Intense Terahertz Excitation Of Semiconductors

This work presents the first comprehensive treatment of high-power terahertz applications to semiconductors and low-dimensional semiconductor structures. Terahertz properties of semiconductors are in the centre of scientific activities because of the need of high-speed electronics.

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.

Terahertz (THz) radiation with frequencies between 100 GHz and 30 THz has developed into an important tool of science and technology, with numerous applications in materials characterization, imaging, sensor technologies, and telecommunications. Recent progress in THz generation has provided ultrashort THz pulses with electric field amplitudes of up to several megavolts/cm. This development opens the new research field of nonlinear THz spectroscopy in which strong light-matter interactions are exploited to induce quantum excitations and/or charge transport and follow their nonequilibrium dynamics in time-resolved experiments. This book introduces methods of THz generation and nonlinear THz spectroscopy in a tutorial way, discusses the relevant theoretical concepts, and presents prototypical, experimental, and theoretical results in condensed matter physics. The potential of nonlinear THz spectroscopy is illustrated by recent research, including an overview of the relevant literature.

This book presents the state-of-the-art of Terahertz spectroscopy. It is a modern source for a beginners and researcher interested in THz spectroscopy. The basics and physical background of THz spectroscopy and technology are explained, and important applications are described. The book presents the highlights of scientific research in the field of THz science and provides an excellent overview of the field and future directions of research. Over the last decade the field of terahertz spectroscopy has developed into one of the most rapidly growing fields of spectroscopy with large impact across a wide range of scientific disciplines. Due to substantial advances in femtosecond laser technology, terahertz time-domain spectroscopy (THz-TDS) has established itself as the dominant spectroscopic technique for experimental scientists interested in measurements in this frequency range. In solids and liquids terahertz radiation is at resonance with both phonon modes and hydrogen bonding modes which makes it an ideal tool to study the interaction between molecules in a unique way, thus opening a wealth of opportunities for research in physics, chemistry, biology, materials science and pharmaceuticals. This book provides an easy access to scientists, engineers and students alike who want to understand the theory and applications of modern terahertz spectroscopy.

This book provides an in-depth review of the rapidly developing field of spintronic semiconductors. It covers a broad range of topics, including growth and basic physical properties of diluted magnetic semiconductors based on II-VI, III-V and IV semiconductors, recent developments in theory and experimental techniques and potential device applications; its aim is to provide postgraduate students, researchers and engineers a comprehensive overview of our present knowledge and future perspectives of spintronic semiconductors.

The development of new sources and methods in the terahertz spectral range has generated intense interest in terahertz spectroscopy and its application in an array of fields. Presenting state-of-the-art terahertz spectroscopic techniques, Terahertz Spectroscopy: Principles and Applications focuses on time-domain methods based on femtosecond laser sources and important recent applications in physics, materials science, chemistry, and biomedicine. The first section of the book examines instrumentation and methods for terahertz spectroscopy. It provides a comprehensive treatment of time-domain terahertz spectroscopic measurements, including methods for the generation and detection of terahertz radiation, methods for determining optical constants from time-domain measurements, and the use of femtosecond time-resolved techniques. The last two sections explore a variety of applications of terahertz spectroscopy in physics, materials science, chemistry, and biomedicine. With chapters contributed by leading experts in academia, industry, and research, this volume thoroughly discusses methods and applications, setting it apart from other recent books in this emerging terahertz field.

An international team of experts describes the optical and electronic properties of semiconductors and semiconductor nanostructures at picosecond and femtosecond time scales. The contributions cover the latest research on a wide range of topics. In particular they include novel experimental techniques for studying and characterizing nanostructure materials. The contributions are written in a tutorial way so that not only researchers in the field but also researchers and graduate students outside the field can benefit.

This two-volume work covers ultrafast structural and electronic dynamics of elementary processes at solid surfaces and interfaces, presenting the current status of photoinduced processes. Providing valuable introductory information for newcomers to this booming field of research, it investigates concepts and experiments, femtosecond and attosecond time-resolved methods, as well as frequency domain techniques. The whole is rounded off by a look at future developments.