This book presents the most recent updates in the field of photon and optical materials research. It is devoted to various interdisciplinary subjects such as fundamental photon physics, bio and medical photon physics, ultrafast non-linear optics, quasiparticle excitation and spectroscopy, coherent mid-infrared (IR) light sources, functional optoelectronic materials and optical fibres, and quantum nano-structured devices for various important technological applications. It contains 19 authoritative peer-reviewed chapters regarding experimental and theoretical research in these fields, contributed by young scientists and engineers (assistant or associate professor level) along with well-established experts. The response of materials to electromagnetic fields, namely light-matter interaction, has been of special concern in fundamental optical sciences. The ability to fabricate and/or engineer new materials and structures is giving rise to revolutionary changes in the field, which also includes soft condensed matter and nanoscale systems. On the other hand, significant demand for new coherent light sources at mid-IR ranges has caused new laser technologies to flourish, which utilise innovative optical materials and quantum structures. Important mid-IR applications include IR countermeasures, optical telecommunications, broadband Internet, environmental monitoring, remote sensing of chemical and biological species, minimally invasive medical surgery and medical diagnosis, and direct imaging of biological structures and relevant processes. Most of the chapters presented in this book address key issues on light-matter interaction in various systems and the next generation of light sources aiming at practical applications. Included with these applications are some fundamental topics such as photon Bose-Einstein condensation, vacuum polarisation field, and polarisational bremsstrahlung. This collection of fresh updates on such stimulating areas makes this book quite unique, as it provides an in-depth review on the most recent progress as well as suggesting future research directions.