Surface-radiative modes and longitudinal excitons in the spectra of exciton–polariton luminescence

Low-temperature ($T=2$ K) spectra of exciton–polariton luminescence in the vicinity of the exciton resonance frequency $A_{n=1}$ of $\mathrm{CdS}$ crystals are studied with allowance for the damping $\hbar\Gamma$ of mechanical excitons. The results of numerical calculations of the partial and interference contributions of bulk and surface-radiative spectral modes in the geometry of the radiation of additional waves into a vacuum with $s$ and $p$ polarizations are analyzed. It is shown that the contributions of purely longitudinal excitons and their interference with polaritons of the upper dispersion branch in luminescence in the vicinity of longitudinal frequency $\omega_L$ are small $(\sim10-30\%)$. Nevertheless, these contributions should be taken into account to achieve a quantitative agreement with the experiment; they determine the formation of an additional $A'_{L}$ line under oblique radiation.