruct the information extraction, and directing the beamformers toward the steering vector as done in conventional beamformers does not solve this problem, especially for extended targets. In this letter, an iterative algorithm is presented to solve this problem using alternative minimization, dividing it into two blocks. The first block is solving for the transmit beamformers successively using block coordinate descent, and the second one is solving for the receiver beamformers using the minimum variance distortionless response. We also show the effect of using our beamformers on the waveform design problem. Numerical results indicate that this algorithm can achieve substantially higher MI than the existing conventional methods. Thus, except for some degenerate cases, having fixed beamformers instead of optimized ones lead to significant performance degradation. © 2004-2012 IEEE.
    view abstract10.1109/LGRS.2018.2876714
  • Mutually coupled dielectric resonators for on-chip antenna efficiency enhancement
    Sievert, B. and Svejda, J.-T. and Erni, D. and Rennings, A.
    2019 2nd International Workshop on Mobile Terahertz Systems, IWMTS 2019 (2019)
    Dielectric Resonator Antennas (DRA) are designed for high efficiency operation utilizing the low dielectric losses of appropriately chosen materials. Although the mode and thus the resonance frequency is determined by the dielectric resonator, the (metal) structure used for excitation needs to fit the resonance frequency as well. This paper presents a novel method of using dielectric resonators as coupling elements into free space. The resonance frequency is determined by the exciting grounded λ-dipole, the dielectric resonators are driven approximately at eigenresonance. A close proximity of the dielectric resonators (DR) enforces a coupling which leads to fringing fields contributing to the radiation. Finally, an increased radiation efficiency is predicted for the on-chip enviroment at 220 GHz. © 2019 European Union
    view abstract10.1109/IWMTS.2019.8823752
  • New approach for the simulation of bent and crumpled antennas on a flexible substrate
    Neumann, K. and Rennings, A. and Erni, D.
    IMWS-AMP 2019 - 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (2019)
    This paper presents a new approach for simulating bent or crumpled antenna structures. A combination of two different simulation tools is used. At first a textile simulator is used to reproduce crumpling actions according to the fabric's material parameters. The resulting model can then be imported into a boundary-element based electromagnetic solver. Such an approach offers the flexibility to fold the device in any possible form, without distorting the shape, that means the surface area remains constant. © 2019 IEEE.
    view abstract10.1109/IMWS-AMP.2019.8880127
  • Reducing the divergence of vortex waves with a lens tailored to the utilized circular antenna array
    Haj Hassan, M. and Sievert, B. and Rennings, A. and Erni, D.
    2019 2nd International Workshop on Mobile Terahertz Systems, IWMTS 2019 (2019)
    In this paper, we propose a tailored lens in order to overcome the obstacle of the large beam divergence inherent to vortex waves that are generated by a Uniform Circular Patch Antenna Array (UCA) at 10 GHz. We implemented two lenses. The first one is a combination of 8 identical lenses, and will be called sectorized lens, where each of the sectors conforms to an underlying patch antenna element. The second lens is a smoothed version of the former one, which will be called the rotationally symmetric lens. The simulation results of the two tailored lenses are in agreement with the design expectations and show a rotationally symmetry, which is essential for an unperturbed vortex beam emission. The performance of these two designs will be compared to the patch antenna array without lens and to an ordinary extended elliptical lens. Both designed lenses have a gain of about 15.3 dBi, which is about 5.5 dBi higher than the gain of the bare patch antenna array. © 2019 IEEE
    view abstract10.1109/IWMTS.2019.8823656
  • Resonant Antenna Periodically Loaded with Series Capacitances for Enhanced Radiation Efficiency
    Sievert, B. and Erni, D. and Rennings, A.
    GeMiC 2019 - 2019 German Microwave Conference (2019)
    A periodic series-capacitance loaded patch antenna for increased radiation efficiency at 250 GHz in on-chip environment is presented. The antenna is described as a periodic high pass transmission line. An exemplary candidate with increased efficiency is compared to a conventional patch antenna for broadside radiation. An essential part of this paper is an equivalent circuit model explaining the increased efficiency as well as the decreased bandwidth of the structure. A dispersion diagram analysis is motivated for the design process as well as for physical insight. © 2019 IMA - Institut fur Mikrowellen- und Antennentechnik e.V.
    view abstract10.23919/GEMIC.2019.8698181
  • Semiperiodic ultra-broadband double-grating to improve c-si thin-film solar cell’s optical absorption, through numerical structural optimization
    Jalali, M. and Nadgaran, H. and Erni, D.
    Crystals 9 (2019)
    Plasmonic gratings provide effective photon management techniques in thin-film solar cells, capable of extending the optical thickness of the solar cell’s active layer. However, the ultra-broadband nature of such application makes an optimal design of the grating structure quite challenging, since a fully periodic grating operates only in specific spectral ranges. To achieve a more broadband design, semiperiodicity is introduced, which, due to having controllable disorder, is an apt solution in broadband optical applications. In this work, semiperiodic double gratings as a broadband photon management technique are introduced in order to improve the optical absorption of c-Si thin-film solar cells, and optimized through numerical structural optimization. Physical parameters of both front and back gratings are determined taking the spectrally integrated optical absorption as the figure of merit and subsequently a semiperiodic double grating is established through adding defects to the fully periodic structure. It is shown that such semiperiodic structure is capable of enhancing the spectrally integrated optical absorption 88.6% compared to a reference structure without gratings. © 2019 by the authors
    view abstract10.3390/cryst9050264
  • Theory of soliton propagation in nonlinear photonic crystal waveguides
    Jandieri, V. and Khomeriki, R. and Berakdar, J. and Erni, D.
    Optics Express 27 (2019)
    Propagation of the temporal soliton in Kerr-type photonic crystal waveguide is investigated theoretically and numerically. An expression describing the evolution of the envelope of the soliton based on the full-wave modal analysis, taking into account all space-harmonics, is rigorously obtained. The nonlinear coefficient is derived, for the first time, based on a modification of the refractive indices for each space-harmonic due to the Kerr-type nonlinearity. For illustrating the general formulation and results, we performed extensive computational electromagnetics simulations for the propagation of gap solitons in an experimentally feasible photonic crystal waveguides, endorsing the correctness and usefulness of the proposed formalism. © 2019 Optical Society of America.
    view abstract10.1364/OE.27.029558
  • Transmitarray element design for subharmonic injection-locked RTD oscillators in THz band
    Zhang, M. and Rennings, A. and Clochiatti, S. and Arzi, K. and Prost, W. and Weimann, N. and Erni, D.
    2019 Photonics and Electromagnetics Research Symposium - Fall, PIERS - Fall 2019 - Proceedings (2019)
    In this paper, a transmitarray element (TE) is designed for wireless subharmonic injection-locked triple barrier (TB) resonant tunneling diode (RTD) oscillators. It adopts a receiver antenna (RA)-transmitter antenna (TA) structure. The RA is a u-slotted patch antenna, and we use a cubic silicon block at top of this patch, so as to increases the RA gain and radiation efficiency. A fat monopole structure with a slot-like counterpoise is used as the TA. In this design, the RA can receive 100 GHz subharmonic injection signal (SIS). Meanwhile, the TA will radiate the 300 GHz fundamental oscillation signal (FOS) generated by the TB RTD. Moreover, the TA structure can isolate the 300 GHz FOS coming into the RA but couple the received 100 GHz SIS to the TB RTD, which performs like a filter-antenna. In the simulation, the transmission loss in the TA structure is higher than 15 dB around 300 GHz and only about 1.5 dB around 100 GHz. The gain of RA is 6 dBi with 65% radiation efficiency at 100 GHz and the gain of TA is 14 dBi at 300 GHz when applying a 1 mm radius silicon lens at the backside of the InP substrate. © 2019 IEEE.
    view abstract10.1109/PIERS-Fall48861.2019.9021805
  • Triple-Barrier Resonant-Tunnelling Diode THz Detectors with on-chip antenna
    Arzi, K. and Clochiatti, S. and Suzuki, S. and Rennings, A. and Erni, D. and Weimann, N. and Asada, M. and Prost, W.
    GeMiC 2019 - 2019 German Microwave Conference (2019)
    Signal detection at (sub-)mm-wave frequencies via a single chip size component is discussed. The monolithic integration consists of high current density InP-based Triple Barrier Resonant Tunneling Diode into an on-chip antenna. The asymmetrical current voltage characteristic of the Triple Barrier Resonant Tunneling Diode enables signal detection at zero bias. A very high responsivity above 250 GHz is experimentally demonstrated. Low temperature DC rectification factor of the diode is investigated and a thermionic current contribution over the temperature is presented. © 2019 IMA - Institut fur Mikrowellen- und Antennentechnik e.V.
    view abstract10.23919/GEMIC.2019.8698124
  • Beam Switching Antenna Based on a Reconfigurable Cascaded Feeding Network
    Wang, P.-Y. and Jin, T. and Meng, F.-Y. and Lyu, Y.-L. and Erni, D. and Wu, Q. and Zhu, L.
    IEEE Transactions on Antennas and Propagation 66 (2018)
    In this paper, a beam switching antenna (BSA) based on a reconfigurable cascaded feeding network (RCFN) is presented. The RCFN is engineered by cascading several varactor-loaded quasi-lumped couplers (QLCs). As the output port of the RCFN, the coupled port of every QLC is connected to an endfire radiator. BSAs based on two types of RCFN, the open-loop RCFN and closed-loop RCFN, are designed, simulated, and measured. The open-loop design features flexible beam switching strategies with the dynamic control of operation modes allocating the input power to the intended radiator(s), and single-/multibeam switching is achieved. In the case of the closed-loop RCFN, a power-recycling circuit is introduced to improve the efficiency of the single-beam switching mode of the BSA. Measured results show that the BSA operating in single-beam mode is able to radiate six switched beams each with a 3 dB beamwidth of 60°, covering 360° in the azimuth plane. The return losses and gain fluctuation of both prototypes are better than 15 dB and less than 1 dB, respectively. The corresponding experiments agree well with both the theoretical analysis and the simulation results. © 1963-2012 IEEE.
    view abstract10.1109/TAP.2017.2786298
  • Circular high impedance surface for radially polarized fields
    Sievert, B. and Erni, D. and Rennings, A.
