Light-matter interactions are able to drive a system out of equilibrium and reveal new phases that are otherwise unobtainable by more conventional temperature/pressure methods. Optical cavities can be used as a platform to enhance the light-matter coupling due to their confined dimensions, and gain greater control over those emergent phases. Recent examples include the proposal of a superconducting state with a non-BCS pairing mechanism or the stabilisation of a ferroelectric ground state in SrTiO₃ via coupling to the vacuum fluctuations of the photons. More recently, control over the metal-insulator transition has been demonstrated for 1T-TaS₂ in the absence of a driving field, where the dimensions of the cavity itself play a governing role. There is also the embedding of 2D semiconductor heterojunctions in optical microcavities to create high-brightness polaritonic light sources. Furthermore, theoretical work has indicated the possibility of tuning magnetic phases in quantum spin liquids and stabilising flat bands in twisted bilayer graphene while loosening the constraints on the need for a finely tuned magic angle. These few examples demonstrate that optical-cavities are quickly becoming a promising tool for altering the properties of a material, and to engineer the electronic, magnetic, phononic, or optical properties of quantum materials and their associated macroscopic phenomena.

This cross-journal Focus Collection between Communications Physics, Nature Communications and Scientific Reports aims to showcase advances in both experimental and theoretical work illustrating how optical cavities are increasingly being considered as an effective platform for tuning between different phenomena in range of condensed-matter systems, as well as provide opportunities to investigate the underlying physics.