Regensburg 2010 – scientific programme

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DS: Fachverband Dünne Schichten

DS 1: Nanophotonics - Devices I (Focused Session together with HL)

DS 1.3: Topical Talk

Monday, March 22, 2010, 11:15–11:45, H2

Nanostructures for Novel Quantum Cascade Structures — •K. Unterrainer, W. Parz, T. Moldaschl, A. Benz, G. Fasching, A.M. Andrews, and G. Strasser — Photonik Institut und Zentrum für Mikro&Nanostrukturen, Technische Universität Wien, A-1040 Wien

In this contribution we will discuss carrier relaxation in quantum wells and quantum dots and its importance for THz quantum cascade lasers (QCLs). THz QCLs have shown rapid improvements of their emission power and wavelength control. However, the operation temperature is still limited to cryogenic temperatures. All attempts to overcome this limitation involve the control of carrier relaxation. Optical phonon relaxation together with other elastic processes determine the non-radiative recombination. We use time-resolved near-infrared pump and THz probe experiments to study carrier dynamics and find that carrier-carrier scattering causes fast relaxation in quantum wells. The subsequent reduction of the doping concentration in QCLs has led to considerably reduced threshold currents. However, the temperature performance has only increased marginal. A further reduction of scattering is only possible by reducing the phase space. As a first step, we have applied a strong magnetic field causing in-plane quantization. The emission intensity of THz QCLs as function of the applied magnetic field shows an increase of the emission power and a decrease of the threshold. Thus, quantum dots should improve the performance of QCLs significantly. We investigate carrier relaxation between sub-levels and study the design requirements for quantum dot cascade structures.