Two-photon excited room-temperature luminescence of CdS in the femtosecondregime

We present a dynamic study of the room-temperature, two-photon excited lumi nescence in CdS. When exciting the medium with red (2 eV), 100-fs-lasting p ulses of intensity 9.4 GW/cm(2), a strong two-photon absorption, with coeff icient alpha(2)=2.7 cm/GW, creates 1.6x10(16) cm(-3) highly out-of-equilibr ium carrier pairs. The overall kinetic energy relaxation time of these carr iers has been measured to be of 350 fs and their final states have been det ermined as being shallow residual localized states in the band gap. From th ese states, potential energy relaxation gives rise to a stimulated emission of green (2.45 eV) light. The dynamics of this emission has been studied u sing a time resolved gain measurement experiment, which indicates a 200-fs duration for the highest excitation intensities (20 GW/cm(2)). [S0163-1829( 99)10631-3].