Non-Linear Terahertz Spectroscopy of Quantum Materials

Linear THz spectroscopy has become well developed in recent years, utilising techniques such as FTIR spectroscopy and TDS to acquire material characteristics and study physical phenomena. Recently, owing to advances in high field THz sources, nonlinear responses in materials can now be studied, such as coherent and incoherent material processes, transport dynamics in graphene and biological processes. These measurements are often performed using a FEL as they generate monochromatic radiation, tuneable between 1-100 THz with fields in the region of 100 MV/cm. These are exclusively contained in large, costly facilities, restricting the availability and practicality of the technology. There is therefore much appeal in the development of tabletop systems to perform these experiments, improving accessibility.

In this thesis, the development of a 2D THz TDS experimental system utilising a tabletop amplified laser as the source is discussed. The system can generate fields greater than 100 kV/cm, sufficient for inducing nonlinear effects within certain samples. This field is spread over a broad bandwidth (~0.1-5 THz), allowing for multiple spectral features to be acquired simultaneously. The system uses both photoconductive arrays and electro-optic crystals to generate the high field, broad bandwidth pulses and investigations of both types of sources are presented in this thesis.

Also presented in this work are measurements of two samples using the system developed, the first being a doped semiconductor impurity centre (germanium doped with arsenic) and the other being a semiconductor quantum well structure designed to exhibit the self-induced-transparency effect. These measurements obtained values of the transition polarisation lifetimes (T2) directly on the few ps timescales. Knowledge of this lifetime is important for performing coherent control experiments and is difficult to measure with other spectroscopy techniques.