Damping Mechanism of Polaritons in GaP

Abstract

We have measured the temperature dependence of the polariton damping function ${\Gamma }_{\pi }({\omega }_{\pi },T)$ for the polariton frequency ${\omega }_{\pi }\simeq 345$ ${\mathrm{cm}}^{-1\mathrm{}}$ in GaP in the temperature range 2-300 °K. The temperature dependence of ${\Gamma }_{\pi }({\omega }_{\pi },T)$ 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 ${\mathrm{cm}}^{-1\mathrm{}}$ via the third-order anharmonic interaction. Inelastic-neutron-scattering data show that these frequencies correspond to acoustic phonons, $\mathrm{TA}\mathrm{}\left(X\right)\mathrm{}$ and $\mathrm{LA}\mathrm{}\left(X\right)\mathrm{}$, 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.