Optical gain spectra of ZnO epitaxial thin films have been measured by using a pump–probe technique. The optical gain is thought to be due to electron–hole plasma. In the differential absorption spectra, we observed saturation of the exciton absorption, band-gap renormalization, as well as the optical gain. From the temporal changes of these structures, the dynamical properties of the photoexcited carriers are discussed.

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Renormalized bandgap energy due to high-density carriers in ZnO is calculated and compared with our experimental results. Two different reported treatments for the correlation energy are employed for the calculation, and we prove that Beni and Rice's model sufficiently explains the experimental data. Furthermore, we calculate optical gain spectrum, which reproduces well the experimental data.