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

The longitudinal acoustic (LA) mode of bulk GeₓSe_1-x glasses is examined in Brillouin scattering (BS) over the $0.15<x<1/3$ range using near band-gap radiation (λ=647.1 nm). The LA-mode frequency (νLA) softens with increasing laser power in an athermal and reversible manner by nearly 30% (=ΔνLA/νLA) near x=xc=0.19(1) or mean coordination number, rc=2+2xc=2.38(2), close to the mean-field rigidity percolation transition (rₜ=2.40). BS is a bulk probe of elasticity, and photosoftening is maximized here when network stress is minimized near the elastic phase transition.