In this thesis, I describe work aimed at understanding nonlinear material responses initiated by strong terahertz (THz) field excitation. I discuss two aspects of nonlinear THz spectroscopy in condensed-matter materials: developments of experimental THz capabilities and spectroscopy methods and their applications in investigating ultrafast nonlinear dynamics in different classes of materials. I first describe the THz generation, detection and spectroscopy methods, which are the basis of all of our studies. We have generated strong single- and multi-cycle THz pulses covering several spectral ranges using inorganic and organic crystals and developed linear and nonlinear THz spectroscopy techniques to interrogate light-matter interactions based on different observables and/or symmetry criteria. We have demonstrated a new method for studying time-domain electron paramagnetic resonance that allows us to measure THz-frequency fine structures of spin energy levels on a tabletop and have developed nonlinear two-dimensional (2D) magnetic resonance spectroscopy to distinguish nonlinear THz-spin interaction pathways. We also show that THz-pump, optical-probe spectroscopy, including THz field-induced second-harmonic generation spectroscopy and THz Kerr effect spectroscopy, can be extended to study phase transitions in quantum paraelectric and topological materials. We have employed the THz methods to drive and detect nonlinear responses from several degrees of freedom in the materials. We have demonstrated collective coherent control over material structure by inducing a quantum paraelectric to ferroelectric phase transition using intense THz electric fields in strontium titanate. We show that a single-cycle THz field is able to drive ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast timescale. We have driven highly nonlinear lattice and electronic responses in a topological crystalline insulator by dynamically perturbing the protecting crystalline symmetry through THz phonon excitation. We have observed oscillations in optical reflectivity that may be associated with electronic gap opening and modulation in the topological surface states. Finally, we have demonstrated nonlinear manipulation of collective spin waves in a canted antiferromagnet using strong THz magnetic fields and we have observed full sets of the second- and third-order nonlinear responses in 2D THz magnetic resonance spectra, which are accurately reproduced in our numerical simulations.

Intense Terahertz Excitation of Semiconductors presents the first comprehensive treatment of high-power terahertz applications to semiconductors and low-dimensional semiconductor structures. Terahertz properties of semiconductors are in the center of scientific activities because of the need of high-speed electronics. This research monograph brigdes the gap between microwave physics and photonics. It focuses on a core topic of semiconductor physics providing a full description of the state of the art of the field. _ The reader is introduced to new physical phenomena which occur in the terahertz frequency range at the transition from semi-classical physics with a classical field amplitude to the fully quantized limit with photons. The book covers a wide range of optical, optoelectronic, and nonlinear transport processes, presenting experimental results, clearly visualizing models and basic theories. Background information for future work and exhaustive references of current literature are given. A particularly valuable feature is through the discussion of various technical aspects of the terahertz range like the generation of high-power coherent radiation, optical components, instrumentation, and detection schemes of short intense radiation impulses. The book complements, for the first time in form of a monograph, previous books on infrared physics which dealt with low-power optical and opto-electronic processes. It will be useful not only to scientists but also to advanced students who are interested in terahertz research.

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.

Englische Version: This thesis exploits techniques of terahertz (THz) spectroscopy to investigate nonlinear low-frequency excitations of condensed matter. In particular, application of two-dimensional (2D) THz spectroscopy allows to disentangle different nonlinear signal contributions. The nonlinear polaronic response of solvated electrons and their surrounding solvent molecules in the polar liquid isopronal is studied. Solvated electrons are generated via multiphoton ionization. Longitudinal polaron oscillations with THz frequencies are impulsively excited during the ultrafast localization of the electrons. Perturbation of such polaron oscillations with an external THz pulse induces nonlinear changes of the transverse polaron polarizability, reflected in distinct modifications to the oscillation phase as mapped in 2D-THz experiments. Further, the generation of mono-cycle THz pulses from asymmetric semiconductor quantum wells upon resonant intersubband excitation in the mid-infrared (MIR) range is demonstrated. The temporal shape of the emitted THz electric field is modified by controlling pulse duration and peak electric field of the MIR driving pulses. Phase-resolved 2D-MIR experiments confirm that the THz emission is predominantly due to a nonlinear shift current generated upon femtosecond intersubband excitation. The influence of combined intra- and interband currents on symmetry properties, which opens novel quantum pathways for phonon excitation in narrow-band-gap materials, is demonstrated by 2D-THz experiments on bismuth. Nonperturbative long-wavelength excitation of charge carriers close to the L points leads to an anisotropic carrier distribution, reflected in a six-fold azimuthal angular dependence of the pump-induced change of THz transmission.[...].

