  In this paper, we present interesting results on the quantification of
photodarkening (PD), photobleaching (PB) and transient PD (TPD) in a-
GexAs35-xSe65 thin films as a function of network rigidity. Composition
dependent light-induced responses of these samples indicate that there exist
two parallel competing mechanisms of instantaneous PD arising from the As part
of the network, and PB arising from the Ge part of the network. Raman spectra
of the as-prepared and illuminated samples provide first direct evidence of the
light-induced structural changes: an increase in AsSe3/2 pyramidal and GeSe4/2
corner-sharing tetrahedra units together with new Ge-O bond formation and
decrease in energetically unstable edge sharing GeSe4/2 tetrahedra.
Importantly, for a fixed Se concentration, Ge:As ratio plays the critical role
in controlling the net light-induced response rather than the much believed
rigidity of the glassy network.