    Asia-Pacific Microwave Conference Proceedings, APMC (2018)
    In this paper, a circular high impedance surface (HIS) with radially adjustable properties for broadband antenna applications is presented. This metasurface is intended to be used as an antenna shield based on cylindrical coordinates. Antennas as the rounded-bowtie, which fit organically into the cylindrical coordinate system, can benefit from a circular HIS which interacts with the radial electric field component. The broadband operation of such an antenna should be supported using an inhomogeneous HIS shield with an in-phase reflection frequency which is radially dependent. The HIS is characterized by the in-phase reflection at certain frequencies, at which it mimics the behavior of a perfect magnetic conductor (PMC). An equivalent circuit (EC) for the reflection phase is given for the proposed structure. Within the demonstrated and explained limitations, the presented EC offers a toolbox for the design of circular HIS. © 2017 IEEE.
    view abstract10.1109/APMC.2017.8251710
  • Compact planar array antenna with electrically beam steering from backfire to endfire based on liquid crystal
    Ma, S. and Zhang, S.-Q. and Ma, L.-Q. and Meng, F.-Y. and Erni, D. and Zhu, L. and Fu, J.-H. and Wu, Q.
    IET Microwaves, Antennas and Propagation 12 (2018)
    In this study, a novel electrically steerable planar array antenna based on liquid crystal (LC) is elaborately designed, fabricated and measured. The occupied area and undesired coupling of the meander line phase shifter are reduced by chamfering appropriately. Combined with the electrically tunable LC material, the miniaturised phase shifter achieves 146° phase shift and 122.7°/dB frequency-dependent figure-of-merit. Utilising in series phase shifters, power dividers and in parallel patches, the proposed antenna features compact configuration 41° continuous wide beam scanning range from forward to backward at the satellite communication band 12.5 GHz. At the same time, it maintains satisfactory reflection coefficient and impedance bandwidth at each angle. Finally, the measurements validate the scanning of the beam by electrically tuning the phase shift between the radiating elements. © 2018, The Institution of Engineering and Technology.
    view abstract10.1049/iet-map.2017.1070
  • Efficient analysis method of light scattering by a grating of plasmonic nanorods
    Jandieri, V. and Okropiridze, L. and Yasumoto, K. and Erni, D. and Pistora, J.
    COMPEL - The International Journal for Computation and Mathematics in Electrical and Electronic Engineering 37 (2018)
    Purpose: The purpose of this paper is to develop a rigorous self-contained formulation for analyzing electromagnetic scattering by grating of plasmonic nanorods. The authors investigate this structure from the viewpoint of the practical application as a refractive index plasmonic sensor. The presented rigorous formulation is accompanied with a neat implementation providing a high computation efficiency and could be considered as an important tool for designing and optimizing compact sensors. Design/methodology/approach: Scattering of an incident plane wave by grating made of a periodic arrangement of metal-coated dielectric nanocylinders on a dielectric slab is rigorously investigated using the recursive algorithm combined with the lattice sums technique. As a dielectric slab, the authors consider glass material, which is widely used in experiments, whereas silver (Ag) is used as a low loss metal suitable to excite plasmon resonances. The main advantage of the developed self-contained formulation is that first the authors extract the reflection and transmission matrices of a single planar array from a separate analysis of the grating and the slab and then obtain the scattering characteristics of the whole structure by using a recursive formula. The method is computationally fast. Findings: Dependence of the surface plasmon resonance wavelength on the refractive index of the surrounding medium is carefully investigated. The resonance peaks are red-shifted with respect to an increasing refractive index of surrounding medium showing an almost linear behavior. Near field distributions are analyzed at the resonance wavelengths of the spectral responses. Special attention is paid to the formation of the dual-absorption bands because of the excitation of the localized surface plasmons. The authors give physical insight to the coupling between grating and the glass slab. The authors found that a strong enhancement of the field inside the slab occurs when the scattered wave of the grating is phase-matched to the guided modes supported by the slab. Originality/value: In the authors’ formulation, they do not use any approximation and it is rigorous and accurate. The authors use their original method. The method is based on the lattice sums technique and uses the recursive algorithm to calculate the generalized reflection and transmission characteristics by grating. Such fast and accurate method is an effective tool apt for designing and optimizing tailored sensors, for e.g. advanced biomedical applications. © 2018, Emerald Publishing Limited.
    view abstract10.1108/COMPEL-08-2017-0354
  • IEEE Workshop on Industrial and Medical Measurement and Sensor Technology-SENSORICA 2017
    Himmel, J. and Erni, D. and Fischerauer, A. and Kanoun, O. and Seeger, T. and Thelen, K.
    Technisches Messen 85 (2018)
    view abstract10.1515/teme-2018-0013
  • Metamaterial-based transmit and receive system for whole-body magnetic resonance imaging at ultra-high magnetic fields
    Herrmann, T. and Liebig, T. and Mallow, J. and Bruns, C. and Stadler, J. and Mylius, J. and Brosch, M. and Svedja, J.T. and Chen, Z. and Rennings, A. and Scheich, H. and Plaumann, M. and Hauser, M.J.B. and Bernarding, J. and Erni, D.
    PLoS ONE 13 (2018)
    Magnetic resonance imaging (MRI) at ultra-high fields (UHF), such as 7 T, provides an enhanced signal-to-noise ratio and has led to unprecedented high-resolution anatomic images and brain activation maps. Although a variety of radio frequency (RF) coil architectures have been developed for imaging at UHF conditions, they usually are specialized for small volumes of interests (VoI). So far, whole-body coil resonators are not available for commercial UHF human whole-body MRI systems. The goal of the present study was the development and validation of a transmit and receive system for large VoIs that operates at a 7 T human whole-body MRI system. A Metamaterial Ring Antenna System (MRAS) consisting of several ring antennas was developed, since it allows for the imaging of extended VoIs. Furthermore, the MRAS not only requires lower intensities of the irradiated RF energy, but also provides a more confined and focused injection of excitation energy on selected body parts. The MRAS consisted of several antennas with 50 cm inner diameter, 10 cm width and 0.5 cm depth. The position of the rings was freely adjustable. Conformal resonant right-/ left-handed metamaterial was used for each ring antenna with two quadrature feeding ports for RF power. The system was successfully implemented and demonstrated with both a silicone oil and a water-NaCl-isopropanol phantom as well as in vivo by acquiring whole-body images of a crab-eating macaque. The potential for future neuroimaging applications was demonstrated by the acquired high-resolution anatomic images of the macaque's head. Phantom and in vivo measurements of crab-eating macaques provided high-resolution images with large VoIs up to 40 cm in xy-direction and 45 cm in z-direction. The results of this work demonstrate the feasibility of the MRAS system for UHF MRI as proof of principle. The MRAS shows a substantial potential for MR imaging of larger volumes at 7 T UHF. This new technique may provide new diagnostic potential in spatially extended pathologies such as searching for spread-out tumor metastases or monitoring systemic inflammatory processes. © 2018 Herrmann et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
    view abstract10.1371/journal.pone.0191719
  • Multi-functional RF coils for ultra-high field MRI based on 1D/2D electromagnetic metamaterials
    Erni, D. and Rennings, A. and Svejda, J.T. and Sievert, B. and Chen, Z. and Liebig, T. and Froehlich, J.
    Journal of Physics: Conference Series 1092 (2018)
    Electromagnetic metamaterials have already proven very valuable for the enhancement and molding of RF magnetic fields within ultra-high field MRI scanners at 7T. We report on our development of coil elements based on composite right-/left-handed (CRLH) 1D electromagnetic (EM) metamaterial transmission lines (metalines) operating in the zeroth order resonance (ZOR) to foster uniform RF magnetic field distributions along the scanner axis. Tailored EM metalines supporting full-wave or quarter-wave resonances are used either as metamaterial ring antenna or as dual-band coil elements for simultaneous 1H/23Na imaging. The EM metalines are key to the MetaBore, which is a fully adaptive RF field control scheme based on a periodic axial arrangement of conformal metamaterial ring antennas in the framework of high-field traveling-wave MRI. With the 2D EM metamaterials (metasurfaces) we realized high-impedance surfaces (HIS) in order to enhance the uniformity and directivity of the RF magnetic field from e.g. Overlaid (elongated) dipole elements towards the probe volume. The designs include simulation studies of the overall multichannel coil systems, which are carried out with our home-made, open source electromagnetic 3D EC-FDTD solver openEMS supporting conformal cylindrical inhomogeneous meshing. Experimental verifications of our coils have been carried out within 7T MRI scanners (Siemens Magnetom). © 2018 Institute of Physics Publishing.All Rights Reserved.
    view abstract10.1088/1742-6596/1092/1/012031
  • Numerical investigation of nematic liquid crystals in the THz band based on EIT sensor
    Wang, P.-Y. and Jin, T. and Meng, F.-Y. and Lyu, Y.-L. and Erni, D. and Wu, Q. and Zhu, L.
    Optics Express 26 (2018)
    This paper introduces the concept of electromagnetically induced transparency (EIT) into the permittivity extraction of an anisotropic material—nematic liquid crystal (NLC). A novel two-step strategy is presented to extract the complex permittivity of the NLC at the THz band, which evaluates the relative permittivity tensor from the resonant frequencies and then determines the loss tangent from the quality factor Q of the EIT sensor. The proposed method features high accuracy due to the sharp resonance of the EIT sensor and also high robustness to the thickness of the NLC layer because only amplitude rather than phase information of the transmission coefficients is required. The NLC filled EIT sensor shows a sensitivity of 56.8 μm/RIU (the resonance wavelength shift over the refractive index change unit (RIU)) and Figure of Merit (FoM) of 6.92. The uncertainty of the proposed technique in the relative permittivity and loss tangent is 3% and 8.2%, respectively. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstract10.1364/OE.26.012318
  • Probing the SERS brightness of individual Au nanoparticles, hollow Au/Ag nanoshells, Au nanostars and Au core/Au satellite particles: Single-particle experiments and computer simulations
    Tran, V. and Thiel, C. and Svejda, J.T. and Jalali, M. and Walkenfort, B. and Erni, D. and Schlücker, S.
    Nanoscale 10 (2018)
    Different classes of plasmonic nanoparticles functionalized with the non-resonant Raman reporter molecule 4-MBA are tested for their SERS signal brightness at the single-particle level: gold nanoparticles, hollow gold/silver nanoshells, gold nanostars, and gold core/gold satellite particles. Correlative SERS/SEM experiments on a set of particles from each class enable the unambiguous identification of single particles by electron microscopy as well as the characterization of both their elastic (LSPR) and inelastic (SERS) scattering spectra. Experimental observations are compared with predictions from FEM computer simulations based on 3D models derived from representative TEM/SEM images. Single gold nanostars and single gold core/gold satellite particles exhibit a detectable SERS signal under the given experimental conditions, while single gold nanoparticles and single hollow gold/silver nanoshells are not detectable. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr06028b
  • Realization of true all-optical AND logic gate based on nonlinear coupled air-hole type photonic crystal waveguides
    Jandieri, V. and Khomeriki, R. and Erni, D.
