Résumé
Hyper Raman scattering (HRS) is a non-linear optical spectroscopy in which two incident photons at frequency ωI give a scattered one at ωS after interaction with an excitation of frequency ω in the media. The main interest of this technique arises from its selection rules different from the classical Raman scattering and Infrared absorption. The hyper-Raman spectroscopy of vitreous silica has been performed and the results have been compared to Raman and Infrared absorption data. The spectral responses have been analyzed within a simple structural model considering motions of Si-O-Si bonds and of SiO4 tetrahedra. Most of the HRS bands can be assigned by motions from one or the other of these two elementary structural units. It includes the boson peak (BP), an excess of vibrational modes at low frequency, characteristic of the disordered state and whose origin remains strongly debated. The HRS BP has been successfully decomposed into libration motions of rigid tetrahedra and into motions originating from fluctuating charges in the glass. Interestingly, both contributions are different in shape and frequency. The former, which largely dominates the hyper-Raman signal, superposes the neutron BP while the latter mimics the Raman signal. This result clarifies the nature of the modes underlying the BP of v-SiO2 and gives a natural explanation for its different signatures in neutron and Raman. The spectroscopy also highlights a new polar component in the high frequency phonon spectrum of v-SiO2, and supports previous numerical simulations predicting that the modes associated to the lowest frequency polar band are strongly delocalized.