In-situ study of athermal reversible photocrystallization in a chalcogenide glass

Abstract

The time-resolved Raman measurements reveal a three-stage mechanism of the photostructural changes in Ge25.0Ga9.5Sb0.5S65.0 (containing 0.5 at. % of Er3+) glass under continuous-above-bandgap illumination. These changes are reversible and effectively athermal, in that the local temperature rises to about 60% of the glass-transition temperature and the phase transitions take place in the glass/crystal and not in an equilibrium liquid. In the early stages of illumination, the glassy-network dimensionality changes from a predominantly 3-D to a mixture of 2-D/1-D represented by an increase in the fraction of edge-sharing tetrahedra and the emergence of homonuclear (semi)metallic bonds. This incubation period of the structural rearrangements, weakly thermally activated with an energy of ∼0.16 eV, facilitates a reversible photocrystallization. The photocrystallization rate in the glass is comparable to that achieved by thermal crystallization from supercooled liquid at large supercooling. Almost complete re-amorphization can be achieved in about an hour by reducing the incident laser-power density by a factor of ten. Glass-ceramic composites—with varying glass-to-crystal fraction—can be obtained by ceasing the illumination during re-amorphization. Microstructural imaging reveals photoinduced mass transport and the formation of columnar-porous structures. This shows the potential for a bond-specific engineering of glassy structures for photonic applications with a spatial resolution unachievable by thermal annealing.

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

“…Two general themes have evolved to explain the light-induced effects: the first one emphasizes the aspects of the local structure and the second one stresses on the global structure [16]. In the former theme, bandgap light illumination produces electron-hole states known as excitons that get self-trapped in the deformable glassy network.…”

Athermal Photoelectronic Effects in Non-Crystalline Chalcogenides: Current Status and Beyond

“…A recent detailed study of athermal photoinduced transitions between the amorphous and the crystalline states has been conducted by Benekou et al [143] for the multicomponent bulk glass Ge25.0Ga9.5Sb0.5S65.0 containing 0.5 at.% of Er 3+ using above-bandgap laser illumination. Essentially, this the first study reporting Raman shift [cm 1 ]…”

“…Figure 17 (left) shows the Raman spectrum "1" of the well-equilibrated glass recorded at low light fluence unable to engender photostructural changes. The spectrum is fitted with various lines, each one assigned to particular species that can be found in the structure at reasonable concentrations for the glass composition under study [143]. CST and EST stand respectively for corner-sharing and edge-sharing tetrahedra.…”

“…Reversible, optically-driven athermal phase changes between a crystal and its disordered state have been demonstrated in a few cases. Recent findings [143] of an all-optical-based mechanism eliciting the reversible transition: glassy(amorphous) ↔ (metastable) crystalline ↔ reamorphized solid, are of particular interest and certainly deserve more systematic studies. The interest particularly arises from our poor, yet, understanding of the second step of the transition stated above.…”