Abstract
Light-induced effects in a-GexSe100-x chalcogenide glasses, i.e., photobleaching (PB), photodarkening (PD), and photoinduced structural transformations, have been investigated as a function of composition across the glass-forming region by an optical two-laser-beam technique, Raman analysis, and first-principles simulations. It was found that there is a critical concentration of Ge x≈30% that corresponds to the crossover from transient PB to the mixture of transient PD and metastable PB. At the microscopic level, this corresponds to the change in the photoexcitation process. At low-Ge concentration ( 30%) favors breakage of the Ge-Ge bonds upon photoexcitation and formation of light-induced 3D nanostructures. The bond conversion process is verified by Raman analysis.
Highlights
Photoinduced effects (PIEs) in chalcogenide glasses (ChG) have a long history of continuing interest due to the potential they hold for new, unexplored applications
The observed diversity of PIEs can be ascribed to the unique electronic and atomic structures and lack of periodicity in ChG [7,8,9,10,11,12], as well as to a unique feature of the availability of states in the band gap associated with the lone pair (LP) electrons located at chalcogen atoms [7]
By means of the two-laser-beam technique, the critical concentration of Ge x≈30% was found, reflected in the crossover from transient PB to the mixture of transient PD and metastable PB; this can be linked to the transition from the close-to-intermediate phase to the stress-rigid phase
Summary
Photoinduced effects (PIEs) in chalcogenide glasses (ChG) have a long history of continuing interest due to the potential they hold for new, unexplored applications. There are two alternative models proposed: one model exploits the excitation-triggered, local structural reordering of homopolar bonds to heteropolar ones, and the second model links the effect to the photooxidation, i.e., Ge-O bond formation [1,18,19] The latter effect is difficult to observe directly with available methods of structural analysis such as Raman spectroscopy because of the limited amount and low vibration intensity of the oxygen-containing bonds. Not rejecting the surface photo-oxidation process, we confirm with first-principles simulations that experimentally observed metastable PB in Ge-rich glasses (x>30%) can be accounted for by photoinduced structural transformations. The validity of this assumption is confirmed with Raman analysis tracking the photo-structural changes
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