  Little is known about the role of network rigidity in light-induced
structural rearrangements in network glasses due to a lack of supporting
experiments and theories. In this report, we demonstrate for the first time the
ultrafast structural rearrangements manifested as induced absorption (IA) over
a broad spectral range in a-GexAs35-xSe65 thin films above the mean-field
rigidity percolation transition, quantified by the mean coordination number <r>
= 2.40. The IA spectrum arising from self-trapped excitons, induced structural
rearrangements by softening the glass network that strikingly reveal two
relaxation mechanisms which differ by one order of magnitude. The fast kinetics
of electron-lattice interaction occurs within 1 ps, exhibits a weak dependence
on rigidity and dominates in the sub-bandgap region. In a stark contrast, the
slow kinetics are associated with the structural changes in the bandgap region
and depends strongly on network rigidity. Our results further demonstrate that
amplitude of IA scales a linear relationship with excitation fluence which
provides a unique way to induce structural rearrangements in over-coordinated
network to exploit it for practical purposes. Our results modify the
conventional concept of rigidity dependence of light-induced effects in network
glasses, when excited with an ultrafast laser.

Materials Science