    Optics Express 26 (2018)
    In this manuscript we propose an easily scalable true all-optical AND logic gate for pulsed signal operation based on band-gap transmission within nonlinear realistic air-hole type coupled photonic crystal waveguides (C-PCW). We call it "true" all-optical AND logic gate, because all AND gate topologies operate with temporal solitons that maintain a stable pulse envelope during the optical signal processing along the different C-PCW modules yielding ultrafast full-optical digital signal processing. We directly use the registered (output) signal pulse as new input signal between multiple concatenated nonlinear C-PCW modules (i.e. AND gates) to setup a multiple-input true all-optical AND logic gate. Extensive full-wave computational electromagnetic analysis proves the correctness of our theoretical studies and the proposed operation principle of the multiple-input AND logic gate is vividly demonstrated for realistic C-PCWs. © 2018 Optical Society of America.
    view abstract10.1364/OE.26.019845
  • RF Shimming and Improved SAR Safety for MRI at 7 T With Combined Eight-Element Stepped Impedance Resonators and Traveling-Wave Antenna
    Elabyad, I.A. and Herrmann, T. and Bruns, C. and Bernarding, J. and Erni, D.
    IEEE Transactions on Microwave Theory and Techniques 66 (2018)
    A remote transmission and detection for magnetic resonance imaging (MRI) at 7 T was validated by using a patch antenna that excites the traveling-wave (TW) modes guided by the radio frequency (RF) shield of the MRI system. In this paper, RF simulations were performed at 298 MHz for an eight-element stepped impedance resonators (SIRs) head volume coil combined with a patch antenna. The combined structure is loaded with both a cylindrical phantom and the Duke full-body human voxel model. An optimization routine was developed in MATLAB to provide homogenous B&#x2081;&#x207A; field distributions with minimal 10 g averaged specific absorption rate (SAR) values within the phantom and the Duke human biological model. Before optimization, the TW approach achieved a more homogenous B&#x2081;&#x207A; field distribution providing a large field of view along the propagation direction compared to the SIRs head volume coil. However, the corresponding peak local SAR values for the TW approach is about 2.4 times higher than those of the SIRs head volume coil. The B&#x2081;&#x207A; transmission efficiency for the TW approach is higher than that of the SIRs head volume coil by 10-30&#x0025; due to the coupling of the TW modes into the volume coil. The RF-shimming technique improves the B&#x2081;&#x207A; field homogeneity for the combined approach by 42&#x0025;, 48&#x0025;, and 47&#x0025; in the central axial, sagittal, and coronal slices, respectively, and satisfies the constraints of maximum local SAR in the human head for MRI at 7 T. IEEE
    view abstract10.1109/TMTT.2017.2708707
  • Simulation and optimization of post-wall waveguide based compact circuits for micro and millimeter waves
    Jandieri, V. and Erni, D. and Maeda, H. and Yasumoto, K. and Akopiani, A.
    GeMiC 2018 - 2018 German Microwave Conference 2018-January (2018)
    Functional post-wall waveguide (PWW) based passive circuits are formed by introducing specific arrangements of additional posts into the PWW structure. Each arrangement represents a circuit element and consists of a circular array of cylindrical posts periodically distributed on the circumference of the corresponding ring. The circular arrays (i.e. the circuit elements) provide enough degrees of freedom to tailor the spectral response in the framework of an ultra-compact planar filter topology. The PWW circuit is analyzed using image theory in combination with the lattice sums technique yielding a highly-efficient semi-analytical computational scheme that is perfectly suitable for the numerical structural optimization of mm-wave band-pass filters. Along an exemplary study we demonstrate the feasibility of steep skirt selectivity within a device footprint of around 0.75A2. © 2018 IMA.
    view abstract10.23919/GEMIC.2018.8335078
  • Spatially modulated high impedance surface based on a multilayer approach
    Sievert, B. and Erni, D. and Rennings, A.
    GeMiC 2018 - 2018 German Microwave Conference 2018-January (2018)
    This paper presents a toolbox for the dimensioning and design of high impedance surfaces (HIS) with spatially varying resonance frequency. An application of those surfaces is a reflector with spatial dependent phase shifting properties. Such types of reflectors use a spatial variation of the resonance frequency of different patches for achieving different phase shifts. This work concentrates on the equivalent circuit (EC) representation of the structure and compares the result to numerical solutions. © 2018 IMA.
    view abstract10.23919/GEMIC.2018.8335079
  • A metamaterial based dual-resonant coil element for combined sodium/hydrogen MRI at 7 Tesla
    Svejda, J.T. and Rennings, A. and Erni, D.
    Technisches Messen 84 (2017)
    A single coil element based on a composite right/left-handed (CRLH) metamaterial transmission line for combined sodium and hydrogen MRI is presented. It provides the particular feature of equal quarter-wavelength current and electromagnetic (EM) field distributions at the two distinct Larmor frequencies of 23Na- and 1H-nuclei at 7 T magnetic flux density. The functionality of a manufactured prototype is validated through near-field measurements that are compared with finite-difference time-domain (FDTD) simulations. Finally, results of a first successful MRI experiment are presented for a canister phantom including ping-pong ball insets filled with different liquids of high sodium concentrations. © 2016 Walter de Gruyter Berlin/Boston 2016.
    view abstract10.1515/tem-2016-0039
  • Analysis of post-wall waveguides and circuits using a model of two-dimensional photonic crystals
    Jandieri, V. and Maeda, H. and Yasumoto, K. and Erni, D.
    Progress In Electromagnetics Research M 56 (2017)
    A semi-analytical method to analyze post-wall waveguides and circuits based on the model of two-dimensional photonic crystals formed by layered periodic arrays of circular cylinders is presented. The propagation constant of the fundamental TE mode, the attenuation constant due to the leakage loss and the effective width of an equivalent rectangular waveguide are calculated. Using the concept of the effective width, the original structure is replaced by an equivalent rectangular structure. When additional metallic posts are loaded in the rectangular waveguide, functional post-wall waveguide-based passive circuits are formed. The S-parameters of the post-wall circuits, which act as bandpass filters, are calculated using the image theory combined with the lattice sums technique. © 2017, Electromagnetics Academy. All rights reserved.
    view abstract10.2528/PIERM17012303
  • Comparison between periodicity and randomness from an effective refractive index point of view; Applicable to thin-film solar cells
    Jalali, M. and Nadgaran, H. and Erni, D.
    Scientia Iranica 24 (2017)
    In this paper, embedding plasmonic nanoparticles inside the solar cell's active layer, both in a periodic and in a random manner, is extensively investigated. The aim of this study is to investigate optical mechanisms inside the active layer as a consequence of nanoparticle inclusion as well as to compare periodicity and randomness in such structures, where the intended maximization of the ultra-broadband absorption renders the analysis complicated. To perform such study, an effective refractive index analysis is employed to simultaneously cover the influential parameters. The results show that although fully periodic structures are more desirable in narrow-band applications such as grating-assisted waveguide coupling, random inclusion of plasmonic nanoparticles in the solar cell's active layer yields a much higher optical absorption. Furthermore, random inclusion of nanoparticles is easier and much cheaper than periodic inclusion to implement in solar cell fabrication. © 2017 Sharif University of Technology. All rights reserved.
    view abstract10.24200/sci.2017.4596
  • Electrically Controllable Composite Right/ Left-Handed Leaky-Wave Antenna Using Liquid Crystals in PCB Technology
    Che, B. and Jin, T. and Erni, D. and Meng, F. and Lyu, Y. and Wu, Q.
    IEEE Transactions on Components, Packaging and Manufacturing Technology 7 (2017)
    A design method for electrically controllable composite right/left-handed (CRLH) leaky-wave antennas (LWAs) with large beam-steering range employing liquid crystal (LC) in printed circuit board technology is proposed. It is demonstrated with detailed mathematical derivation that the design principle enables the LC-CRLH-LWA to keep the balanced condition with all bias states applied to the LC, yielding LC-CRLH-LWAs that feature a steady balanced condition and a broadband property. Based on this principle, an LC-CRLH-LWA prototype is designed, simulated, optimized, and experimentally validated. According to the simulation results, the designed LC-CRLH-LWA operates in the band from 11.14 to 12.77 GHz with a frequency-agile radiation direction. By tuning the permittivity of LC, the radiation direction of the designed antenna scans from -21° to +23° at the fixed operating frequency of 12.4 GHz. The experimental results agree well with the simulated data. Furthermore, sidelobe level suppression of the designed antenna is achieved through decreasing the reflection between the unit cells of the antenna.
    view abstract10.1109/TCPMT.2017.2680469
  • Electrically Controllable Composite Right/Left-Handed Leaky-Wave Antenna Using Liquid Crystals in PCB Technology
    Che, B.-J. and Jin, T. and Erni, D. and Meng, F.-Y. and Lyu, Y.-L. and Wu, Q.
