  We report the first experimental observations of two-photon emission from
semiconductors, to the best of our knowledge, and develop a corresponding
theory for the room-temperature process. Spontaneous two-photon emission is
demonstrated in optically-pumped bulk GaAs and in electrically-driven
GaInP/AlGaInP quantum wells. Singly-stimulated two-photon emission measurements
demonstrate the theoretically predicted two-photon optical gain in
semiconductors - a necessary ingredient for any realizations of future
two-photon semiconductor lasers. Photon-coincidence experiment validates the
simultaneity of the electrically-driven GaInP/AlGaInP two-photon emission,
limited only by detector's temporal resolution.
