Resonant Brillouin scattering in CdS. I. Experiment

High-resolution resonant Brillouin-scattering (RBS) experiments were performed in the vicinity of the $A$ exciton in cadmium sulfide. The spectroscopic system used in these investigations consisted of a single-mode dye laser which was frequency tuned through the exciton resonance, and a triple-pass Fabry-Perot interferometer combined with a tandem grating spectrometer which was used to analyze the scattered light. This experimental arrangement enabled Brillouin shift, linewidth, and intensity measurements to be made on several different Brillouin components which result from the scattering by acoustic phonons of both inner- and outer-branch exciton-polaritons in the crystalline medium. Very good agreement was obtained between the Brillouin shift data and theory resulting in revised values for the exciton effective mass, transverse frequency, background dielectric constant, and oscillator strength. However, the Brillouin linewidth data indicated that the exciton damping constant $\Gamma$ is not constant in value but instead increases monotonically for frequencies greater than ${\omega }_T$. In an attempt to identify the correct additional boundary condition (ABC) which is needed to describe the simultaneous propagation of two degenerate exciton-polariton modes, a theoretical external scattering cross-section calculation was developed (see the following paper) and compared with the measured Brillouin intensities. Although the RBS intensity measurements did not fit very well with theory for any of the three ABC's, agreement between theory and experiment was considerably better with either ABC2 or ABC3 than with ABC1. The RBS results were also checked for self-consistency by reflectivity and absorption experiments on the same CdS sample.