Electrodynamics of Topological Insulator Thin Films Revealed by Time-Domain THz Spectroscopy

This PhD thesis uses time-domain THz spectroscopy to study the electrodynamics of topological insulator thin films. These materials are insulating in the bulk but conduct along their surfaces through spin-polarized Dirac states. First, it develops a high-sensitivity THz polarimetry set-up, using a fibre-coupled THz spectrometer, high-extinction-ratio wire-grid polarizers and a thorough calibration scheme, reaching a polarization precision of 0.02 milliradians. Applied to magnetically doped topological insulators, the THz Hall conductance hysteresis loops agree qualitatively with dc transport, but the frequency dependence contradicts models with a clean magnetic gap. The thesis explains this with a spatially inhomogeneous magnetic gap, likely caused by chromium clustering at the surface. In non-magnetic topological insulators it finds sub-quantized Hall conductances and mismatched carrier densities, attributed to electron and hole puddles from non-uniform doping.