Abstract-Imaging and Spectroscopic Sensing with Low-Repetition-Rate Terahertz Pulses and GaN TeraFET Detectors

• Daniel VoßEmail author
• Wissem Zouaghi
• Mehran Jamshidifar
• Sebastian Boppel
• Cormac McDonnell
• James R. P. Bain
• Nils Hempler
• Graeme P. A. Malcolm
• Gareth T. Maker
• Maris Bauer
• Alvydas Lisauskas
• Adam Rämer
• Sergey A. Shevchenko
• Wolfgang Heinrich
• Viktor Krozer
• Hartmut G. Roskos

https://link.springer.com/article/10.1007%2Fs10762-017-0447-1

Aiming for non-destructive testing and security applications, we investigate transmission-mode imaging and spectroscopic sensing using terahertz (THz) pulses from a commercial optical parametric oscillator (OPO) in combination with THz detectors based on antenna-coupled field-effect transistors (TeraFETs). The Q-switched OPO generates quasi-continuous-wave THz pulses with a peak power of up to 1 W at a repetition rate between 12 and 90 Hz. The pulses are frequency-tunable between 0.7 and 2.6 THz with a typical linewidth of 50 GHz. We explore detection with fast GaN/AlGaN TeraFETs which hold the potential for multi-pixel and homodyne detection.

Abstract-Nonlinear spectroscopy of impurity states in semiconductors driven by intense THz fields

 Fanqi Meng,  Mark D. Thomson,  Hartmut G. Roskos

http://ieeexplore.ieee.org/document/8066892/

We present an experimental investigation of the nonlinear response of acceptor impurities in semiconductors to coherent excitation with intense THz fields. In Zn-doped GaAs, we observed a well-defined saturation of the transition from the impurity ground state to the excited states and valence band. In B-doped Si, however, there is a clear indication of field-induced distortion of the impurity energy states: in the highly B-doped sample, an absorption band due to boron-carbon pairs (boron-X center) emerges and shows strong field-induced broadening and splitting. Similar effects were also observed in the substitutional boron impurity states at lower doping density

Abstract-Efficient Detection of 3 THz Radiation from Quantum Cascade Laser Using Silicon CMOS Detectors

Kęstutis Ikamas, Alvydas Lisauskas, Sebastian Boppel, Qing Hu, Hartmut G. Roskos

https://link.springer.com/article/10.1007/s10762-017-0407-9

In this paper, we report on efficient detection of the radiation emitted by a THz quantum cascade laser (QCL) using an antenna-coupled field effect transistor (TeraFET). In the limiting case when all radiated power would be collected, the investigated TeraFET can show up to 230 V/W responsivity with the noise equivalent power being as low as 85 pW/Hz−−−√$\sqrt{\text{Hz}}$ at 3.1 THz, which is several times lower than that of the typical Golay cell. A combination of the QCL and a set of off-axis parabolic mirrors with 3-inch and 2-inch focal lengths was used to measure the signal-to-noise ratio (SNR) of the TeraFET. The practically achieved SNR was five times lower than that of the Golay cell and two orders of magnitude lower than a bolometer’s. However, TeraFETs are much faster and do not need a signal modulation, thus can be used both in a continuous mode for power monitoring or for investigation of transient processes on a sub-microsecond time scale.

Abstract-Relativistic Doppler frequency up-conversion of terahertz pulses via reflection from photo-induced plasma fronts in solid-state media

Mark D. Thomson; Fanqi Meng; Hartmut G. Roskos

http://spie.org/Publications/Proceedings/Paper/10.1117/12.2186752

We have recently proposed and investigated the use of the relativistic Doppler reflection to up-shift the frequency of incident THz pulses, where the reflecting boundary is realized by a charge-carrier plasma front generated by a counter-propagating optical pump pulse in a semiconductor medium. In light of experimental results with high-resistivity silicon as the medium, here we employ numerical simulations to examine the effects of (i) the scattering time and (ii) pre-excitation of the plasma before the main pulse, which both can have a profound impact on the frequency up-conversion. These results also suggest that the initial effective Drude scattering time in silicon (before thermalization) may be below 10 femtoseconds, exemplifying the use of the Doppler reflective geometry as a novel probe of initial charge-carrier dynamics.

Abstract-Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe

Fanqi Meng, Mark D. Thomson, Bo E. Sernelius, Michael Jörger, and Hartmut G. Roskos
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.075201

We employ ultrabroadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (1015–1019cm−3) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of ∼200fs at low densities, reaching ∼20fs for densities of ∼1019cm−3, where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial subpicosecond temporal development of the Drude response and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.

DOI: http://dx.doi.org/10.1103/PhysRevB.91.075201

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• Published 4 February 2015
• Received 25 November 2014

©2015 American Physical Society

Abstract-Terahertz frequency upconversion via relativistic Doppler reflection from a photoinduced plasma front in a solid-state medium

http://prb.aps.org/abstract/PRB/v87/i8/e085203

Mark D. Thomson, Slava M. Tzanova, and Hartmut G. Roskos 
Physikalisches Institut, J. W. Goethe-Universität, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany

Received 15 October 2012; published 15 February 2013

We propose and simulate the relativistic Doppler reflection of terahertz (THz) radiation normally incident on a plasma front in a semiconductor medium, resulting in a significant frequency upshift for the reflected radiation. The plasma front is generated by linear interband excitation of the semiconductor by a counterpropagating femtosecond optical pulse. High-resistivity silicon is identified as an ideal medium for experiments, as it possesses a desirable optical penetration depth, upshift factor, and low THz absorption and dispersion. The depletion of the optical pump pulse results in a spatiotemporal plasma profile, which leads to results that go beyond the existing analytic theory. We employ one-dimensional finite-difference time-domain simulations to predict the reflected THz pulses vs a range of realistic experimental parameters. The results indicate that a significant frequency upshift can be expected for both conventional and ultrabroadband THz pulses, and that this technique may be suitable to provide higher-bandwidth THz radiation extending into the midinfrared.

©2013 American Physical Society

URL:

http://link.aps.org/doi/10.1103/PhysRevB.87.085203

DOI:

10.1103/PhysRevB.87.085203

PACS:

42.65.Re, 78.47.J-, 42.72.Ai, 78.20.Bh