This dissertation is an exploration of the material response to Terahertz (THz) radiation. Specifically we will explore the ultrafast electron dynamics in the nonperturbative regime in semiconductors that have been patterned with nanoantenna arrays using broadband, high intensity, THz radiation. Three main semiconductor materials will be studied in this work. The first is VO2 which undergoes a phase transition from an insulator, when it is below 67° C, to a metal, when it is above 67° C. The second and third materials are Si and GaAs which are two of the most commonly used semiconductors. We study the insulator to metal transition (IMT) of VO2 and its response to high field THz radiation. The near room temperature IMT for VO2 makes it a very promising material for electrical and photonic applications. We demonstrate that with high field THz the IMT transition can be triggered. This transition is induced on a sub-cycle timescale. We also demonstrate a THz field dependent reduction in the transition temperature for the IMT when transitioning from both below T[subscript c] to above as well as from above T[subscript c] to below. This transition is not equal for the above and below cases and leads to a narrowing of the hysteresis curve of the IMT. The thin film Fresnel coefficients, along with a phenomenological model developed for the nanoantenna patterned VO2, are also used to calculate the sheet conductivity of the VO2 sample. We show, using this sheet conductivity and its relation to the band gap, that the bang gap in the insulating phase has a strong dependence on the incident THz radiation with larger fields reducing the band gap from 1.2 eV at low incident THz fields to 0.32 eV at high incident THz fields. The ultrafast, non-equilibrium, electron dynamics of GaAs and Si were also explored. GaAs and Si are the two most prevalent semiconductors in use today and with the decrease in size and increase in clock speeds of transistors a deep understanding of the ultrafast high field electron dynamics of these materials is of vital importance. Using THz time domain spectroscopy we investigate the transition rates of electrons excited by a 800 nm optical pump. We show that the optically induced transition for GaAs happens on a shorter timescale than that of Si. We also investigate the THz transmission dependence on the indecent THz field. We show that intense THz fields enhance the transmission through the sample. The increase in transmission is due to intervalley scattering which increases the effective mass of the electrons resulting in a decrease in the conductivity the sample.

The Encyclopedia of Modern Optics, Second Edition, Five Volume Set provides a wide-ranging overview of the field, comprising authoritative reference articles for undergraduate and postgraduate students and those researching outside their area of expertise. Topics covered include classical and quantum optics, lasers, optical fibers and optical fiber systems, optical materials and light-emitting diodes (LEDs). Articles cover all subfields of optical physics and engineering, such as electro-optical design of modulators and detectors. This update contains contributions from international experts who discuss topics such as nano-photonics and plasmonics, optical interconnects, photonic crystals and 2D materials, such as graphene or holy fibers. Other topics of note include solar energy, high efficiency LED’s and their use in illumination, orbital angular momentum, quantum optics and information, metamaterials and transformation optics, high power fiber and UV fiber lasers, random lasers and bio-imaging. Addresses recent developments in the field and integrates concepts from fundamental physics with applications for manufacturing and engineering/design Provides a broad and interdisciplinary coverage of specialist areas Ensures that the material is appropriate for new researchers and those working in a new sub-field, as well as those in industry Thematically arranged and alphabetically indexed, with cross-references added to facilitate ease-of-use

In a new branch of physics and technology, called spin-electronics or spintronics, the flow of electrical charge (usual current) as well as the flow of electron spin, the so-called "spin current", are manipulated and controlled together. This book is intended to provide an introduction and guide to the new physics and applications of spin current.