Nonlinear terahertz (THz) spectroscopy maps the nonlinear response of matter to terahertz waves, thus giving insight into elementary excitations and charge transport under nonequilibrium conditions and on ultrashort time scales. Nonlinear terahertz spectroscopy covers a broad range of basic research in physics, chemistry, and materials science and is relevant for applications in optical technologies.

THz radiation with frequencies between 100 GHz and 30 THz has developed into an important tool of science and technology, with numerous applications in basic science, 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 has opened the 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 the relevant basic concepts of light-matter interactions and nonlinear optics, discusses methods of THz generation and nonlinear THz spectroscopy, and presents prototypical experimental and theoretical results in condensed matter physics and chemistry. The potential of nonlinear THz spectroscopy is illustrated by recent research on low-frequency excitations in solids, on high-frequency and high-field charge transport, and on the nonlinear THz response of molecular liquids and proteins. Each chapter includes an overview of relevant recent literature.