Linear and non-linear spectroscopy of semiconductors using synchrotron infrared

Abstract

This thesis deals with linear and non-linear optical gain of quantum cascade lasers (QCLs) and the development of unique spectroscopic tools at the infrared (IR) beamline X01DC of the Swiss Light Source (SLS) synchrotron. These tools consist of (i) an ultra-broadband pumpprobe system, and (ii) a diffraction-limited micro-spectroscopy transmission setup. These tools enable the investigation of the IR response of condensed matter systems and devices under conditions of strong electrical and/or optical excitation. It is shown that infrared synchrotron sources complement conventional infrared sources by offering a high-brilliance, broadband infrared spectrum and pulsed time-structure. The first part of this thesis focusses on all-optical pump-probe experiments. A pulse of synchrotron infrared with a length of 100ps overlaps on the samples with a broadly tuneable pump pulse generated by non-linear generation from an Nd:YAG laser. This setup enables the investigation of dynamic processes down to the 100 ps limit. As a showcase, the optical gain and losses in Germanium-on-Silicon layers are investigated. A discussion is given on the possibility to realise a laser on Si using strained and doped Ge layers. Also, a brief introduction is given on the time-resolved investigation of photocatalytic processes using the here introduced pump-probe tool. The second part of the thesis deals with the characterisation of QCLs both in the linear and the non-linear regime. QCLs are based on quantum wells and emit light in the midand far-Infrared. Their structure allows to design the spectral properties of the emission with a high degree of flexibility and tuneability. Light generation results from optical intersubband transitions between electron states in the conduction band of the semiconductor heterostructure. Because such laser devices contain single-mode waveguides (typically