Photoinduced metastability in Ag-containing chalcogenide glasses

Photodoping of Ag metal (photodissolution), photoinduced surface deposition (PSD) of Ag deposits and photoinduced chemical modification (PCM) of the Ag concentration have been reported for Ag-containing chalcogenide glasses. These phenomena are closely related to the thermodynamic metastability of the glasses, since Ag+ migration by illumination causes the structural change. For photodoping of Ag/As–S sample, the solid-state reaction between Ag metal and As–S glass decreases the total free energy of the system (unstable → stable). The segregation of Ag metal by PSD decreases the total free energy and the process is also represented by ‘unstable → stable’. Inversely, PCM increases the total free energy and the process is ‘stable → unstable’, in contrast to the other phenomena. The results agree with the data that photodoping and PSD are irreversible reactions but PCM is reversible. The other basic data can also be explained in a coherent way from the photoinduced free-energy change.