Design and Simulation of THz Quantum Cascade Lasers

Strategies and concepts for the design of THz emitters based on the quantum cascade scheme are analyzed and modeled in terms of a fully three-dimensional Monte Carlo approach; this allows for a proper inclusion of both carrier-carrier and carrier-phonon scattering mechanisms. Starting from the simulation of previously published far-infrared emitters, where no population inversion is achieved, two innovative designs are proposed. The first one follows the well-established chirped-superlattice scheme whereas the second one employs a double-quantum well superlattice to allow energy relaxation through optical phonon emission. For both cases a significant population inversion is predicted at temperatures up to 80 K.Comment: 4 pages, 2 figures, 2 table

Electron density stratification in two-dimensional structures tuned by electric field

A new kinetic instability which results in formation of charge density waves is proposed. The instability is of a purely classical nature. A spatial period of arising space-charge and field configuration is inversely proportional to electric field and can be tuned by applied voltage. The instability has no interpretation in the framework of traditional hydrodynamic approach, since it arises from modulation of an electron distribution function both in coordinate and energy spaces. The phenomenon can be observed in thin 2D nanostructures at relatively low electron density.Comment: 4 pages, 2 figure

Spectral characterization of a 2.5 THz multi-mode quantum cascade laser

Recently, terahertz-range semiconductor quantum cascade lasers (QCL) have attracted attention as potential local oscillators in heterodyne receivers. The basic characterization of the lasers, such as beam profiles, frequency and power stability as well as temperature and current-related frequency tunability has been carried out. Precise spectral analysis is required if the device is going to be used for applications requiring very accurate absolute frequency, spectral purity and fine frequency tunability, such as in heterodyne spectroscopy. By combination of Fourier transform spectroscopy with heterodyne and homodyne mixing measurements it is possible to create a complete picture of the QCL spectral output. We will report on the characterization of a 2.5 THz multi-mode QCL with respect to mechanisms determining the laser emission frequency and mode spectrum. The laser mechanism is based on a bound-to-continuum laser design. Laser emission spectra at different drive currents and heat sink temperatures have been measured by homodyne (QCL-QCL) and heterodyne (QCL-THz gas laser) mixing experiments. The analysis shows that variations of the laser frequency and modifications of the mode spectra are caused by fast processes which apparently control the gain of the device. The emission frequencies of the QCL based on a bound-to-continuum design can be described by a formalism similar to a Fabry-Perot-type cavity with additional corrections, depending on the QCL frequency and drive current. The detailed study of the spectral characteristics provides basic data about the direction, magnitude, and range of the frequency tunability of QCLs of this family/design. The data are necessary for implementation of the laser in a THz heterodyne receiver for example on board of SOFIA

Terahertz-Heterodynempfänger mit Quantenkaskadenlaser und supraleitendem Mischer

Die hochauflösende Spektroskopie atomarer und molekularer Emissions- und Absorptionslinien im Terahertz (THz)-Spektralbereich liefert Informationen über den stellaren-interstellaren Lebenszyklus (z. B. Fragen der Sternentstehung), chemische Prozesse im inter-stellaren Medium (ISM), Planetenatmosphären und die Gaskoma von Kometen. Eine hinreichend gute spektrale Auflösung (Δν/ν » 10-6) bei gleichzeitig hoher Detektionsempfindlich-keit ist nur mit einem Heterodynspektrometer realisierbar. Diese Eigenschaften des THz-Heterodynspektrometers eröffnen auch Anwendungen in der Sicherheitstechnik, bei denen verborgene gefährliche Objekte oder Substanzen in mehreren Metern Entfernung detektiert werden sollen. Wichtige Bestandteile eines Heterodynspektrometers sind der Lokaloszillator und der Mischer. Der Lokaloszillator muss in der Lage sein, den Mischer mit ausreichend Leistung zu versorgen, sowie eine hinreichend gute Frequenzstabilität besitzen. Die Anforderungen an den Mischer sind zum einen ein niedriges Eigenrauschen und zum anderen eine möglichst hohe Konversionseffizienz. Wir berichten über ein THz-Heterodynspektrometer mit einem Quan-tenkaskadenlaser (QCL) als Lokaloszillator sowie einem Hot Electron Bolometer (HEB) als Mischer. Die Ausgangsleistung des Quantenkaskadenlasers betrug mehrere Milliwatt und war ausreichend, um das HEB (Rauschtemperatur ca. 2700 K) mit genügend Pumpleistung zu versorgen. Die Kombination aus supraleitendem HEB und QCL Lokaloszillator eröffnet die Möglichkeit ein kompaktes und robustes THz – Heterodynspektrometer für Anwendungen in der Astronomie oder Sicherheitstechnik zu realisiere

Terahertz semiconductor-heterostructure laser

Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10^12 Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications—for example, chemical detection, astronomy and medical imaging[1, 2, 3, 4]. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources[5, 6, 7, 8, 9]. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm-2 up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems