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.

Niccolò Sellati is a theoretical condensed matter physicist and postdoctoral researcher. He earned his Ph.D. in Physics at Sapienza University of Rome under the supervision of Prof. Lara Benfatto, with research focused on the optical response of hybrid light-matter modes in quantum materials. His research primarily employs analytical theoretical frameworks to investigate collective excitations such as Josephson plasmons, phonon-polaritons, and magnetic spin waves, while also exploring light-induced nonequilibrium states with the aim of understanding and controlling the optical properties of complex quantum materials. Working in close collaboration with several experimental groups, he aims at a fundamental theoretical understanding of the ultrafast dynamics of correlated electron systems and high-temperature superconductors, with a particular focus on terahertz spectroscopy and nonlinear optical phenomena.