Stimulated Emission Spectra of CdS Platelets under Various Excitation Levels

Abstract

The stimulated emission of CdS platelets excited by light pulses from a N₂ laser is investigated at 2° and 77°K, as a function of the level of excitation and for differently prepared crystals. Pure and perfect crystals show peaks due to exciton‐exciton interactions (P), and due to the recombination of a free exciton with simultaneous emission of one or two LO phonons [(Ex‐LO) and (Ex‐2L)]. Platelets grown in an atmosphere of excess S or Cd show up to five additional stimulated emission lines, located between P and Ex‐LO. Imperfect crystals show a broad emission band extending from P to beyond Ex‐LO. At low excitation levels only P appears in the stimulated emission spectra. With increasing level of excitation the intermediate impurity lines (JJ) in Cd‐ and S‐rich crystals also show stimulated emission and the maximum emission intensity shifts from the P peak sequentially through all JJ lines always toward the next peak at lower wavelengths. At still higher excitation levels, Ex‐LO, the next peak at lower wavelengths, gains maximum intensity and in one observation even Ex‐2LO grew to become the dominant stimulated emission line. In imperfect crystals the maximum of the broad emission band shifts from a position near P to Ex‐LO with increasing level of excitation. Further, the P line is found to split in a magnetic field $\mathrm{H\perp C}$ . The intensity ratio of the upper to the lower P branch changes with the excitation intensity, such that with lower excitation intensity the higher energetic branch dominates the emission while at higher excitation intensity the lower energetic branch has higher emission intensity than the other branch. A reasonable explanation of the dependence on excitation intensity is given in terms of electron‐assisted transitions which increase the absorption in the tail region of the absorption edge, and thus spoil the necessary gain for stimulated emission.