Pump-probe measurement of ZnO epitaxial thin films

Time-resolved optical absorption spectra of ZnO epitaxial thin films have been measured by using a pump-probe technique with subpicosecond time resolution. We have observed following three structures in the differential absorption spectra: saturation of the exciton absorption, bandgap renormalization and optical gain. The optical gain is thought to be due to electron-hole plasma. From the temporal changes of these structures, the dynamical properties of the photo-excited carriers are discussed