The processes involved in the laser effect in CdS excited by electron bombardment have been studied with the aid of an experimental technique allowing the spectral dependence of the optical gain to be obtained. It is shown that at least three different processes can lead to a laser effect in CdS. The first, which corresponds to a low gain, is due to the annihilation of a free exciton with the emission of a photon and an LO phonon. The second, yielding a medium gain, is due to an exciton-exciton interaction; and the third, which results in a high gain, involves an exciton-electron interaction. These last two processes have been studied theoretically, and the results are compared with the experimental data.

• Received 18 December 1967

DOI:https://doi.org/10.1103/PhysRev.177.567