Pump-probe measurement of ZnO epitaxial thin films

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