Resonant hyper-Raman scattering due to the forbidden LO phonon in SrTiO3

The resonant behavior of the hyper-Raman scattering in a solid is reported for the first time. The scattering efficiency for the LO phonon normally forbidden in the backward geometry in SrTi${\mathrm{O}}_3$ is found to increase strikingly as twice the incident photon energy $2\hslash {\omega }_i$ approaches the lowest direct band gap around 3.46 eV from below. It turns out that the polarizations of incident and scattered light are not parallel to each other, which contrasts with the corresponding Raman case. An explanation in terms of the intraband Frohlich interaction is tentatively presented.