- Thermodynamic and structural characterization.
We developed a set-up to perform calorimetric studies of chalcogenide samples under laser irradiation. A series of laser diodes in the visible range can be used in a setup allowing selection of the beam spot size and of the light intensity, and thus of the power density at the sample position. The sample is mounted on a Mettler-Toledo Flash DSC (FDSC) chip enclosed in an external cell in inert-gas atmosphere. The use of the FDSC allows us to prepare easily and reproducibly samples with the desired quenching rate. The calorimetric trace collected up to above Tg in comparison to that collected out on the pristine sample is then used to extract the change in enthalpy of the sample following irradiation. The experiments are carried out both as a function of the irradiation time (dose) and as a function of the quenching rate used to prepare the glass. The results obtained so far show that laser irradiation leads to an increase (rejuvenation) of the enthalpy of the glass, i.e. to glass rejuvenation. Moreover, the enthalpy of all samples, upon sufficient irradiation, converges towards that of unique, stationary state. This is an ergodic state with a viscosity characteristic of a supercooled liquid, and is a nice example of a non-equilibrium state where flow is due to the continuous generation of defects in the material induced by absorption of the laser radiation and not by temperature. Temperature remains fixed in fact close to the ambient value during the whole process.
In parallel to this activity, we are carrying out a structural characterization of these irradiated glasses using X-ray diffraction at the ESRF and Petra III synchrotron radiation sources in Grenoble (F) and Hamburg (D), respectively. The data collected so fa confirm the trend seen in the thermodynamic measurements. In particular, upon irradiation we observe continuous structural changes that, for a long enough irradiation, converge towards a stationary state. These changes correspond to a progressive broadening and decrease in intensity of the peaks of the structure factor, which signals a more and more disordered structure.
2. Dynamical characterization.
The goal here is to obtain information on dynamical properties of chalcogenide glasses upon IR irradiation which, when coupled to the thermodynamic and structural information discussed above, can help us to go deeper into the mechanisms behind glass stability and the effect of laser irradiation. In order to do so, we have designed, realized and commissioned a novel setup for dynamic light-scattering in the IR range (the chalcogenide glasses of interest here are opaque in the visible range). Experiments based on this setup are currently underway.