Evidence of Microscopic-Scale Modifications in Optical Glasses Supporting Second Harmonic Generation

TL;DR

This study investigates microscopic modifications in optical glasses caused by thermal poling, revealing increased ordering and changes in vibrational properties that support second harmonic generation.

Contribution

It provides evidence that thermal poling induces increased microscopic order in vitreous silica, affecting vibrational states without creating new localized vibrational modes.

Findings

Excess vibrational modes in specific frequency regions

Narrowing of the 100 meV vibrational peak

Increased ordering evidenced by reduced inter-tetrahedral angle spread

Abstract

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.