"Reducing the dimensionality of a material from 3D to 2D introduces new quantum effects that can be exploited to see new physics and produce new devices. Two examples of such materials are graphene, a single layer of carbon atoms, and quantum well heterostructures confining charges to a planar potential well. A powerful method for observing charge transport in these 2D materials is time-resolved terahertz (THz) spectroscopy, using picosecond pulses of far-infrared light to measure the complex valued ac conductivity. In this thesis, THz spectroscopy is used to study the linear and nonlinear electrodynamic response of two 2D systems: graphene and self-assembled metal halide perovskite quantum wells. In graphene, THz field-induced bleaching is studied, found to be well described by simple phenomenological reduction in intraband absorption using a free carrier Drude model that includes neutral and charged impurity scattering as well as optical phonon scattering. The Fermi level dependence of this nonlinearity is studied for the first time, which allows us to separate the contributions of long and short range momentum scattering and map their dependence on the field amplitude. We find, contrary to previous work, that the increase in scattering cannot be explained by an increase in the lattice temperature alone and that other mechanisms such as carrier-carrier scattering must be accounted for to explain the effect. Our results provide a deeper understanding of transport in graphene devices operating at THz frequencies and in modest kV/cm field strengths present in most devices. Second, time-resolved multi-THz spectroscopy was used to investigate carrier dynamics in 2D organic metallic halide perovskite (OMHP) systems. The 2D OMHPs are promising photovoltaic materials showing reduced sensitivity to environmental degradation, however their photoconductivity response has been unexplored on ultrafast time scales. We observed a rapid decay of injected photocarriers on a 10-100 ps time scale, indicating a significant trap/decay channel that limits device performance. Supporting hyperspectral photoluminescent imaging measurements confirm the band gap shift of the naturally formed quantum wells, however also show significant surface structural defects serving as monomolecular traps. We show how mono-molecular, bimolecular and Auger constants are sensitive to the changes in the electronic confinement of the quantum wells, of importance to devices attempting to exploit their tunable optical properties and environmental stability." --

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.

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.

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 book presents the latest results of quantum properties of light in the nanostructured environment supporting surface plasmons, including waveguide quantum electrodynamics, quantum emitters, strong-coupling phenomena and lasing in plasmonic structures. Different approaches are described for controlling the emission and propagation of light with extreme light confinement and field enhancement provided by surface plasmons. Recent progress is reviewed in both experimental and theoretical investigations within quantum plasmonics, elucidating the fundamental physical phenomena involved and discussing the realization of quantum-controlled devices, including single-photon sources, transistors and ultra-compact circuitry at the nanoscale.

This book presents an overview of the most recent advances in nonlinear science. It provides a unified view of nonlinear properties in many different systems and highlights many new developments. While volume 1 concentrates on mathematical theory and computational techniques and challenges, which are essential for the study of nonlinear science, this second volume deals with nonlinear excitations in several fields. These excitations can be localized and transport energy and matter in the form of breathers, solitons, kinks or quodons with very different characteristics, which are discussed in the book. They can also transport electric charge, in which case they are known as polarobreathers or solectrons. Nonlinear excitations can influence function and structure in biology, as for example, protein folding. In crystals and other condensed matter, they can modify transport properties, reaction kinetics and interact with defects. There are also engineering applications in electric lattices, Josephson junction arrays, waveguide arrays, photonic crystals and optical fibers. Nonlinear excitations are inherent to Bose-Einstein Condensates, constituting an excellent benchmark for testing their properties and providing a pathway for future discoveries in fundamental physics.

This book covers the latest advances in the techniques employed to manage the THz radiation and its potential uses. It has been subdivided in three sections: THz Detectors, THz Sources, Systems and Applications. These three sections will allow the reader to be introduced in a logical way to the physics problems of sensing and generation of the terahertz radiation, the implementation of these devices into systems including other components and finally the exploitation of the equipment for real applications in some different field. All of the sections and chapters can be individually addressed in order to deepen the understanding of a single topic without the need to read the whole book. The THz Detectors section will address the latest developments in detection devices based on three different physical principles: photodetection, thermal power detection, rectification. The THz Sources section will describe three completely different generation methods, operating in three separate scales: quantum cascade lasers, free electron lasers and non-linear optical generation. The Systems and Applications section will take care of introducing many of the aspects needed to move from a device to an equipment perspective: control of terahertz radiation, its use in imaging or in spectroscopy, potential uses in security, and will address also safety issues. The text book is at a level appropriate to graduate level courses up to researchers in the field who require a reference book covering all aspects of terahertz technology.