Abstract
Time-resolved luminescence and optical gain spectra due to an electron-hole plasma (EHP) in ZnO epitaxial thin films are studied through optical Kerr gate luminescence spectroscopy. Above the critical Mott density, free electron-hole pairs are created instantaneously by intense pump pulses and bandgap is reduced without any detectable delay time. Cooling of the hot carriers toward the bottom of the renormalized bandgap and subsequent buildup of the quasi-thermalized EHP state take place. Then the recovery of the renormalized bandgap toward higher energy with decreasing the carrier density follows. From the lineshape analysis of the time-resolved optical gain spectra, we find that the decrease of the carrier density is governed by a bimolecular recombination process which represents radiative recombination of electrons with holes in the EHP state. The effective temperature as well as the carrier density of the EHP are also determined.