Spontaneous and Optically Amplified Luminescence from Exciton-Exciton Collisions in GaSe at Liquid-He Temperature

We report on our investigations of the excitonic luminescence spectra of photoexcited GaSe crystals immersed in superfluid He. We measured the spontaneous and optically amplified luminescence due to exciton-electron and to direct-direct as well as to direct-indirect exciton-exciton scattering. The intensity of the exciton-exciton recombination lines is proportional to the squared intensity of the luminescence from simple excitonic recombination. We find good agreement, within experimental uncertainties, between our experimental results and theory. Gain due to exciton scattering processes is weak and difficult to observe in GaSe. From our measurements, we estimate net gain values of up to about 50 cm-1