Ultrafast light-induced softening of chalcogenide thin films above the rigidity percolation transition

TL;DR

This study reveals ultrafast light-induced structural changes in chalcogenide thin films above the rigidity percolation threshold, showing two distinct relaxation mechanisms and a linear relationship between absorption amplitude and excitation fluence.

Contribution

It provides the first experimental evidence of ultrafast structural rearrangements related to network rigidity in chalcogenide glasses, challenging previous assumptions.

Findings

Ultrafast structural rearrangements occur within 1 ps.

Two relaxation mechanisms with different dependence on rigidity.

Absorption amplitude scales linearly with excitation fluence.

Abstract

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.