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- ItemHigh-spectral-resolution terahertz imaging with a quantum-cascade laser(Washington, DC : Optical Society of America, 2016) Hagelschuer, Till; Rothbart, Nick; Richter, Heiko; Wienold, Martin; Schrottke, Lutz; Grahn, Holger T.; Hübers, Heinz-WilhelmWe report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.
- ItemA compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler(Washington, DC : Optical Society of America, 2010) Richter, H.; Greiner-Bär, M.; Pavlov, S.G.; Semenov, A.D.; Wienold, M.; Schrottke, L.; Giehler, M.; Hey, R.; Grahn, H.T.; Hübers, H.-W.We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.
- ItemTerahertz quantum-cascade lasers as high-power and wideband, gapless sources for spectroscopy(Washington, DC : Optical Society of America, 2017) Röben, Benjamin; Lü, Xiang; Hempel, Martin; Biermann, Klaus; Schrottke, Lutz; Grahn, Holger T.Terahertz (THz) quantum-cascade lasers (QCLs) are powerful radiation sources for high-resolution and high-sensitivity spectroscopy with a discrete spectrum between 2 and 5 THz as well as a continuous coverage of several GHz. However, for many applications, a radiation source with a continuous coverage of a substantially larger frequency range is required. We employed a multi-mode THz QCL operated with a fast ramped injection current, which leads to a collective tuning of equally-spaced Fabry-Pérot laser modes exceeding their separation. A continuous coverage over 72 GHz at about 4.7 THz was achieved. We demonstrate that the QCL is superior to conventional sources used in Fourier transform infrared spectroscopy in terms of the signal-to-noise ratio as well as the dynamic range by one to two orders of magnitude. Our results pave the way for versatile THz spectroscopic systems with unprecedented resolution and sensitivity across a wide frequency range.
- ItemFrequency modulation spectroscopy with a THz quantum-cascade laser(Washington, DC : Optical Society of America, 2013) Eichholz, R.; Richter, H.; Wienold, M.; Schrottke, L.; Hey, R.; Grahn, H.T.; Hübers, H.-W.We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.
- ItemDoppler-free spectroscopy with a terahertz quantum-cascade laser(Washington, DC : Optical Society of America, 2018) Wienold, M.; Alam, T.; Schrottke, L.; Grahn, H.T.; Hübers, H.-W.We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.
- ItemTerahertz quantum-cascade lasers for spectroscopic applications(Berlin : Technische Universität Berlin, 2018) Röben, Benjamin Malte; Grahn, Holger T.Terahertz (THz) quantum-cascade lasers (QCLs) are unipolar semiconductor heterostructure lasers that emit in the far-infrared spectral range. They are very attractive radiation sources for spectroscopy, since they are very compact and exhibit typical output powers of severalmWas well as linewidths in the MHz to kHz range. This thesis presents the development of methods to tailor the emission characteristics of THz QCLs and employ them for spectroscopy with highest resolution and sensitivity. In many cases, these spectroscopic applications require that the far-field distribution of the THz QCLs exhibits only a single lobe. However, multiple lobes in the far-field distribution of THz QCLs were experimentally observed, which were unambiguously attributed to the typically employed mounting geometry and to the cryogenic operation environment such as the optical window. Based on these results, a method to obtain a single-lobed far-field distribution is demonstrated. A critical requirement to employ a THz QCL for high-resolution spectroscopy of a single absorption or emission line is the precise control of its emission frequency. This long-standing problem is solved by a newly developed technique relying on the mechanical polishing of the front facet. A QCL fabricated in this manner allows for spectroscopy at a maximal resolution in the MHz to kHz range, but its accessible bandwidth is usually limited to a few GHz. In contrast, a newly developed method to utilize QCLs as sources for THz Fourier transform spectrometers enables highly sensitive spectroscopy over a significantly larger bandwidth of at least 72 GHz with a maximal resolution of typically 100 MHz. The application of QCLs as sources for THz Fourier transform spectroscopy leads to a signal-to-noise ratio and dynamic range that is substantially increased by a factor of 10 to 100 as compared to conventional sources. The results presented in this thesis pave the way to routinely employ THz QCLs for spectroscopic applications in the near future.