RESONANT SCATTERING OF EXCITON POLARITONS IN ZNP2 CRYSTALS

Secondary emission of monoclinic zinc diphosphide is investigated under excitation into the exciton resonance region at 2 K. Distinct emission lines are observed in the vicinity of the is exciton energy, shifted from the excitation energy by the longitudinal-optical (LO) phonon energies. It is shown that these lines are caused by inelastic scattering of exciton polaritons by the LO phonons with nearly equal energies of 32.2 and 31.3 meV. The LO lines show the so-called ''outgoing resonance''; the lines are enhanced resonantly as they approach the energy of a transverse or longitudinal exciton. The observed phonon energies are related to the exciton damping constants derived from temperature dependence of the reflection spectra. It is concluded that among numbers of phonons, the LO phonons of 32 meV govern the relaxation processes of exciton polaritons in ZnP2.