Resonant scattering of exciton polaritons by LO and acoustic phonons

Abstract

A change in the shape of the Raman scattering spectrum in CdS is observed when the exciting laser frequency ${\omega }_i$ is tuned across the $A$-exciton resonance. When ${\omega }_i$ is below resonance a single LO peak is present in the forbidden Raman spectrum [$x\left(\mathrm{yy}\right)\mathrm{}\overline{x}$]. Under resonance conditions two contributions to the spectrum can be distinguished: (i) narrow peaks whose Raman shifts change with the exciting frequency superimposed on (ii) a broad luminescence band which is stationary in frequency. The narrow peaks are interpreted as acoustic-phonon interbranch exciton-polariton scattering followed by LO phonon emission. Their frequency shifts and shapes are consistent with this explanation when the anisotropy of the piezoelectric acoustic phonon-exciton interaction is included. The sudden appearance of these two phonon modes when the exciting frequency is slightly above the $A$-exciton frequency can only be understood within an exciton-polariton picture. This picture must also be invoked in order to explain the shape of the broad luminescence band which is the LO replica of the free-exciton luminescence.