RESONANT PHENOMENA OF HYPER-RAMAN-SCATTERING OF OPTIC PHONONS IN A TIO2 CRYSTAL

We report two-photon-resonant phenomena of hyper-Raman-scattering by optic phonons in TiO2. It is found that the scattering efficiency of the allowed mode in the 90-degrees-scattering geometry increases remarkably as twice the incident photon energy, 2HBAR-omega(i), approaches the direct forbidden energy gap from below. In addition, the configuration-forbidden LO mode in the backward-scattering geometry is found to show a sharp two-photon-resonant behavior at the band edge. The prominent polarization dependence of the phenomena is observed, reflecting the anisotropic nature of TiO2. Further, the absolute value of the scattering efficiency is estimated as a function of 2HBAR-omega(i). It is shown that these results can be interpreted quantitatively with a model on the basis of the electronic band structure. In the course of the analysis, we have estimated the second-lowest-direct-gap energy as 3.10 eV. Finally, we emphasize that the resonant hyper-Raman spectroscopy would be useful as a method for studying the band structure in solids.