Abstract
Time evolution of a high density electron-hole plasma (EHP) and excitons in ZnO epitaxial thin films was studied under resonant excitation of the excitonic state using pump-probe absorption and optical Kerr gate luminescence techniques with sub-picosecond time resolution in order to elucidate dynamics of the photo-excited high density carriers. The bleaching of the exciton absorption occurs without any measurable delay time, suggesting that the excitonic state instantaneously becomes unstable because of the dissociation of the high density excitons into the EHP due to the screening of Coulomb interaction. This leads to an instantaneous bandgap reduction. Then the recovery of the renormalized bandgap as well as that of the exciton bleaching takes place with decreasing the carrier density due to radiative recombination of electrons with holes. From the time-resolved absorption and luminescence experiments, the recovery time is determined to be 25 v ps.