Quantum fountain infrared light sources based on intersubband emissions inquantum wells

Recent achievements on quantum fountain infrared sources relying on intersu bband emission under optical pumping of coupled quantum well structures are reviewed. Population inversion can be achieved at midinfrared wavelengths using a careful engineering of the scattering rates between subbands. Obser vations of intersubband luminescences under either intersubband or interban d optical pumping is reported. At low temperatures, the spontaneous emissio n is shown to result from direct optical excitation of the electrons in the upper subband. However, under interband excitation at room-temperature, th e luminescence is the manifestation of the black-body emission of the two-d imensional electron gas. Population inversion at 77 K is then demonstrated for the first time using optical pumping by a free-electron laser. Large st imulated gains at 12.5 mu m wavelength are reported. Lasing action appears to be feasible under infrared optical pumping.