Resonant Raman scattering and free-exciton emission in CuGaS2 crystals

Origins of lines appearing in the exciton region spectra of CuGaS2 crystals at low temperatures under 4579, 4765 or 4880 A excitation of an Ar+ laser are discussed in terms of emissions from upper- and lower-branch excitonic polaritons (UBPs and LBPs) and multi-phonon Raman processes. The line at 2.500 eV is an emission from LBPs. The line at 2.504 eV seen in our samples under 4765 A excitation is shown to be interpretable as a three-phonon Raman process in resonance with UBP states, while the line reported at the same energy by Shirakata is likely to be an emission from UBPs.