Study of hyper-Raman scattering excited by dye-laser for spectroscopic use in solids

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1. For the first time, we have succeeded in observing the hyper-Raman signals of optic phonon in solids by using a repetitively pulsed dye laser. For this purpose, we have developed a high-sensitive detection-system.2. Thereby, we have for the first time studied experimentally the frequency dispersion of the hyper-Raman scattering efficiency in Tio_2 and SrTiO_3 crystals, in such a range that the relevant photon energies, i.e., twice the incident one, 2hwi and the scattered one, hws are across the fundamental absorption edge. As the result, it is found that the efficiency of the allowed phonon modes in the 90ﾟ scattering geometry increases remarkably, as 2hwi approaches the band gap Eg from below. This fact will enable one to perform more sensitive hyper-Raman experiments in the future.3.We have also shown that the above result can be reasonably interpreted in terms of a two-photon ongoing resonance with Eg.4. We have found that the forbidden longitudinal phonon normally forbidden in the backward scattering geometry in a rutile crystal shows a sharp and striking two-photon resonant behavior at the band gap. The origin may be attributed to the resonance with the excitons due to the q-dependent- Fro^^"lich interaction. we have also observed a similar resonant behavior due to the LO phonon in SrTiO_3. we have interpreted this phenomenon as arising from the Fro^^"lich interaction.5. As a consequence, the resonant hyper-Raman spectroscopy in the above sense is a promising technique to unravel specific critical points of electronic band structure in solids, particularly in the ultraviolet region. It is noted that a photon energy of half the optical transition energy is sufficient.