High harmonic generation in condensed matter

When solids are exposed to intense laser fields whose forces are comparable to the binding forces of valence electrons in crystals, nonlinear optics of solids advances to the nonperturbative or extreme nonlinear regime. A hallmark effect of extreme nonlinear optics is the emission of high-order harmonics of the laser from the bulk of materials. The discovery and detailed study of this phenomenon over the course of the past decade has offered a broad range of possibilities and seen the dawn of a new field of extreme solid-state photonics. In particular, it opens the way to previously inaccessible spectral ranges, as well as the development of novel solid-state spectroscopy and microscopy techniques that enable detailed probing of the electronic structure of solids. Here we review recent experimental and theoretical advances in this emerging area of study, and highlight the potential future directions and applications.