Time-Resolved and Non-Linear Studies of Terahertz Quantum Cascade Lasers
9th January 2017, PHYSIKALISCHES KOLLOQUIUM, Hörsaal H-NB, Ruhr-Universität Bochum
Start: 9th January 2017 12:00
End: 9th January 2017 01:00 p.m.
Dr. Nathan Jukam, Ruhr-Universität Bochum
Prof. Dr. A. D. Wieck, Ruhr-Universität Bochum
The terahertz (THz) frequency range has been historically under-developed due to a lack of practical sources. This is despite the fact that THz radiation can be used in many applications such as nondestructive testing, detection of illicit substances, and pharma-ceutical inspection. Furthermore, THz sources are of interest for fundamental science since many rotational and vibrational molecular transitions, as well as collective excitations in solid state systems, occur at THz frequencies. Due to their small size and potential for mass fabrication, quantum cascade lasers (QCLs) are promising sources of THz radiation. In contrast to interband laser diodes, in QCLs the laser transition takes place between intersubband levels. Since their realization  THz QCLs have shown considerable improvement in threshold current, operating temperature, power output, and low frequency operation.
In the first half of this talk, I give an overview of how THz QCLs function. In the second half of the talk, I describe research activities at the Ruhr-Universität Bochum involving THz QCLs. First, I describe how injection seeding can be used to control the spectral emission of THz QCLs. The emission from QCLs is typically multi-mode. The spectral emission of a multi-mode laser can be controlled by injecting a tunable seed pulse into the laser cavity before the laser is turned on. Narrow-band THz seeds are generated by optical rectification of a femtosecond laser in a periodically poled lithium niobate (PPLN) crystal and are used to injection seed a THz QCL . In addition, injection seeding with THz pulses whose bandwidth is much greater than the laser’s gain bandwidth is also investigated. For broad-band injection seeding, a single seed pulse has no effect on the seeded laser emission, since all longitudinal modes are simultaneously selected. However, double pulse injection seeding  can systematically modify the QCL spectra when the time delay between the two pulses is varied. Lastly, I discuss recent non-linear measurement on THz QCLs. By injecting two coherent THz pulses with a variable time-delay into a QCL, two-dimensional spectroscopy is performed. This permits the observation of a 2nd, 3rd, and 4th order non-linear response in the 2D spectra of the QCL. Sum frequency and second harmonic generation are directly detected in the emission spectra of a THz QCL. Both sum frequency generation and second harmonic generation are enhanced when a novel THz waveguide is used to enhance the internal laser field at the QCL facet.