    IEEE Transactions on Components, Packaging and Manufacturing Technology 7 (2017)
    A design method for electrically controllable composite right/left-handed (CRLH) leaky-wave antennas (LWAs) with large beam-steering range employing liquid crystal (LC) in printed circuit board technology is proposed. It is demonstrated with detailed mathematical derivation that the design principle enables the LC-CRLH-LWA to keep the balanced condition with all bias states applied to the LC, yielding LC-CRLH-LWAs that feature a steady balanced condition and a broadband property. Based on this principle, an LC-CRLH-LWA prototype is designed, simulated, optimized, and experimentally validated. According to the simulation results, the designed LC-CRLH-LWA operates in the band from 11.14 to 12.77 GHz with a frequency-agile radiation direction. By tuning the permittivity of LC, the radiation direction of the designed antenna scans from -21° to +23° at the fixed operating frequency of 12.4 GHz. The experimental results agree well with the simulated data. Furthermore, sidelobe level suppression of the designed antenna is achieved through decreasing the reflection between the unit cells of the antenna. © 2017 IEEE.
    view abstract10.1109/TCPMT.2017.2680469
  • Field distribution and coupling investigation of an eight-channel RF coil consisting of different dipole coil elements for 7 T MRI
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE Transactions on Biomedical Engineering 64 (2017)
    In this contribution, we investigate the B1 distribution and coupling characteristics of a multichannel radio frequency (RF) coil consisting of different dipole coil elements for 7 T MRI, and explore the feasibility to achieve a compromise between field distribution and decoupling by combining different coil elements. Two types of dipole elements are considered here: the meander dipole element with a chip-capacitor-based connection to the RF shield which achieves a sufficient decoupling between the neighboring elements; and the open-ended meander dipole element which exhibits a broader magnetic field distribution. By nesting the open-ended dipole elements in between the ones with end-capacitors, the B1 distribution, in terms of field penetration depth and homogeneity, is improved in comparison to the dipole coil consisting only of the elements with end-capacitors, and at the same time, the adjacent elements are less coupled to each other in comparison to the dipole coil consisting only of the open-ended elements. The proposed approach is validated by both fullwave simulation and experimental results. © 2016 IEEE.
    view abstract10.1109/TBME.2016.2602441
  • Improving B1 Efficiency and Signal-to-Noise-Ratio of a Surface Coil by a High-Impedance-Surface RF Shield for 7-T Magnetic Resonance Imaging
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE Transactions on Microwave Theory and Techniques 65 (2017)
    In this paper, we present a fundamental investigation to improve the B₁ efficiency and the signal-to-noise ratio (SNR) of a radio frequency (RF) surface coil for ultrahigh-field magnetic resonance imaging (MRI) by utilizing a high-impedance surface (HIS) as the RF shield. An analytical investigation indicates that a circular loop backed by a perfect magnetic conductor (PMC), which is the ideal case of an HIS, suggests an improved magnetic field compared with the case of a perfect electric conductor (PEC) and the case without any shield. This improvement is verified by a full-wave simulation, where the surface coil is modeled by an ideal impressed current model with azimuthal component (Jsurf,α = 1 A/m). The electromagnetic field is effectively shielded out behind the PEC and PMC shields compared with the case without any shield. Furthermore, the surface coil with uniform current distribution and the PMC shield is realized by a series resonant loop structure and a 2-D HIS structure, respectively. Since the normal component of the magnetic field is supported at the surface of an HIS, whereas suppressed by a conventional PEC, the B₁ field in the vicinity of the HIS shield is enhanced compared with the case with a PEC shield. Hence, an improvement on SNR and B₁ efficiency is achieved by utilizing an HIS shield, especially in the regions adjacent to the surface coil. It has been found that the improvement of B₁ efficiency is more prominent than the improvement of SNR due to different normalizations. The difference of peak SAR between considered shields, which is used for B₁ efficiency normalization, is considerably larger than the difference of the power loss within the phantom, which is used for the SNR normalization. The proposed approach is validated by full-wave finite-element method simulations and near-field measurements, which reveal good agreement with each other.
    view abstract10.1109/TMTT.2016.2631169
  • Liquid Crystal Leaky-Wave Antennas With Dispersion Sensitivity Enhancement
    Ma, S. and Yang, G. and Erni, D. and Meng, F. and Zhu, L. and Wu, Q. and Fu, J.
    IEEE Transactions on Components, Packaging and Manufacturing Technology 7 (2017)
    A novel design, fabrication, and packaging technology is proposed for liquid crystal (LC)-based beam-scanning leaky-wave antennas (LWAs). Different from conventional ones, extra dispersion sensitivity enhancement (DSE) components are introduced to increase the slope of effective phase constant versus frequency of LWAs and, hence, the beam scanning range. CST MW Studio software package is used to validate the design method. It is shown that, by adding the DSE components, the electrical beam scanning range is extended by more than 56% in the Ku satellite communication band, where good impedance matching and balanced condition are kept for both extreme tuning states of LCs. Prototypes of the designed LC-composite right/left-handed LWAs with and without the DSE components are fabricated and measured. The fabrication methodology incorporating printed circuit board and precision metal processing technology ensures the mechanical stability, flexible design, perfect packaging, and low cost. Finally, the feature-selective validation technique validates that the measured and simulated results are in good agreement.
    view abstract10.1109/TCPMT.2017.2683529
  • Periodic SIW Leaky-Wave Antenna with Large Circularly Polarized Beam Scanning Range
    Lyu, Y.-L. and Meng, F.-Y. and Yang, G.-H. and Erni, D. and Wu, Q. and Wu, K.
    IEEE Antennas and Wireless Propagation Letters 16 (2017)
    In this letter, we propose and demonstrate a periodic leaky-wave antenna (P-LWA) by loading transverse slot pairs (TSPs) and longitudinal slots (LSs) along substrate integrated waveguide. The proposed P-LWA is able to yield a circularly polarized beam, which scans continuously from backward, through broadside, and to forward. Furthermore, it is shown that a large circularly polarized beam scanning range requires maximally coinciding radiation patterns of TSPs and LSs. Hence, within the design and optimization of the proposed P-LWA, the radiation pattern of the TSP should be adjusted to match the radiation pattern of the LS. A resulting large circularly polarized beam scanning range from -40° to 25° has been predicted by simulations and experimentally verified. This circularly polarized beam scanning range turns out to be larger than in most of the previously reported works. © 2002-2011 IEEE.
    view abstract10.1109/LAWP.2017.2726089
  • Conversion of UEP Signatures between Different Environmental Conditions Using Shaft Currents
    Schaefer, D. and Doose, J. and Pichlmaier, M. and Rennings, A. and Erni, D.
    IEEE Journal of Oceanic Engineering 41 (2016)
    In this paper, we investigate an approach that allows naval vessels to continuously estimate their underwater electric potential (UEP) signature strength, by converting a beforehand measured reference UEP signature to the present environmental conditions. The approach is intended to improve risk assessment during mine counter measure (MCM) operations and to be used within integrated ship signature management systems (ISSMS). It is therefore designed to depend only on data available in these scenarios and to be simple and fast, rather than just being as accurate as possible. After explaining the concept and deducing the conversion relations, the approach is tested using numerical simulations and ranged UEP data of a ship (CFAV Quest). The results show that it is well suited to convert UEP signatures between different water conductivities and sensor depths, while it is not capable of converting between different seabed conductivities. The signature conversion is best for intact hull coatings, while hull-to-anode currents lower the correlation between signature and shaft currents, and hence impair the conversion quality. © 1976-2012 IEEE.
    view abstract10.1109/JOE.2015.2401991
  • Coupling investigation between RF coil array elements backed by surface impedance characterized shields for 7 Tesla MRI
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    GeMiC 2016 - 2016 German Microwave Conference (2016)
    In this paper we present a coupling investigation between RF coil array elements which are backed by surface impedance characterized RF shields for 7 Tesla magnetic resonance imaging (MRI). Two simulation models for the RF coil elements are considered here: An ideal impressed current model for an initial 2-D investigation, and a symmetrically fed dipole with meander terminals for the 3-D investigation. The RF shield, which is placed behind the coil element, is characterized by a surface impedance boundary condition (SIBC), where different surface impedances are defined. An optimal surface impedance of the RF shield can be found to achieve minimum coupling between neighboring coil elements. Different spatial arrangements (e.g.The shape of the phantom, the edge-To-edge separation between coupled coil elements, the separation from coil element to RF shield) are considered. In general, a large surface impedance provides a higher coupling level in comparison to a small surface impedance. As the separation distance from the coil element to the RF shield increases, a reduced surface impedance selectivity of the coupling behavior is observed. The proposed fundamental investigation reveals a new approach to modify the coupling characteristics of the dipole coil elements for MRI. © 2016 Institut fur Mikrowellen und Antennentechnik-IMA.
    view abstract10.1109/GEMIC.2016.7461569
  • Electromagnetic Field Analysis of a Dipole Coil Element with Surface Impedance Characterized Shielding Plate for 7-T MRI
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE Transactions on Microwave Theory and Techniques 64 (2016)
    In this paper, we systematically investigate the electromagnetic (EM) field of a stripline dipole coil element backed by various shielding plates, which are characterized by surface impedance. The initial analysis is based on a 2-D finite-element-method model, where the considered surface impedance was categorized in terms of magnitude and phase. It has been demonstrated that the shielding plate can be approximately modeled by the magnitude of a complex surface impedance if the absolute EM field distribution is considered. Additionally, as the magnitude of the surface impedance increases, the magnetic and electric fields excited by the stripline tend to distribute in a broader manner. Thus, the transversal homogeneity of the B1 field of a stripline coil can be improved by a shielding plate with a high surface impedance, which has been verified by 3-D models based on single-and multi-coil elements. For the experimental validation, two shielding plates-a copper-plated substrate and a high-impedance surface, which exhibits a small and large surface impedance, respectively-are considered. An excellent agreement of field distributions between numerical simulation and measurement has been observed. © 1963-2012 IEEE.
    view abstract10.1109/TMTT.2016.2518168
  • Improved field homogeneity for multi-channel stepped impedance microstrip transceiver arrays and travelling wave for MRI at 7t
    Elabyad, I.A. and Herrmann, T. and Bruns, C. and Bernarding, J. and Erni, D.
    European Microwave Week 2016: "Microwaves Everywhere", EuMW 2016 - Conference Proceedings; 46th European Microwave Conference, EuMC 2016 (2016)
    A multi-element microstrip transceiver array is a promising approach for magnetic resonance imaging (MRI) at 7T (298 MHz). Travelling wave (TW) MRI is another method where the RF magnetic field B1 + is carried by propagating circularly polarized modes excited from an antenna located at one end of the MRI system bore. In this paper, full-wave electromagnetic (EM) simulations were performed for an 8-element stepped impedance resonators (SIRs) coil array combined with a circularly polarized patch antenna loaded with a spherical dielectric phantom (Ø = 17 cm). An optimization routine was developed to homogenize and maximize the B1 + field inside the phantom. The results yield a combined B1 + field that is 1.3 times higher than the field of a sole 8-element SIRs volume coil and 3.5 times higher compared to the TW approach only. The B1 + field homogeneity for the 8-element SIRs volume coil is much better than for the TW approach only, where the E-field is about 2.3 times higher than the use of a sole 8-element SIRs coil array. After optimizing the combined B1 + field with amplitude/phase control, the B1 + field homogeneity was improved by 67% and 91% in the central transversal and sagittal slices, respectively. © 2016 EuMA.
    view abstract10.1109/EuMC.2016.7824568
  • Leaky-Wave Antennas Based on Noncutoff Substrate Integrated Waveguide Supporting Beam Scanning from Backward to Forward
    Lyu, Y.-L. and Liu, X.-X. and Wang, P.-Y. and Erni, D. and Wu, Q. and Wang, C. and Kim, N.-Y. and Meng, F.-Y.
