Evidence of microscopic-scale modifications in optical glasses supporting second harmonic generation

We explore the extent of changes in dynamic correlations of vitreous SiO2 induced by treatments required to enable frequency doubling of infrared light (poling) by means of inelastic neutron scattering. These appear as an excess of modes in certain regions of the frequency spectrum as well as a narrowing of the 100 meV peak. Such alterations are better ascribed to a change in ordering of the material than to the emergence of "defect" modes, absent in the native sample. (C) 1998 Elsevier Science B.V.