  In this work, nonlinear optical (NLO) studies of nickel oxide doped (ranging
from 0.2 to 1.0 mol%) Li2SO4-MgO-P2O5 glasses are reported. A combination of
femtosecond (fs) laser, as a pumping light source and a high-accuracy
polarimeter with low power probing laser, is used to investigate the
light-induced optical anisotropy (OA) in these glass materials. The
light-induced birefringence (LIB) exhibits slow relaxation tendency up to about
10 s suggesting on anisotropic photorefractive effect evidently dominated fast
Kerr effect. This behavior is evaluated in the light of other results reported
recently and is explained by the glass polymerization mechanisms. The
photorefractive birefringence increases with increase of the quantity of NiO up
to 0.8 mol% and it is attributed to the enhanced degree of depolymerization of
the glass network due to the hike in the concentration of Ni2+ ions that occupy
octahedral (Oh) positions. Further increase of NiO content (>0.8 mol%) causes,
however, a certain decrease of the photorefractive birefringence. Notable
change in concentration trend is interpreted by rising of phonon losses due to
increasing portion of the nickel ions occupying tetrahedral (Th) sites that
facilitates the polymerization of the glass network. The doped glasses with the
NiO content of about 0.8 mol% may be considered as optimal in the sense of
photorefractive efficiency. Relevant samples exhibit largest magnitudes of the
photorefractive birefringence and appear to be favorable for potential
applications.
