A number of possible nonlinear light effects in solids are discussed where the optical properties of a medium depend upon the value of the incident $E$ and $H$ radiation field; as a consequence of the nonlinear properties of these effects and their intrinsic fast response times, the possibility exists of producing harmonic generation and mixing at infrared and visible frequencies. The theory of multiple-photon excitation of an electron from the valence to the conduction in a solid is developed. It is shown that this process exhibits an intensity-dependent absorption edge at photon energies below the minimal energy gap of a solid. The experimental conditions for observing such transitions are considered; presently available optical maser sources of radiation are of sufficient intensity to enable double-quantum processes to be experimentally observable in semiconductors and insulators.

• Received 12 September 1961

DOI:https://doi.org/10.1103/PhysRev.125.475