Frequency and temperature dependence of the TO phonon-polariton decay in GaP

The temperature and frequency dependence of the decay time of the transverse optical (TO) phonon polariton in GaP has been measured by impulsive stimulated Raman scattering with 25-fs optical pulses. Tuning of the polariton frequency from 365 to $361 {\mathrm{cm}}^{\ensuremath{-}1}$ reveals drastic changes of the polariton decay time with temperature. This result is explainable by third-order anharmonic decay of polaritons into two acoustic phonons taking into account the temperature-dependent shift of the van Hove singularity in the two-phonon density of states of the acoustic phonons at the edge of the Brillouin zone. The frequency dependence of the TO phonon-polariton decay times measured at different temperatures allows one to directly observe the shift of this singularity with temperature.