Abstract
We report on our detailed measurements of the gain spectra in the region just below the transition of the direct free exciton ground state. We use the very sensitive method recently introduced by Hvam. The crystals, immersed in superfluid He, are excited by green laser light (532 nm) the intensity of which is varied from 0.1 to 3 MW cm −2. At excitation intensities above about 0.3 MW cm −2, all samples show a wide band which starts at about 5970 Å and peaks at round 6000 Å. The high energy edge of this well-known emission is independent of excitation intensity; its peak, however, shifts to low energies as the excitation increases. Induced absorption is virtually absent. In addition, our gain spectra show a structure of several weak lines which varies from sample to sample. The measured gain spectra are compared with the predictions of the recently developed theories of stimulated emission of strongly excited semiconductors.
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