  Structural alterations in v-SiO2 induced by "thermal poling", a treatment
which makes the glass able to double the frequency of an impinging infrared
light, are revealed by neutron diffraction as a breakdown of the macroscopic
isotropy. This leads to concomitant changes in the vibrational density of
states measured by inelastic neutron scattering. The observations are found to
be consistent with the emergence of partial ordering within the glassy matrix
along the direction of an electrostatic field applied during the poling
treatment.
