Evidence of Microscopic-Scale Modifications in Optical Glasses   Supporting Second Harmonic Generation
  We explore, by means of inelastic neutron scattering, the extent of changes in dynamic correlations induced by thermal poling of vitreous $SiO_2$. The measured vibrational density of states shows an excess of modes in certain frequency regions as well as a narrowing of the 100 meV peak. These findings indicate that such alterations cannot be ascribed to the appearance of new well defined vibrational modes, such as those coming from localized topological defects, but rather arises from an increase in ordering in the material as attested in a reduced spread of the inter-tetrahedral angles. 
C. Cabrillo, G.J. Cuello, P. Garcia-Fernandez, F.J. Bermejo, V.   Pruneri, F. Samoggia, P. G. Kazansky and S.M. Bennington