Damping Mechanism of Polaritons in GaP

Abstract

We have measured the temperature dependence of the polariton damping function ${\ensuremath{\Gamma}}_{\ensuremath{\pi}}({\ensuremath{\omega}}_{\ensuremath{\pi}},T)$ for the polariton frequency ${\ensuremath{\omega}}_{\ensuremath{\pi}}\ensuremath{\simeq}345$ ${\mathrm{cm}}^{\ensuremath{-}1}$ in GaP in the temperature range 2-300 \ifmmode^\circ\else\textdegree\fi{}K. The temperature dependence of ${\ensuremath{\Gamma}}_{\ensuremath{\pi}}({\ensuremath{\omega}}_{\ensuremath{\pi}},T)$ shows that the predominant mechanism of polariton damping at this frequency is a decay process of the polaritons into two acoustic phonons of approximate frequencies 242 and 103 ${\mathrm{cm}}^{\ensuremath{-}1}$ via the third-order anharmonic interaction. Inelastic-neutron-scattering data show that these frequencies correspond to acoustic phonons, $\mathrm{TA}(X)$ and $\mathrm{LA}(X)$, near the $X$ point of the Brillouin zone. Thus the present experiment establishes the predominant source of the contributions to the imaginary part of the phonon self-energy.