Strong light-matter interactions in microcavities have been long known to provide means to alter optical and nonlinear properties of the coupled system. As a result of this interaction, one typically observes the emergence of new polaritonic eigenstates of the coupled system.

Dielectric and metallic nanostructures can be tailored to provide unusual interaction with light waves. For example, they support localized resonances highly sensitive to the surroundings, complex scattering patterns and ultrafast nonlinearities.

When photoactive molecules strongly interact with confined light, for instance inside optical cavities or near (localised) surface plasmons, new hybrid light-matter states may form, the so-called polaritons or plexcitons.