Two-photon excited room-temperature luminescence of CdS in the femtosecond regime

We present a dynamic study of the room-temperature, two-photon excited luminescence in CdS. When exciting the medium with red (2 eV), 100-fs-lasting pulses of intensity 9.4 GW/${\mathrm{cm}}^2,$ a strong two-photon absorption, with coefficient ${\alpha }_2=2.7\mathrm{}$ cm/GW, creates $1.6\times {10}^{16}$ ${\mathrm{cm}}^{-3}$ highly out-of-equilibrium carrier pairs. The overall kinetic energy relaxation time of these carriers has been measured to be of 350 fs and their final states have been determined as being shallow residual localized states in the band gap. From these states, potential energy relaxation gives rise to a stimulated emission of green (2.45 eV) light. The dynamics of this emission has been studied using a time resolved gain measurement experiment, which indicates a 200-fs duration for the highest excitation intensities (20 GW/${\mathrm{cm}}^2).\mathrm{}$