Coherent THz-emission from current-driven plasma instabilities

Abstract

The terahertz regime is still relatively unexplored in terms of sources and detectors. Research approaches either from the high frequency electronics side, or from the long wavelength end of photonics. The operation frequency of conventional electronic oscillators is limited by the time constant, whereas optoelectronic devices are limited by the thermal background. A novel way to generate THz-radiation is to use coherent microcharge oscillations. The many-body nature of this process could make it less sensitive to temperature.<br />Collective excitations of electrons in bounded quantum systems can lead under certain conditions to generation of a stimulated plasma instability. In a three-level quantum well structure, this can be achieved by tuning two intersubband plasmon modes via depolarization shifts into resonance. In this work, an advanced three-level structure consisting of an InGaAs/AlAs quantum well with a resonant tunneling diode, acting as an energy filter for injection or extraction, is used.<br />With this structure, a crossing of two intersubband plasmons is realized. The subject of this thesis is to get experimental evidence for emission of THz-radiation from current-driven plasma instabilities. For the optimization of the quantum levels and transitions of these heterostructures, a self-consistent Schrödinger-Poisson solver is used.<br />Samples with 1, 10 and 20 cascades are realized. The heterostructures are grown on GaAs-wafers by molecular beam epitaxy, and the compositions and thicknesses are controlled by high-resolution X-ray diraction scans. Device fabrication, using an emission-optimized mask layout with square mesas, is done with standard optical lithography, reactive ion etching and metal deposition using plasma sputtering or e-beam evaporation.<br />The samples are cooled down to temperatures of liquid helium and characterized both, electrically and optically. In current-voltage measurements, a steep increase of the current is observed. This is a result of an attractive crossing of an absorption mode with an emission mode. For optical characterization, many devices are connected in parallel to enhance the total emission. A Fourier-Transform infrared spectrometer in step scan mode together with a bolometer detector is used to record the emitted radiation. Narrowband emission of THz-radiation at 3.4 THz is observed from the 10-periodic intersubband plasmon emitter. While the frequency is independent of the bias, the intensity of the emitted narrowband radiation shows a clear resonance peak, which is also somewhat weaker observed in the linewidth. The linewidth of the measured peak is considerably narrower than it has been previously observed from spontaneous intersubband transitions. From this we conclude that an inversion is present in the sample, which leads to the line narrowing.