    IEEE Transactions on Antennas and Propagation 64 (2016)
    In this paper, we propose an approach to realize substrate integrated waveguide (SIW)-based leaky-wave antennas (LWAs) supporting continuous beam scanning from backward to forward above the cutoff frequency. First, through phase delay analysis, it was found that SIWs with straight transverse slots support backward and forward radiation of the-1-order mode with an open-stopband (OSB) in between. Subsequently, by introducing additional longitudinal slots as parallel components, the OSB can be suppressed, leading to continuous beam scanning at least from-40 ° through broadside to 35°. The proposed method only requires a planar structure and obtains less dispersive beam scanning compared with a composite right/left-handed (CRLH) LWA. Both simulations and measurements verify the intended beam scanning operation while verifying the underlying theory. © 2016 IEEE.
    view abstract10.1109/TAP.2016.2550054
  • Microfluidic chip architectures for a cell sorter based on the electrowetting technology
    Schreiber, F. and Kahnert, S. and Goehlich, A. and Greifendorf, D. and Bartels, F. and Janzyk, U. and Lennartz, K. and Kirstein, U. and Rennings, A. and Küppers, R. and Erni, D.
    Technisches Messen 83 (2016)
    In this contribution we present the development of various physical sorting algorithms, each of which is represented by a corresponding sorter topology for a microfluidic-based cell sorter chip using electrowetting on dielectric (EWOD) as transport mechanism. One of the main tasks of this novel cell sorter is devoted to the study of specific cell mechansims using cell selection in the research on tumor genesis with respect to e.g. the development of leukemias or lymphomas. The development of the multi-layer chip has been carried out in the framework of the EU founded (EFRE) joint research project .,MINAPSO" (Mikrochip Navigierte Parallel Sortier-Anlage) and encompasses a control chip in standard CMOS technology together with the corresponding mictrofluidic packaging. The hereby realized sorter topology consists of an optimized so-called .,2-3-Sequential-Divider-Sorter" with a numerically estimated performance (i.e. cell throughput) between 0.6 bis 1.85 cells/clock (future chip architectures such as the so-called .,Smart-Diffusion-Sorter" achieve even higher cell throughputs up to 5 cells/clock). For the modeling and optimization of the sorter architectures (aka sorter algorithms) a simulation platform has been developed and implemented in MATLAB. Here the EWOD-based droplet manipulations and subsequent operators have been represented and handled within a sort of data paradigm in order to closely conform to the essentials of digital microfluidics. To estimate the characteristic time delays and the impact of the EWOD-based operators on the overall cell throughput together with the quest for hidden optimization potentials the previous numerical logistic analysis is paralleled by extensive computational fluid dynamics (CFD) simulations using COMSOL Multiphysics as well as by corresponding droplet actuation experiments. It's worth noting that the presented cell sorter stands-to the best of our knowledge-for the currently most complex EWOD microfluidic chip. © 2016 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/teme-2015-0054
  • Modelling of electron beam induced nanowire attraction
    Bitzer, L.A. and Speich, C. and Schäfer, D. and Erni, D. and Prost, W. and Tegude, F.J. and Benson, N. and Schmechel, R.
    Journal of Applied Physics 119 (2016)
    Scanning electron microscope (SEM) induced nanowire (NW) attraction or bundling is a well known effect, which is mainly ascribed to structural or material dependent properties. However, there have also been recent reports of electron beam induced nanowire bending by SEM imaging, which is not fully explained by the current models, especially when considering the electro-dynamic interaction between NWs. In this article, we contribute to the understanding of this phenomenon, by introducing an electro-dynamic model based on capacitor and Lorentz force interaction, where the active NW bending is stimulated by an electromagnetic force between individual wires. The model includes geometrical, electrical, and mechanical NW parameters, as well as the influence of the electron beam source parameters and is validated using in-situ observations of electron beam induced GaAs nanowire (NW) bending by SEM imaging. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4945674
  • Near-field measurements and dual-Tuned matching of two CDRA versions for combined 1H/23Na 7T-MRI
    Svejda, J.T. and Erni, D. and Rennings, A.
    GeMiC 2016 - 2016 German Microwave Conference (2016)
    First near-field measurement results of two different congeneric dual resonant antennas (CDRAs) are presented in this paper. Each CDRA generates a half-wavelength current distribution at two desired resonance frequencies tailored to the magnetic resonance frequency of 1H-and 23Na-nuclei for 7 Tesla magnetic resonance imaging (MRI). Furthermore, the dual-Tuned matching and feeding networks for these antenna types are discussed and shown in this paper as well. The presented CDRAs are based on composite right/left-handed (CRLH) transmission lines terminated with open or short circuits providing two different current and field distributions between both CDRA versions. © 2016 Institut fur Mikrowellen und Antennentechnik-IMA.
    view abstract10.1109/GEMIC.2016.7461567
  • Rigorous analysis of light scattering by a grating of nanocylinders coupled to a dielectric substrate
    Jandieri, V. and Yasumoto, K. and Erni, D.
    2016 URSI International Symposium on Electromagnetic Theory, EMTS 2016 (2016)
    Light scattering by a grating made of the Ag nanocylinders onto a dielectric substrate is investigated using an accurate and rigorous formalism based on a recursive algorithm combined with the lattice sums technique. Physical insight is given to the localization of the field along the interfaces of the Ag nanocylinders, to the strong reflected field by the grating and the field enhancement at the surface of the dielectric substrate. The accuracy of the numerical analysis has been validated based on the principle of the energy conservation. © 2016 IEEE.
    view abstract10.1109/URSI-EMTS.2016.7571450
  • Scattered field leapfrog ADI-FDTD method for drude dispersive media
    Rouf, H.K. and Erni, D.
    IEEE Antennas and Wireless Propagation Letters 15 (2016)
    This letter presents a new unconditionally stable scattered field (SF) finite-difference time-domain (FDTD) scheme for Drude dispersive medium based on leapfrog alternating direction implicit (ADI) technique. The proposed scheme introduces polarization current density and analytical incident field terms in the leapfrog scheme without adding extra computational complexities except causing minor changes in the coefficients for the central spatial locations. Unconditional stability and accuracy of the scheme are validated by numerical tests. When the Courant number is set to three or a higher value, the scheme can perform faster than the explicit FDTD scheme. © 2002-2011 IEEE.
    view abstract10.1109/LAWP.2016.2517572
  • Semiperiodicity versus periodicity for ultra broadband optical absorption in thin-film solar cells
    Jalali, M. and Nadgaran, H. and Erni, D.
    Journal of Nanophotonics 10 (2016)
    We propose the use of one-dimensional semiperiodic front and back gratings based on Thue-Morse, Fibonacci, and Rudin-Shapiro (RS) binary sequences as promising photon management techniques for enhancing ultra-broadband optical absorption in thin-film solar cells. The semiperiodicity allows an aggregate light in-coupling into the active layer within the range of the solar spectrum that is less weak compared to an inherently broadband random grating, but has a much larger bandwidth than the strong in-coupling via a periodic grating configuration. The proper design procedure proposed here deviates from a canonical double grating synthesis as it adheres to an ultra-broadband design where the spectrally integrated absorption in the active material is the proper subject to optimization, leaving the grating perturbations just a measure to perturb and mold the trapped light field in the active layer accordingly. It is shown that by using a well-defined RS double grating in a 400-nm thick crystalline silicon solar cell, a 110.2% enhancement of the spectrally integrated optical absorption can be achieved relative to the reference case without grating. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/1.JNP.10.036018
  • Unconditionally stable FDTD scattered field formulation for dispersive media
    Rouf, H.K. and Erni, D.
    Microwave and Optical Technology Letters 58 (2016)
    A new unconditionally stable scattered field (SF) formulation for the finite difference time domain (FDTD) technique applied to dispersive media is presented. The scheme is based on alternating direction implicit (ADI) principle but, unlike ADI, it does not require computation and storage of field values at the intermediate time steps which reduces computational costs and memory. Dispersive media characterized by Debye and Lorentz models are incorporated in the proposed SF FDTD scheme by auxiliary differential equation (ADE) method. For each of the dispersive media unconditional stability and accuracy of the scheme are validated by numerical tests. The scheme can perform faster than the explicit FDTD scheme at Courant number as low as three. © 2016 Wiley Periodicals, Inc.
    view abstract10.1002/mop.29963
  • A Method of Using Nonidentical Resonant Coils for Frequency Splitting Elimination in Wireless Power Transfer
    Lyu, Y.-L. and Meng, F.-Y. and Yang, G.-H. and Che, B.-J. and Wu, Q. and Sun, L. and Erni, D. and Li, J.L.-W.
    IEEE Transactions on Power Electronics 30 (2015)
    In this paper, an efficient method is proposed to eliminate frequency splitting in nonradiative wireless power transfer via magnetic resonance coupling. In this method, two nonidentical resonant coils (NIRCs) are used as wireless power transmitter and receiver, respectively. According to the elliptic integral term in the analytical expression, the pole of the mutual inductance function with respect to transfer distance can be eliminated by using the two NIRCs, and hence overcoupling between transmitter and receiver with close transfer distance is avoided. Therefore, frequency splitting caused by overcoupling can be suppressed and stable output power can be achieved. The NIRCs are analytically calculated, numerically simulated and finally, fabricated and tested to verify the theory. All the calculated and experimental results show that frequency splitting is completely eliminated and uniform voltage across the load is achieved. Furthermore, lateral misalignment between the NIRCs barely introduces frequency splitting, and the suppression level of frequency splitting can also be controlled freely. © 1986-2012 IEEE.
    view abstract10.1109/TPEL.2014.2387835
  • Improved B1 distribution of an MRI RF coil element using a high-impedance-surface shield
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    2015 German Microwave Conference, GeMiC 2015 (2015)
    In this paper we propose an approach to improve the B<inf>1</inf> distribution in terms of homogeneity and penetration depth of a coil element by utilizing a high impedance surface (HIS) as the RF shield for 7 T magnetic resonance imaging (MRI). The transverse magnetic field distribution in the case of a HIS and a perfect electrical conductor (PEC) being the shielding plate are compared for different separation distances from the dipole coil to the shielding plate. As the PEC shield is adjacent to the dipole coil, an undesired surface current is induced on the PEC shielding plate by the dipole coil, whereas the induced surface current on the HIS shield is sufficiently suppressed due to the high surface impedance. As a result, the dipole coil with a HIS shield exhibits a broader and stronger field distribution, and thus achieves an improvement on the transverse B<inf>1</inf> homogeneity as well as the penetration depth. As the separation distance increases, the impact of the induced current is weakened and thus variations on the field distribution with different shielding scenarios (HIS and PEC) are reduced. The proposed approach has been validated by numerical simulations and experimental measurements, which show a good agreement. © 2015 IMA: IMATech e.V.
    view abstract10.1109/GEMIC.2015.7107765
  • Nonlinear all-optical digital amplification of light pulses based on coupled photonic crystal guiding nanostructures
    Jandieri, V. and Khomeriki, R. and Schettini, G. and Erni, D.
