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Stimulated Emission and Pump-Probe studies were performed in GaN, InGaN, and AlGaN epilayers as well as GaN/AlGaN separate confinement heterostructures. We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below approximately 150 K, whereas at elevated temperatures electron-hole plasma is the dominant gain mechanism. An analysis of the relative shift between the spontaneous emission and lasing peaks in SCH samples, combined with the temperature dependence of the lasing threshold, reveals that exciton-exciton scattering is the dominant gain mechanisms leading to low-threshold UV lasing in the GaN/AlGaN SCH over the entire temperature range studied. We further performed optical pumping of AlGaN epilayers at different temperatures. Stimulated emission has been observed in AlxGa1-xN thin films for Al concentrations as high as x = 0.26, with a resultant stimulated emission wavelength as low as 328 nm at room temperature. This result indicated that AlGaN-based structures are suitable not only for use in deep-UV detectors, but also as a potential source of deep-UV laser radiation. The interband optical transitions in GaN and InGaN have also been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well- resolved features were seen in the absorption and reflection spectra corresponding to the 1s A and B exciton transitions. Broadening and decrease in intensity of these features were studied as the function of excitation pump density. We found that values of induced transparency and induced absorption are extremely large in GaN epilayers. The pump-probe results in GaN epilayers were directly compared to ones obtained from InGaN films. Significant differences in near-bandedge absorption changes were clearly observed between the two materials.
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