Near-forward Raman selection rules for the phonon-polariton in (Zn, Be)Se alloys

The Raman selection rules of the (Zn-Se, Be-Se)-mixed phonon-polariton ( PP) created by “alloying” in the three-mode [1 × (Zn-Se), 2 × (Be-Se)] Zn1−xBexSe system, whose dramatic S-like dispersion (∼200 cm−1) covers the large frequency gap between the Zn-Se and Be-Se spectral ranges, are studied in its wave vector ( q) dependence by near-forward Raman scattering. Both the “collapse” regime away from the Γ point (q = 0) and the “reinforcement” regime near Γ are addressed, using appropriate laser lines and Be contents. We find that in both regimes the considered PP, in fact a transverse mode with the mixed mechanical-electrical character, obeys the same nominal Raman selection rules as its purely mechanical variant observed in the standard backscattering geometry. The discussion is supported by contour modeling of the multi- PP Raman lineshapes in their q-dependence within the linear dielectric approach.