    Proceedings of the 2015 International Conference on Electromagnetics in Advanced Applications, ICEAA 2015 (2015)
    All-optical amplification of the light pulse in a weakly coupled nonlinear photonic crystal waveguides (PCWs) is proposed. We consider pillar-type PCWs, which consist of the periodically distributed circular rods made from a Kerr-type dielectric material. The operating frequency is properly chosen to be located at the edge of the dispersion diagram of the modes. In case of nonlinear medium when the amplitude of the injected signal is above some threshold value, solitons are formed and they are propagating inside the coupled nonlinear PCWs. Near field distributions of the light pulse and the output powers of the registered signals are studied in a detail. A good agreement between the numerical and theoretical studies is observed. © 2015 IEEE.
    view abstract10.1109/ICEAA.2015.7297073
  • A 4-channel RF coil based on a novel dipole-element with eigen-resonant shielding plate for 7-Tesla magnetic resonance imaging
    Chen, Z. and Kraff, O. and Solbach, K. and Erni, D. and Rennings, A.
    European Microwave Week 2014: Connecting the Future, EuMW 2014 - Conference Proceedings; EuMC 2014: 44th European Microwave Conference (2014)
    First imaging tests of a 4-channel RF coil using novel 41 cm-long dipole elements with an eigen-resonant shielding plate in a 7-Tesla magnetic resonance imaging (MRI) system are presented. The proposed 4-channel coil is loaded by a homogeneous phantom to model the human body. Gradient echo images on the transverse, sagittal and coronal planes are acquired in CP+ mode and compared to the more established 25 cm-long elements. Relative B<inf>1</inf>+ maps for each channel show good separation of transmit sensitivities. The proposed coil exhibits improved image homogeneity on transverse planes due to the broader field distribution, and a large longitudinal field-of-view. © 2014 European Microwave Association.
    view abstract10.1109/EuMC.2014.6986474
  • A compact electromagnetic bandgap structure based on multi-layer technology for 7-Tesla magnetic resonance imaging applications
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    European Microwave Week 2014: "Connecting the Future", EuMW 2014 - Conference Proceedings; EuMIC 2014: 9th European Microwave Integrated Circuits Conference (2014)
    A compact electromagnetic bandgap (EBG) structure based on multi-layer technology for 7-Tesla magnetic resonance imaging (MRI) applications is presented. The proposed structure introduces an additional floating metallic layer between the top surface and the ground plane of the conventional single-layer uni-planar EBG structures, which provides more capacitive coupling between the adjacent unit cells. Thus, for the desired operating frequency (300 MHz), a miniaturized dimension of the unit cell, about 2.6% of the free-space wavelength, is achieved. The proposed structure is characterized by the reflection phase and the stop-band properties. The full-wave simulations and experimental results show a good agreement. © 2014 European Microwave Association-EUMA.
    view abstract10.1109/EuMIC.2014.6997936
  • Accurate multiscale skin model suitable for determining the sensitivity and specificity of changes of skin components
    Fröhlich, J. and Huclova, S. and Beyer, C. and Erni, D.
    Computational Biophysics of the Skin (2014)
    view abstract10.4032/9789814463850
  • Coupling investigation of different RF coil elements for 7-tesla magnetic resonance imaging based on characteristic mode analysis
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE MTT-S International Microwave Symposium Digest (2014)
    Here in this paper we propose an approach to investigate the coupling mechanism of different RF coil elements based on characteristic mode analysis. The coil element with lumped-element-connection to the shielding plate focuses the magnetic field below the strip-line due to a loop current from the dominant mode. Denoted by a negative characteristic mode eigenvalue, the coil element without direct connection to the shielding plate behaves more like an electric dipole and the field is distributed mainly above the strip-line. A compromise between these two scenarios which suffers neither from the field below nor above the strip-line achieves an optimal decoupling. FDTD simulation and experimental measurement confirmed the conclusion from characteristic mode analysis. © 2014 IEEE.
    view abstract10.1109/MWSYM.2014.6848473
  • Highly confined photonic nanojet from elliptical particles
    Jalali, T. and Erni, D.
    Journal of Modern Optics 61 (2014)
    Elliptically shaped particles with different size and refractive indices have been studied under plane wave illumination using simulation tools such as 2D-FDTD, 2D-MMP, and 3D-MMP. Owing to careful manipulation, the power distribution in the vicinity of the particles opposite boundary resulted in a tightly focused photonic nanojet. Their waists are significantly smaller than the diffraction limit while propagating over several optical wavelengths without significant divergence. In this paper, we report on the manipulation of the particles elliptical shapes and the underlying refractive indices with respect to a maximally confined power distribution in the resulting nanojet which has been parameterized according to both, the beam waist and the beam divergence. The result that elliptical particles (i.e. oblate spheroids) turned out to be superior to spherical ones was underpinned within a highly accurate and fast 3D-MMP simulation using ring multipoles. © 2014 Taylor & Francis.
    view abstract10.1080/09500340.2014.920056
  • A MIM/coaxial stub-line CRLH zeroth-order series-mode resonator used as an RF coil element for 7-Tesla magnetic resonance imaging
    Rennings, A. and Svejda, J.T. and Otto, S. and Solbach, K. and Erni, D.
    IEEE MTT-S International Microwave Symposium Digest (2013)
    A novel MIM/coaxial-stub composite right/left-handed (CRLH) metamaterial transmission-line (TL) structure is proposed. In addition to the common metal-insulator-metal (MIM) series capacitor, a short-circuited coaxial line, which is vertically aligned to the otherwise planar structure, forms the shunt inductor. The zeroth-order (ZO) series mode with its spatially uniform and longitudinally polarized series current of the short-circuited CRLH TL is utilized in the context of RF field excitation in a 7-Tesla magnetic resonance imaging (MRI) scanner. The usage of coaxial stubs instead of sidewise microstrip lines has several advantages, especially for the MRI application, such as a unit cell with smaller width, the unwanted longitudinal magnetic field components excited by the shunt resonantor are shielded by the outer coaxial connector and the upper MIM plate, yielding a high Qsh-to-Q se-ratio (here 250/100), which implies an extremely uniform series current distribution. Here a 40-cm long ZO resonant element for 300 MHz is proposed. FEM eigenmode and FDTD driven-mode full-wave simulation, together with corresponding near-field measurements on fabricated prototypes are presented to comfirm the approach. © 2013 IEEE.
    view abstract10.1109/MWSYM.2013.6697615
  • Automatic Design of Broadband Gradient Index Metamaterial Lens for Gain Enhancement of Circularly Polarized Antennas
    Meng, F. Y. and Liu, R. Z. and Zhang, K. and Erni, D. and Wu, Q. and Sun, L. and Li, L. W.
    Progress in Electromagnetics Research-pier 141 (2013)
    A broadband gradient index (GRIN) metamaterial lens for gain enhancement of circularly polarized antennas has been automatically designed, fabricated and investigated. The GRIN metamaterial lens consists of an isotropic dielectric plate with a corresponding distribution of deep-subwavelength drill holes each with the same diameter. Such drill holes have a negligible influence on both the polarization state and the spectral response of the electromagnetic wave transmitting through the resulting GRIN metamaterial lens. Therefore, the GRIN metamaterial lens is polarization-insensitive and can efficiently transform spherical waves into planar waves over a very broad frequency range keeping the initial polarization states (e.g., linear or circular) scarcely changed. In the following we have derived analytical formulas that enable the setup of distribution rules for the drill holes on the plate. Based on these formulas, the GRIN metamaterial lens can be automatically designed and easily fabricated using circuit board engraving machines. The proposed GRIN metamaterial lens has been tested by placing it on the aperture of a circularly polarized conical horn antenna. The agreement between simulation and measurement results shows that the gain of the horn antenna has been significantly increased within the whole X-band (i.e., from 8 GHz to 12 GHz) and the largest gain enhancement reaches up to 5.7 dB. In particular, the axial ratio of the horn antenna with the GRIN metamaterial lens is less than 1.6 dB.
    view abstract10.2528/PIER13051104
  • Average light velocities in periodic media
    Kaspar, P. and Kappeler, R. and Erni, D. and Jäckel, H.
    Journal of the Optical Society of America B: Optical Physics 30 (2013)
    Electromagnetic Bloch modes are used to describe the field distribution of light in periodic media that cannot be adequately approximated by effective macroscopic media. These modes explicitly take into account the spatial modulation of the medium and therefore contain the full physical information at any specific location in the medium. For instance, the propagation velocity of light can be determined locally, and it is not aninvariant of space, as it is often implicitly assumed when definitions such as that of the group velocity vgr = dω/dk are used (where ω is the angular frequency and k is the Bloch index of a monochromatic mode). Spatially invariant light velocities can only be expected if the medium is assumed to show an effective behavior similar to a homogeneous material (where a plane-wave ansatz would be more appropriate). This inevitably leads to the question: what exactly is dω/dk of a Bloch mode, if it is not the group velocity? The answer is the average group velocity. This is not a trivial observation, and it has to be taken into account, for instance, when the enhancement of nonlinear effects induced by slow light is estimated. The example of a Kerr nonlinearity is studied, and we show formally that using the average group velocity can lead to an underestimation of the effect. Furthermore, this article critically reviews the concepts of energy and phase velocity. In particular, the different interpretationsof phase velocity that exist in the literature are unified using a generic definition of the quantity. © 2013 Optical Society of America.
    view abstract10.1364/JOSAB.30.002849
  • OpenEMS - A free and open source equivalent-circuit (EC) FDTD simulation platform supporting cylindrical coordinates suitable for the analysis of traveling wave MRI applications
    Liebig, T. and Rennings, A. and Held, S. and Erni, D.
