Light-Induced Giant Softening of Network Glasses Observed near the Mean-Field Rigidity Transition

The longitudinal acoustic (LA) mode of bulk ${\mathrm{Ge}}_x{\mathrm{Se}}_{1-x}$ glasses is examined in Brillouin scattering (BS) over the $0.15<x<1/3$ range using near band-gap radiation ($\lambda =647.1\mathrm{nm}$). The LA-mode frequency (${\nu }_{\mathrm{LA}}$) softens with increasing laser power in an athermal and reversible manner by nearly 30% ($=\Delta {\nu }_{\mathrm{LA}}/{\nu }_{\mathrm{LA}}$) near $x=x_c=0.19(1)$ or mean coordination number, ${\mathbf{r}}_c=2+2x_c=2.38(2)$, close to the mean-field rigidity percolation transition (${\mathbf{r}}_t=2.40$). BS is a bulk probe of elasticity, and photosoftening is maximized here when network stress is minimized near the elastic phase transition.