Large optical nonlinearities in the band gap region of GaN thin films grown by MOCVD on sapphire

We report the results of nanosecond non-degenerate optical pump-probe experiments and single-beam power dependent absorption experiments performed on metalorganic chemical vapor deposition (MOCVD) grown GaN thin films. Changes in the optical transitions near the band gap due to excess photo-generated free carriers were studied as a function of excitation density at 10 K and room temperature using pump-probe spectroscopy. At 10 K, strong, well-resolved features are present in the absorption spectra corresponding to the Is A and B free exciton transitions. These features are shown to broaden and decrease in intensity due to the presence of the high densities of photo-excited free carriers generated by the pump beam, resulting in extremely large values of induced transparency, exceeding 4 x 10(4) cm(-1) as the excitation density (I-exc) approaches 3 MW/cm(2). In addition, large values of induced absorption are observed with increasing pump density in the below-gap region where gain was expected. This induced absorption was also found to be extremely large, exceeding 4 x 10(4) cm(-1) as I-exc was increased to over 3 MW/cm2. At room temperature the resulting induced transparency and induced absorption were found to approach 2 x 10(4) cm(-1) as I-exc approached 3 MW/cm(2). The single-beam power dependent absorption experiments show enhanced bleaching of the excitonic transitions with increasing I-exc compared to the pump-probe experiments, while the below-gap induced absorption is drastically reduced in the single-beam experiments. The large values of induced transparency/absorption observed in this work and the fact that excitons have been shown to persist to over 450 K in GaN1 (indicating that the large optical nonlinearities will persist substantially above room temperature) suggest the possibility of new opto-electronic applications for the group III nitrides.