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.

光与物质之间的相互作用在光谱学，传感，量子信息处理和激光中起着重要作用。在大多数这些应用中，光是根据在真空中以光速传播的电磁平面波来考虑的。结果，光-物质相互作用通常可以被认为是非常弱的，并且在量子电动力学中以最低顺序被捕获。然而，在理解光子与物质准粒子（等离激元，声子和激子）耦合的过程中，需要对光子相互作用的每个核心都具有一个概括的认识。在这张新图片中，光子可以具有截然不同的偏振和色散，并被限制在几纳米的范围内。此类光子准粒子在与束缚电子和自由电子相互作用时，都会产生大量其他无法观察到的光-物质相互作用现象。这篇综述着重于实现与光子准粒子之间新的光-物质相互作用的理论和实验发展。例如，室温强耦合，原子中超快的“禁忌”跃迁和切伦科夫效应的新应用，以及超快电子显微镜的突破和紧凑型X射线源的新概念。