Large optical nonlinearities near the band gap of GaN thin films

The interband optical transitions in single-crystal GaN films grown by metal organic chemical vapor deposition have been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well-resolved features are seen in the absorption and reflection spectra corresponding to the $\text{1s}$ A and B exciton transitions. These features broaden and decrease in intensity due to the presence of a high density of photoexcited free carriers and are completely absent in the absorption and reflection spectra as the excitation density, $I_{\mathrm{exc}},$ approaches $\text{3\hspace{0.17em}}{\mathrm{MW/cm}}^{\mathrm{2}},$ resulting in induced transparency in transmission measurements. The absorption spectra also show induced absorption below the band gap as $I_{\mathrm{exc}}$ is increased. Both the observed induced transparency and induced absorption were found to be extremely large, exceeding ${\text{4×10}}^4\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-1}}$ as the pump density approaches $\text{3\hspace{0.17em}}{\mathrm{MW/cm}}^{\mathrm{2}}$ at 10 K.