    International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 26 (2013)
    In this paper, we present a free and open source platform by using the equivalent-circuit finite-difference time-domain (FDTD) method adapted to cylindrical coordinates to efficiently model cylindrically shaped objects. We will address the special characteristics of a cylindrical FDTD mesh such as the mesh singularity at r = 0 and discuss how cylindrical subgrids for small radii can reduce the simulation time considerably. Furthermore, we will demonstrate the applicability and advantages of this cylindrical equivalent-circuit FDTD method to evaluate new types of conformal ring antennas used in the context of high-field (7T) traveling wave magnetic resonance imaging (MRI). Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.
    view abstract10.1002/jnm.1875
  • Reduced Coulomb interaction in organic solar cells by the introduction of high-k SrTiO3 nanoparticles
    Benson, N. and Engel, M. and Schaefer, D. and Erni, D. and Kern, J. and Deibel, C. and Herzig, E.M. and Muller-Buschbaum, P. and Schmechel, R.
    Conference Record of the IEEE Photovoltaic Specialists Conference (2013)
    A concept is introduced which allows for reduced Coulomb interaction in organic solar cells and as such for enhanced power conversion efficiencies. The concept is based on the introduction of electrically insulating, nanostructured high-k materials into the organic matrix, which do not contribute to the charge carrier transport, however, enhance the effective permittivity of the organic active layer. Using an analytical model, it is demonstrated that even at a distance of 20 nm to the organic/inorganic interface of the nanostructure, the Coulomb interaction in the organic semiconductor can be reduced by more than 15 %. The concept is substantiated experimentally by realizing P3HT:PCBM solar cells with integrated SrTiO3 nanoparticles. It could be demonstrated that in comparison to a reference cells without nanoparticles, the power conversion efficiency is improved by ∼17 %. This effect is interpreted to be the result of an organic active layer effective permittivity enhancement, which is supported by the result of transient absorption as well as grazing incidence wide angle x-ray scattering measurements. © 2013 IEEE.
    view abstract10.1109/PVSC.2013.6745113
  • Reduced Coulomb interaction in organic solar cells by the introduction of inorganic high-k nanostructured materials
    Engel, M. and Schaefer, D. and Erni, D. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    In this paper a concept is introduced, which allows for reduced Coulomb interaction in organic solar cells and as such for enhanced power conversion efficiencies. The concept is based on the introduction of electrically insulating, nanostructured high-k materials into the organic matrix, which do not contribute to the charge transport; however, enhance the effective permittivity of the organic active layer and thereby reduce the Coulomb interaction. Using an analytical model, it is demonstrated that even at a distance of 20 nm to the organic/inorganic interface of the nanostructure, the Coulomb interaction in the organic semiconductor can be reduced by more than 15%. The concept is implemented using P3HT:PCBM solar cells with integrated high-k nanoparticles (strontium titanate). It could be demonstrated that in comparison to a reference cell without integrated nanoparticles, the power conversion efficiencies is improved by ∼17%. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228771
  • RF coil element with longitudinal and transversal two-peak field distribution for low SAR 7-Tesla magnetic resonance imaging
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE MTT-S International Microwave Symposium Digest (2013)
    In ultra high field MRI (B0 ≥ 7 T) the allowed local SAR restricts the power that can be applied to the RF coil. Here in this paper we propose an approach where the power that is absorbed by the body is better distributed, yielding a lower SARmax value, hence higher power level can be used for feeding. The optimized SAR distribution is excited by an extended dipole (41 cm-long), which is longer than the ordinary half-wavelength one, and an eigen-resoanant metal plate used for shielding purpose. At 300 MHz, the excited dipole has an approximate 3λ/2 electrical length where the maximum of current is located away from the feeding position, exciting a longitudinal two-peak SAR distribution, whereas the passive shielding plate exhibits a half-wavelength eigen-resonance, which yields a transveral two-peak SAR distribution. Compared to an established strip line element (25 cm-long), a 29% improvement of figure of merit (Hmean/√SARmax) is obtained based on simulation. © 2013 IEEE.
    view abstract10.1109/MWSYM.2013.6697434
  • Tailored RF magnetic field distribution along the bore of a 7-Tesla traveling-wave magnetic resonance imaging system
    Yang, H. and Liebig, T. and Rennings, A. and Froehlich, J. and Erni, D.
    Proceedings of the 2013 International Conference on Electromagnetics in Advanced Applications, ICEAA 2013 (2013)
    This paper provides a highly efficient method for tailoring the RF magnetic field (B1) distribution along the cylindrical bore of a high-field (7T) magnetic resonance imaging (MRI) scanner operating in the advanced traveling-wave scheme. Here the B1 wave field propagates as a circularly polarized TE11 waveguide mode and is excited, molded and dumped by a lengthwise equidistant array of thin quadrature-fed (metamaterial) ring antennas where each of them perfectly conforms to the inner surface of the cylindrical MRI bore. All individual antenna excitations associated to the desired longitudinal field profile are retrieved from an inverse problem that is efficiently solved in a (weighted) least-squares sense. The electromagnetic modeling is carried out with our equivalent-circuit (EC) FDTD simulation platform openEMS, and a convincing showcase involving a narrow illumination window for larynx diagnostics is presented. © 2013 IEEE.
    view abstract10.1109/ICEAA.2013.6632281
  • Design optimization of an electrowetting cell sorter chip platform
    Goellner, B. and Kerkhoff, D. and Michelsen, U. and Padberg, M. and Schreiber, F. and Erni, D.
    Biomedizinische Technik 57 (2012)
    In this paper fast but simple physical sorting algorithms for high-volume cell sorting based on the microfluidic electrowetting platform are analyzed and further optimized. Relying on both, hardware demonstrations of the underlying sorting operators and simulations of different generic sorter topologies, first design studies for the realization of an efficient cell sorter chip platform are carried out. © 2012 by Walter de Gruyter Berlin Boston.
    view abstract10.1515/bmt-2012-4032
  • Modelling and validation of dielectric properties of human skin in the MHz region focusing on skin layer morphology and material composition
    Huclova, S. and Erni, D. and Fröhlich, J.
    Journal of Physics D: Applied Physics 45 (2012)
    Human skin consists of several layers with distinct dielectric properties. Physiological processes leading to changes in dielectric properties of the specific layers can potentially be non-invasively monitored employing dielectric spectroscopy. So far no comprehensive skin and underlying tissue model is available for this purpose in the frequency range between 1 and 100MHz. Focusing on this dispersion-dominated frequency region, different multilayer skin models are investigated. First, with sublayers obtained from two-phase mixtures, second, three-phase mixtures of shelled cell-like ellipsoids and finally, multiphase mixtures obtained from numerical models of single cells generated using a flexible surface parametrization method. All models are numerically evaluated using the finite-element method and a fringing field sensor on the top of the multilayer system serving as a probe. Furthermore, measurements with the sensor probing skin in vivo were performed. The validity of the models was tested by removing the uppermost skin layer, the stratum corneum (SC). It was found that only a three-phase mixture (extracellular medium, cell membrane and cytoplasm) at least can qualitatively reproduce the measured dispersion still occurring without the SC if the model is set up without a priori knowledge of the dispersive behaviour as e.g. a ColeCole fit to measured data. Consequently, microstructural features of tissue have to be part of any accurate skin model in the MHz region.
    view abstract10.1088/0022-3727/45/2/025301
  • Polarization-independent metamaterial analog of electromagnetically induced transparency for a refractive-index-based sensor
    Meng, F.-Y. and Wu, Q. and Erni, D. and Wu, K. and Lee, J.-C.
    IEEE Transactions on Microwave Theory and Techniques 60 (2012)
    A polarization-independent metamaterial analog of electromagnetically induced transparency (EIT) at microwave frequencies for normal incidence and linearly polarized waves is experimentally and numerically demonstrated. The metamaterial consists of coupled bright split-ring resonators (SRRs) and dark spiral resonators (SRs) with virtually equal resonance frequencies. Normally incident plane waves with linear polarization strongly couple to the SRR, but are weakly interacting with the SR, regardless of the polarization state. A sharp transmission peak (i.e., the transparency window) with narrow spectral width and slow wave property is observed for the metamaterial at the resonant frequency of both, the bright SRR and the dark SR. The influence of the coupling strength between the SRR and SR on the frequency, width, magnitude, and quality factor of the metamaterial's transparency window is theoretically predicted by a two-particle model, and numerically validated using full-wave electromagnetic simulation. In addition, it is numerically demonstrated that the EIT-like metamaterial can be employed as a refractive-index-based sensor with a sensitivity of 77.25 mm/RIU, which means that the resonance wavelength of the sensor shifts 77.25 mm per unit change of refractive index of the surrounding medium. © 1963-2012 IEEE.
    view abstract10.1109/TMTT.2012.2209455
  • Controllable metamaterial-loaded waveguides supporting backward and forward waves
    Meng, F.-Y. and Wu, Q. and Erni, D. and Li, L.-W.
    IEEE Transactions on Antennas and Propagation 59 (2011)
    Rectangular waveguides loaded by anisotropic metamaterials are analyzed to assess the controllability of transmission characteristics of the involved electromagnetic waves. Dispersion relations of TEm0 modes in the metamaterial-loaded waveguide (MLW) are theoretically investigated. It is shown that all propagating modes (the forward wave, the backward wave and the evanescent wave) in the MLW can be realized below the cut-off frequency by changing transverse and longitudinal components of permeability tensors of the loading metamaterials. Numerical simulations are carried out to verify the proposed theory and the controllability. Transmission characteristics and effective constitutive parameters of three MLWs with different cells, which should theoretically support forward waves, backward waves and evanescent waves, respectively, are numerically calculated. Dispersion curves and magnetic field distribution for the backward wave MLW and the forward wave MLW are simulated. It is shown that the simulated results are in a good agreement with theoretical predictions. Implementation of the controllable MLW was achieved by using axially rotating control rods. Rotating the control rods can reconfigure the metamaterial and make propagating modes in the MLW switch from backward waves to forward waves or evanescent waves. © 2006 IEEE.
    view abstract10.1109/TAP.2011.2161540
  • Highly adaptive RF excitation scheme based on conformal resonant CRLH metamaterial ring antennas for 7-Tesla traveling-wave magnetic resonance imaging
    Erni, D. and Liebig, T. and Rennings, A. and Koster, N.H.L. and Frohlich, J.
    Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS (2011)
    We propose an adaptive RF antenna system for the excitation (and manipulation) of the fundamental circular waveguide mode (TE 11) in the context of high-field (7T) traveling-wave magnetic resonance imaging (MRI). The system consists of flat composite right-/left-handed (CRLH) meta-material ring antennas that fully conforms to the inner surface of the MRI bore. The specific use of CRLH metamaterials is motivated by its inherent dispersion engineering capabilities, which is needed when designing resonant ring structures for virtually any predefined diameter operating at the given Larmor frequency (i.e. 298MHz). Each functional group of the RF antenna system consists of a pair of subsequently spaced and correspondingly fed CRLH ring antennas, allowing for the unidirectional excitation of propagating, circularly polarized B 1 mode fields. The same functional group is also capable to simultaneously mold an incoming, counter-propagating mode. Given these functionalities we are proposing now a compound scheme (i.e. periodically arranged multiple antenna pairs) termed as MetaBore that is apt to provide a tailored RF power distribution as well as full wave reflection compensation virtually at any desired location along the bore. © 2011 IEEE.
    view abstract10.1109/IEMBS.2011.6090102
  • Relevance of the light line in planar photonic crystal waveguides with weak vertical confinement
    Kaspar, P. and Kappeler, R. and Erni, D. and Jäckel, H.
    Optics Express 19 (2011)
    The concept of the so-called light line is a useful tool to distinguish between guided and non-guided modes in dielectric slab waveguides. Also for more complicated structures with 2D mode confinement, the light lines can often be used to divide a dispersion diagram into a region of a non-guided continuum of modes, a region of discrete guided modes and a forbidden region, where no propagating modes can exist. However, whether or not the light line is a concept of practical relevance depends on the geometry of the structure. This fact is sometimes ignored. For instance, in the literature on photonic crystal waveguides, it is often argued that substrate-type photonic crystal waveguides with a weak vertical confinement are inherently lossy, since the entire bandgap including the line defect modes is typically located above the light line of the substrate. The purpose of this article is to illustrate that this argument is inaccurate and to provide guidelines on how an improved light line concept can be constructed. © 2011 Optical Society of America.
    view abstract10.1364/OE.19.024344
  • Modelling effective dielectric properties of materials containing diverse types of biological cells
    Huclova, S. and Erni, D. and Fröhlich, J.
    Journal of Physics D: Applied Physics 43 (2010)
    An efficient and versatile numerical method for the generation of different realistically shaped biological cells is developed. This framework is used to calculate the dielectric spectra of materials containing specific types of biological cells. For the generation of the numerical models of the cells a flexible parametrization method based on the so-called superformula is applied including the option of obtaining non-axisymmetric shapes such as box-shaped cells and even shapes corresponding to echinocytes. The dielectric spectra of effective media containing various cell morphologies are calculated focusing on the dependence of the spectral features on the cell shape. The numerical method is validated by comparing a model of spherical inclusions at a low volume fraction with the analytical solution obtained by the Maxwell-Garnett mixing formula, resulting in good agreement. Our simulation data for different cell shapes suggest that around 1MHz the effective dielectric properties of different cell shapes at different volume fractions significantly deviate from the spherical case. The most pronounced change exhibits εeff between 0.1 and 1 MHz with a deviation of up to 35% for a box-shaped cell and 15% for an echinocyte compared with the sphere at a volume fraction of 0.4. This hampers the unique interpretation of changes in cellular features measured by dielectric spectroscopy when simplified material models are used. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/43/36/365405
  • Numerical investigations of a multi-walled carbon nanotube-based multi-segmented optical antenna
    Cui, X. and Dong, L. and Zhang, W. and Wu, W. and Tang, Y. and Erni, D.
    Applied Physics B: Lasers and Optics 101 (2010)
    Motivated by the fabrication potential of multi-walled carbon nanotube structures, we numerically investigated a paired structure consisting of two metallic spheres each grown on one end of a multi-walled nanotube. The paired two-segmented structure is capable to convert free-space radiation into an intense near-field, and, hence, acting as an optical antenna. Vice versa the presence of the two nanotubes enable a current source at the antenna feed to more efficiently energy into the radiation modes, resulting e.g. in correspondingly altered luminescence lifetimes when an excited single molecule is placed in the feed point. Furthermore, the structure represents a mean to localize light on a sub-wavelength scale within different materials, which is interesting in the context of a fabrication technology for integrated nanophotonic components with different material combinations. The optical properties of the nano-antenna are analyzed by means of numerical simulations using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties The structure we propose here carries a great potential for bio-sensing, for tip-enhanced spectroscopy applications, and for interfacing integrated photonic nano circuits. © 2010 Springer-Verlag.
    view abstract10.1007/s00340-010-4220-6
  • Optical properties of a nanomatch-like plasmonic structure
    Cui, X. and Zhang, W. and Erni, D. and Dong, L.
    Journal of the Optical Society of America A: Optics and Image Science, and Vision 27 (2010)
    The optical properties of a match-like plasmonic nanostructure are numerically investigated using full-wave finite-difference time-domain analysis in conjunction with dispersive material models. This work is mainly motivated by the developed technique enabling reproducible fabrication of nanomatch structures as well as the growing applications that utilize the localized field enhancement in plasmonic nanostructures. Our research revealed that due to the pronounced field enhancement and larger resonance tunabilities, some nanomatch topologies show potentials for various applications in the field of, e.g., sensing as well as a novel scheme for highly reproducible tips in scanning near field optical microscopy, among others. Despite the additional degrees of freedom that are offered by the composite nature of the proposed nanomatch topology, the paper also reflects on a fundamental complication intrinsic to the material interfaces especially in the nanoscale: stoichiometric mixing. We conclude that the specificity in material modeling will become a significant issue in future research on functionalized composite nanostructures. © 2010 Optical Society of America.
    view abstract10.1364/JOSAA.27.001783
  • Single-shot electron bunch length measurements using a spatial electro-optical autocorrelation interferometer
    Sütterlin, D. and Erni, D. and Schlott, V. and Sigg, H. and Jäckel, H. and Murk, A.
    Review of Scientific Instruments 81 (2010)
    A spatial, electro-optical autocorrelation (EOA) interferometer using the vertically polarized lobes of coherent transition radiation (CTR) has been developed as a single-shot electron bunch length monitor at an optical beam port downstream the 100 MeV preinjector LINAC of the Swiss Light Source. This EOA monitor combines the advantages of step-scan interferometers (high temporal resolution) [D. Mihalcea, Phys. Rev. ST Accel. Beams 9, 082801 (2006) and T. Takahashi and K. Takami, Infrared Phys. Technol. 51, 363 (2008)] and terahertz-gating technologies [U. Schmidhammer, Appl. Phys. B: Lasers Opt. 94, 95 (2009) and B. Steffen, Phys. Rev. ST Accel. Beams 12, 032802 (2009)] (fast response), providing the possibility to tune the accelerator with an online bunch length diagnostics. While a proof of principle of the spatial interferometer was achieved by step-scan measurements with far-infrared detectors, the single-shot capability of the monitor has been demonstrated by electro-optical correlation of the spatial CTR interference pattern with fairly long (500 ps) neodymium-doped yttrium aluminum garnet (Nd:YAG) laser pulses in a ZnTe crystal. In single-shot operation, variations of the bunch length between 1.5 and 4 ps due to different phase settings of the LINAC bunching cavities have been measured with subpicosecond time resolution. © 2010 American Institute of Physics.
    view abstract10.1063/1.3480997
  • Sphere-on-pillar optical nano-antennas
    Cui, X. and Fan, Z. and Tao, X. and Zhang, W. and Erni, D. and Fan, X. and Zhang, X. and Dong, L.
    2010 IEEE Nanotechnology Materials and Devices Conference, NMDC2010 (2010)
    We propose an optical nano-antenna consisting of a pair of sphere-on-pillar structures. Experiments show that the controlled fabrication of metallic nanospheres on the tip of carbon nanotubes (CNTs) is effective, and numerical investigation revealed that a pair of such structures are capable to convert free space radiation into an intense near-field; hence can function as an optical antenna. The fabrication process, electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB), are based on nanofluidic mass delivery at the attogram scale using metal-filled CNTs. Under the irradiation of a high energy electron beam of a transmission electron microscope (TEM), the encapsulated metal is melted and extruded out from the tip of the nanotube; generating a metallic sphere. In the case that the encapsulated materials inside the CNT have a higher melting point than that of the beam energy can reach, electromigration-based mass delivery is an optional process to apply. Under a low bias (2-2.5V), spherical nanoparticles are formed on the tips of nanotubes. The optical properties of the nano-antenna are analyzed numerically using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties. Sphere-on-pillar optical antennas carry a great potential for bio-sensing, tip-enhanced spectroscopy applications, and interfacing integrated nanophotonic circuits. © 2010 IEEE.
    view abstract10.1109/NMDC.2010.5652241
  • The influence of particle shapes on the optical response of nearly touching plasmonic nanoparticle dimers
    Cui, X. and Erni, D.
    Journal of Computational and Theoretical Nanoscience 7 (2010)
    In this paper we provide a systematic analysis of the optical properties of different nanoscopic dimer structures with relatively small gap distances. In particular, we have focused on two different aspects, namely the feasibility of functionalizing optical nanodimers while analyzing the influence of different particle shapes with respect to the proper contact regions, as well as the impact of potential fabrication imperfections. Both scenarios-functionalization and perturbation-are rooted in the significant variations of the nanodimer's optical properties, such as a dramatically altered field enhancement, together with a significant shift in the resonance wavelength. Referring to a state-of-the-art nanoprocessing technology we have forecasted in our outlook that emergent changes in the chemical composition especially of the metallic part will add a further dimension to the uncertainties that have to be faced in functional nanoparticle desing. Hence, the proper determination of the nanoparticle's shape and the corresponding material properties may become constitutive in any design and optimization procedure of functional plasmonic nanostructures. Copyright © 2010 American Scientific Publishers. All right reserved.
    view abstract10.1166/jctn.2010.1525
  • dielectric materials

  • electromagnetic and optical metamaterials

  • finite element method

  • modelling and simulation

  • nanoelectromagnetics

  • nanomaterials

  • nanophotonics

  • numerical structural optimization

  • optical properties

  • photonics

  • Plasmonics

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