Observation of anomalous acoustic phonon dispersion in SrTiO_3 by broadband stimulated Brillouin scattering

摘要

Femtosecond laser pulses have been used to generate and detect longitudinal acoustic phonons along the [001] direction of SrTiO_3 single crystals coated with a thin absorbing layer. Two-dimensional maps of the reflectivity as a function of probe wavelength and time delay display persistent oscillations, from which the frequency-dependent group velocity of the photoinduced strain pulse can be directly measured. By applying the theory of stimulated Brillouin scattering we found that the longitudinal sound speed varies from 7900 to 7520 m/s in the 60-74 radμm~(-1) wave vector range, thus indicating a clear deviation from a linear phonon dispersion relationship.