Near-forward Raman selection rules of the phonon-polariton created by alloying in (Zn,Be)Se

TL;DR

This study investigates the near-forward Raman selection rules of phonon-polaritons in alloyed (Zn,Be)Se, revealing their wave vector dependence and how electrical field development affects Raman lineshapes in different regimes.

Contribution

It provides new insights into the wave vector dependence of phonon-polariton Raman selection rules in alloyed semiconductors and models their lineshapes using a linear dielectric approach.

Findings

Phonon polaritons obey the same Raman selection rules as mechanical modes.

Electrical field E influences intramode and intermode oscillator strength transfers.

Distinct lineshape differences reveal E development near the Brillouin zone center.

Abstract

The Raman selection rules of the (ZnSe, BeSe) mixed phonon polariton created by alloying in the three mode (1ZnSe, 2BeSe) ZnBeSe system, whose dramatic S like dispersion covers the large frequency gap between the ZnSe and BeSe spectral ranges, is studied in its wave vector dependence by near forward scattering. Both the collapse regime away from the Brillouin zone centre and the reinforcement regime near the Brillouin zone centre are addressed, using appropriate laser lines and Be contents. We find that in both regimes the considered phonon polariton, in fact a transverse mode with mixed mechanical and electrical character, obeys the same nominal Raman selection rules as its purely mechanical variant commonly observed in the backscattering geometry. Besides, marked differences in the phonon polariton Raman lineshapes in the two regimes give a hint about how the phonon polariton electrical field E develops while descending the S like dispersion towards the Brillouin zone centre. In the reinforcement regime E is large, leading to intramode on top of intermode transfers of oscillator strength mediated by E between the two BeSe modes, that both exhibit a fine structure on account of the alloy disorder. In contrast, in the collapse regime E remains weak, as testified by the absence of intramode transfer. The discussion is supported by contour modeling of the multi phonon polariton Raman lineshapes in their wave vector dependence within the linear dielectric approach.