Frequency-dependent damping constant and the linewidth of resonant Brillouin scattering of exciton-polaritons in CdS

The linewidths of Brillouin components involving lower-branch $A$-exciton polaritons in the resonant-Brillouin-scattering (RBS) spectrum of CdS have been carefully measured and analyzed. It was found that the exciton damping constant $\gamma$ must be frequency dependent to explain the observed increase of the linewidth in the resonance region. In particular we suggest that mixing of the ${\Gamma }_5\mathrm{}\left(A\right)\mathrm{}$ polariton and ${\Gamma }_6\mathrm{}\left(A\right)\mathrm{}$ exciton states induced by the $k$-linear interaction is the most probable explanation for the observed sharp increase of the linewidth slightly above the exciton resonance frequency. Interpretation of our linewidth data based on this mechanism led to a coefficient for the $k$-linear term for the $A$ exciton of ${\phi }_A=(1.9\mathrm{}\pm 0.2)\times {10}^{-11\mathrm{}}$ eV cm. The frequency-dependent damping constant $\gamma \left(\omega \right)$ must also be used to analyze the intensity of RBS which is related to the additional-boundary-condition (ABC) problem. Preliminary results of intensity fitting using a generalized form of the ABC are presented.