Light–matter interactions with photonic quasiparticles

Interactions between light and matter play an instrumental role in spectroscopy, sensing, quantum information processing and lasers. In most of these applications, light is considered in terms of electromagnetic plane waves propagating at the speed of light in vacuum. As a result, light–matter interactions can usually be treated as very weak and captured at the lowest order in quantum electrodynamics. However, progress in understanding the coupling of photons to material quasiparticles (plasmons, phonons and excitons) brings the need for a generalized view of the photon at the core of every light–matter interaction. In this new picture, the photon can have greatly different polarization and dispersion and be confined to the scale of a few nanometres. Such photonic quasiparticles enable a wealth of otherwise unobservable light–matter interaction phenomena, in interactions with both bound and free electrons. This Review focuses on the theoretical and experimental developments in realizing new light–matter interactions with photonic quasiparticles. Examples include room-temperature strong coupling, ultrafast ‘forbidden’ transitions in atoms and new applications of the Cherenkov effect, as well as breakthroughs in ultrafast electron microscopy and new concepts for compact X-ray sources.