Coherent scattering of exciton polaritons and acoustic phonons in a ZnO single crystal

The light reflection oscillations resulting from the coherent scattering of polariton states and propagating coherent acoustic phonons (CAPs) in a ZnO single crystal are studied by using a two-color femtosecond pump-probe technique. The oscillation frequency manifested in the photoinduced reflectance shows a nonlinear dependence on the probe-beam wave vector owing to the variation of the refractive index near the band gap. The oscillation amplitude is affected by the refractive index modulation derived from the CAP-detuning polariton states. The larger oscillation for shorter probe-beam wavelengths is consistent with the stronger scattering between the CAP and excitonlike polariton states closer to the band gap.