Browsing by Author "Wienold, Martin"

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    Development of terahertz quantum-cascade lasers as sources for heterodyne receivers
    (Berlin : Humboldt-Universität zu Berlin, 2012) Wienold, Martin
    This thesis presents the development and optimization of terahertz quantum-cascade lasers (THz QCLs) as sources for heterodyne receivers. A particular focus is on single-mode emitters for the heterodyne detection of the important astronomic oxygen (OI) line at 4.75 THz. Various active-region designs are investigated. High-output-power THz QCLs with low operating voltages and emission around 3 THz are obtained for an active region, which involves phonon-assisted intersubband transitions. While these QCLs are based on a GaAs/Al_xGa_(1-x)As heterostructure with x=0.15, similar heterostructures with x=0.25 allowed for very low threshold current densities. By successive modifications of the active-region design, THz QCLs have been optimized toward the desired frequency at 4.75 THz. To obtain single-mode operation, first-order lateral distributed-feedback (DFB) gratings are investigated. It shows that such gratings allow for single-mode operation in combination with high continuous-wave (cw) output powers. A general method is presented to calculate the coupling coefficients of lateral gratings. In conjunction with this method, the lasers are well described by the coupled-mode theory of DFB lasers with two reflective end facets. Single-mode operation within the specified frequency bands at 4.75 THz is demonstrated. Stable operation of THz QCLs is often in conflict with the occurrence of a negative differential resistance (NDR) regime at elevated field strengths and the formation of electric-field domains (EFDs). Stationary EFDs are shown to be related to discontinuities in the cw light-current-voltage characteristics, while non-stationary EFDs are related to current self-oscillations and cause a temporal modulation of the output power. To model such effects, the nonlinear transport equations of weakly coupled superlattices are adopted for QCLs by introducing an effective drift velocity-field relation. Zugriffsstatistik:
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    High-Performance GaAs/AlAs Terahertz Quantum-Cascade Lasers for Spectroscopic Applications
    (New York, NY : IEEE, 2020) Schrottke, Lutz; Lü, Xiang; Röben, Benjamin; Biermann, Klaus; Hagelschuer, Till; Wienold, Martin; Hübers, Heinz-Wilhelm; Hannemann, Mario; van Helden, Jean-Pierre H.; Röpcke, Jürgen; Grahn, Holger T.
    We have developed terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures for application-defined emission frequencies between 3.4 and 5.0 THz. Due to their narrow line width and rather large intrinsic tuning range, these THz QCLs can be used as local oscillators in airborne or satellite-based astronomical instruments or as radiation sources for high-resolution absorption spectroscopy, which is expected to allow for a quantitative determination of the density of atoms and ions in plasma processes. The GaAs/AlAs THz QCLs can be operated in mechanical cryocoolers and even in miniature cryocoolers due to the comparatively high wall-plug efficiency of around 0.2% and typical current densities below 500 A/cm$^2$. These lasers emit output powers of more than 1 mW at operating temperatures up to about 70 K, which is sufficient for most of the abovementioned applications. © 2011-2012 IEEE.
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    High-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-Wilhelm
    We 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.
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    Real-time gas sensing based on optical feedback in a terahertz quantum-cascade laser
    (Washington, DC : Optical Society of America, 2017) Hagelschuer, Till; Wienold, Martin; Richter, Heiko; Schrottke, Lutz; Grahn, Holger T.; Hübers, Heinz-Wilhelm
    We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.