Impulsive excitation of phonon-pair combination states by second-order Raman scattering

The excitation of(100)-oriented KTaO3 with 25-fs laser pulses impulsively d rives phonon-pair combination states via second-order Raman scattering. Osc illations in the phonon-amplitude covariance at the sum and difference freq uency of the two involved phonons are observed in a spectrally and temporal ly resolved pump-probe experiment. Transmission changes of the sample are d ominated by contributions of wave vector conserving phonon-pair combination s from the entire Brillouin zone that have maxims in their combined density of states. For low temperatures the temperature dependence of the covarian ce oscillations of different phonon combinations is reproduced by a quantum -mechanical model.