Is frozen space charge responsible for SHG in poled silicate glasses only?

It was shown that, in thermal poling of silicate glasses, in parallel to the “frozen” electric field created by the “frozen” space charge formed in the depleted subsurface layers due to non-uniformly distributed mobile glass ions, dc electric field induces “frozen” ionic polarization of the media leading to the local symmetry lowering. Both “frozen” space charge and “frozen” ionic polarization contribute to the quadratic optical nonlinearity of the poled glasses (second harmonic generation). It was found that a fast decay of the “frozen” polarization takes place already at room temperature, while no relaxation of the space charge (actually, no leveling of glass composition in depleted layers) at room temperature is observed.