Accurate determination of the low frequency dielectric dispersion in SrTiO₃ by hyper-Raman scattering

Abstract

The hyper-Raman effect is utilized to study infrared-active, but Raman-forbidden modes by light scattering. We have accurately measured the lowest and highest frequency hyper-Raman line of SrTiO₃ at room temperature with a spectral slit width of less than 4 cm⁻¹. Applying the fluctuation-dissipation theorem we relate the hyper-Raman lineshapes to the dielectric function ϵ (Ω). Absolute values of ϵ (Ω) are deduced for the frequency range of the ferroelectric soft mode. We compare our results with far-infra-red data and discuss the soft mode self-energy calculated from our ϵ (Ω).