High-resolution terahertz semiconductor spectroscopy using quantum-cascade lasers: Spectros-copy of impurity transitions in Ge and Si
Final Report Abstract
Due to the close analogy with free atoms, atomic impurities in high-purity semiconductors have seen again an increasing attention in the past years. For quantum information and quantum computing as well as laser applications, a detailed knowledge about the relaxation mechanisms and lifetimes of impurity transitions is required. Most of the present results are obtained by time-domain measurements based on ultra-short and powerful laser pulses. To obtain complementary results in the frequency domain, a low-noise, high-resolution spectroscopic technique is required, which exceeds the capabilities of conventional Fouriertransform spectrometers. The aim of the project was the development of a spectrometer for high-resolution laser spectroscopy of semiconductors at terahertz (THz) frequencies based on narrow-line-width quantum-cascade lasers (QCLs) as well as the investigation of the linewidth, line shape, and the relaxation time of impurity states in Ge and Si. In a first part of the project, the focus was on the development of a spectrometer for high-resolution spectroscopy based on multi-mode QCLs, since such lasers typically combine a large total bandwidth with a high spectral purity of the individual laser modes. For frequency tuning, the driving current and the operating temperature of the QCL are varied. The performance of the spectrometer was confirmed by high-resolution molecular spectroscopy. In order to realize high spectral resolution, we studied effects related to external optical feedback in QCLs and nonlinear spectroscopic techniques. This work resulted in the demonstration of molecular spectroscopy based on laser self-detection and the demonstration of Doppler-free spectroscopy with THz QCLs. In the context of these results, we were able to confirm that the linewidth of continuous-wave THz QCLs is less than 1 MHz even without active frequency stabilization provided that the QCL is properly implemented in the spectrometer. One difficulty arose from the limited frequency tuning ranges of the individual laser modes resulting in spectral gaps. Although the spectrometer is operational with high sensitivity and high spectral resolution, we could not perform the intended semiconductor spectroscopy due to the insufficient frequency coverage for impurity transitions. In order to address the physical problem of the linewidths and lifetimes of semiconductor impurity transitions in the frequency domain, we replaced the QCL with a continuous-wave (cw) photomixer source. This source provides cw emission up to approximately 3 THz with a slightly reduced linewidth of 10 MHz compared to a QCL. A Si bolometer was used as a detector. With this technique, we were able to separate the contributions of inhomogeneous broadening and homogeneous broadening to the total linewidth. The obtained results are in excellent agreement with the lifetimes determined from time domain pumpprobe measurements at a free-electron laser.
Publications
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Fast continuous tuning of terahertz quantum-cascade lasers by rear-facet illumination, Appl. Phys. Lett. 108, 191106, 5 pages (2016)
M. Hempel, B. Röben, L. Schrottke, H.-W. Hübers, and H. T. Grahn
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Terahertz gas spectroscopy through self-mixing in a quantum-cascade laser, Appl. Phys. Lett. 109, 191101, 5 pages (2016)
T. Hagelschuer, M. Wienold, H. Richter, L. Schrottke, K. Biermann, H. T. Grahn, and H.-W. Hübers
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High-resolution terahertz gas-phase spectroscopy based on external optical feedback in a quantum cascade laser, 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Cancun, Mexiko (2017)
T. Hagelschuer, M. Wienold, H. Richter, N. Rothbart, and H.-W. Hübers
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Lamb dip spectroscopy with a terahertz quantum-cascade laser, 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Cancun, Mexiko (2017)
M. Wienold, T. Alam, L. Schrottke, H. T. Grahn, and H.-W. Hübers
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Real-time gas sensing based on optical feedback in a terahertz quantum-cascade laser, Opt. Express 25, 30203‒30213 (2017)
T. Hagelschuer, M. Wienold, H. Richter, L. Schrottke, H. T. Grahn, and H.-W. Hübers
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Doppler-free spectroscopy with a terahertz quantum-cascade laser, Opt. Express 26, 6692‒6699 (2018)
M. Wienold, T. Alam, L. Schrottke, H. T. Grahn, and H.-W. Hübers
