We have investigated the photoluminescence properties of a lightly alloyed In0.02Ga0.98N thin film at 10 K under intense excitation conditions. A photoluminescence band (P band) peculiar to the intense excitation condition has been clearly observed. The excitation-power dependence of the P-band intensity exhibits an almost quadratic behavior, accompanied by a threshold-like appearance. The threshold-excitation power for the P band is very low: ∼3kW∕cm2. At the threshold excitation power, the energy of the P band is lower than the energy of the n=1 A free exciton by the energy difference between the n=1 and n=2 exciton states. The results described above indicate that the P band originates from exciton-exciton scattering. Furthermore, we have confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band by using transmission-type pump-probe spectroscopy.

Dielectric function spectra of purified thallium bromide crystals grown by the Bridgman method were studied over the photon range of 1.5–5.4 eV at room temperature. In addition, density functional theory (DFT)-based calculations of the electronic band structure were performed. Interband optical transitions associated with the obtained dielectric function were determined using standard critical point analysis. The electronic bands involved in each particular interband transition in the accessed photon energy range were identified within the calculated band structure.