Light-matter interaction drives many systems, such as optoelectronic devices like light-emitting diodes and solar cells, biological structures like photosystem II and potential future quantum devices. The absorption or emission of light typically occurs on the sub-nanometre scale and the involved processes take place on attosecond to picosecond timescales. Light-matter interaction can be studied at atomic space-time scales by using a scanning tunnelling microscope and coupling light into or extracting light from the tunnel junction. Electromagnetic radiation couples with matter through the interaction with charge carriers, leading to excitations such as electronic transitions, collective oscillations, excitons and spin flips. These excitations can be studied with high spatial and temporal resolution using approaches in which light interacts with the tunnel junction itself or with a quantum system in the junction. This Review discusses the powerful union of photonics and scanning probe techniques.

The interaction of light with matter probed with a scanning tunnelling microscope reveals dynamics at atomic space-time scales. This Review discusses experimental schemes by which light-matter interaction is explored, taking advantage of light coupled into or extracted from the tunnel junction.