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

23 Jan 2018

Time-resolved Raman measurements reveal a three-stage mechanism of 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 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 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 illumination during re-amorphization. Microstructural imaging reveals photoinduced mass transport and formation of columnar-porous structures. This shows the potential for bond-specific engineering of glassy structures for photonic applications with a spatial resolution unachievable by thermal annealing.