Optical Response of Hybrid Light-Matter Modes

This book provides a comprehensive theoretical exploration of low-energy terahertz (THz) hybrid light–matter excitations in quantum materials, focusing on superconductors and polar crystals. These collective modes play a fundamental role in determining how electromagnetic radiation interacts with complex solids and are central to emerging spectroscopic, photonic, and quantum technologies. Motivated by the rapid development of THz spectroscopy as a powerful probe of low-energy phenomena, this book develops a unified many-body framework for describing both the linear and nonlinear optical response of hybrid excitations. The approach addresses key limitations of conventional theories, particularly in anisotropic materials and nonequilibrium regimes, enabling the interpretation of increasingly sophisticated experimental observations. Particular emphasis is placed on layered superconductors, where the theory reveals the unconventional properties of Josephson plasmons and their spectroscopic signatures, and on polar crystals, where new methodologies are developed for investigating phonon-polaritons through THz pump–optical probe techniques. The text also examines broader phenomena including phonon–plasmon coupling, ultrafast optical control, and light-induced nonequilibrium dynamics. Throughout, analytical developments are closely linked to experimentally accessible observables, providing practical tools for identifying the microscopic origins of optical responses, interpreting state-of-the-art spectroscopy measurements, and guiding the design of future experiments. Combining theoretical rigor with direct relevance to contemporary research, this book offers a coherent framework for understanding hybrid excitations and light–matter interactions in quantum materials. It is an essential resource for graduate students and researchers in condensed matter physics, optical spectroscopy, quantum materials, and ultrafast science.