Light-induced giant softening of network glasses observed near the mean-field rigidity transition.

The longitudinal acoustic (LA) mode of bulk GexSe1-x glasses is examined in Brillouin scattering (BS) over the 0.15<x<1/3 range using near band-gap radiation (lambda=647.1 nm). The LA-mode frequency (nu(LA)) softens with increasing laser power in an athermal and reversible manner by nearly 30% (=Delta nu(LA)/nu(LA)) near x=x(c)=0.19(1) or mean coordination number, r(c)=2+2x(c)=2.38(2), close to the mean-field rigidity percolation transition (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.