Author: Ayesheshim Kebie Ayesheshim

Publisher:

Published: 2015

Total Pages: 233

ISBN-13:

This thesis describes the generation, characterization, and nonlinear application of intense terahertz (THz) pulses. Nonlinear THz spectroscopy has emerged as a powerful tool to study the ultrafast time evolution of high-field charge carrier dynamics in semiconductors and nano-materials. The study of such phenomena in semiconductors and semiconductor structures requires intense THz pulses with high electric-field strengths. We have developed an improved experimental setup for generating high-power, nearsingle cycle THz pulses by tilted-pulse-front optical rectification in LiNbO3 with optimized optical-to-THz conversion efficiency, and proper characterization of the THz pulses in the Ultrafast Nanotools lab at the University of Alberta. We have investigated the effects of optical pump pulse pre-chirping and polarization on THz pulse generation using separate compressors for the optical pulses used for THz generation and detection. By down-chirping the 800 nm optical pump pulses to 385 fs, single-cycle THz pulses with energies up to 3.6 microJ were obtained, corresponding to an energy conversion efficiency of 3x10^{-3}. This high-field THz source is capable of generating electric fields greater than which can induce nonlinear carrier dynamics in semiconductors. We demonstrate novel high-field THz experiments that explore nonlinear processes in doped and photo-excited bulk semiconductors. As a benchmark and consistency check, a nonlinear THz absorption bleaching Z-scan experiment was conducted on an n-doped InGaAs epilayer on a lattice matched InP substrate. This experiment confirmed that the THz pulses generated by our source are adequate for ultrafast nonlinear measurements in the THz frequency range. Even more interesting, we have achieved unprecedented THz field absorption bleaching simply by flipping the face of the sample illuminated by the THz pulse pump. That is, we illuminate the insulating (substrate) side of the sample with the THz pulse in the Z-scan experiment rather than illuminating the usual (conducting) face of the sample. In this study considerable insight has been gained into developing an optical diode. We have also developed a technique to measure transient voltage pulses induced in doped and photoexcited semiconductors due to a shift current generated from the nonlinear THz dynamics of free electrons in the conduction band. This is a fascinating feature with a practical application as an ultrafast and ultra-sensitive THz phtotodetector. A Drude-based dynamic intervalley scattering simulation reveals that the nonlinear THz-induced transient voltage pulses are a result of intervalley scattering driven by high-field THz pulses. It is the first time that THz induced picosecond voltage transients are measured in semiconductors. We find that an intense THz pulse incident on an InGaAs sample excites a transient dipole due to intervalley scattering. Also, THz pulse induced transient voltage signals have been investigated in ZnTe, and doped-Si semiconductors due to a direct flow of free carriers upon THz photon absorption. We have observed nonlinear conductivity responses in Si, ZnTe, photo-excited SI-GaAs, and doped InGaAs, showing the strong THz pulse can heat the electron population and create a momentum distribution leading to saturable absorption in the THz frequency range.