Experimental observations of photo-induced bond conversions in GeAsS thin films

• Photo-induced dichroism is investigated in Ge25As30S45 glassy thin-films.
• Polarized micro-Raman spectra demonstrate heteropolar to homopolar bond conversion.
• The existence of a photo-induced local heating effect is demonstrated experimentally.
• The possibility of dynamic bond conversions by the laser local heating is shown.

Photo-induced anisotropy phenomena have been investigated in Ge25As30S45 chalcogenide glassy thin-films prepared by the electron beam evaporation technique. Thin-films of thicknesses ranging from 3 to 7 μm were prepared, annealed slightly below the glass transition temperature Tg and characterized by means of optical transmission, thermal analysis, elemental microanalysis through energy dispersive X-ray spectroscopy and by micro-Raman spectroscopy. Photo-induced dichroism (PID) was studied using band gap 514 nm laser excitation. Its kinetics was studied by observing the transmission and reflection of the film at different excitation intensities and temperatures. The conversion of homopolar (Ge–Ge, As–As) toward heteropolar (Ge–S, As–S) bonds was confirmed by polarized micro-Raman spectroscopic studies.A phenomenological unipolar model, that suggests the presence of photo induced bond changes through an energetic barrier (estimated at 14 kJ/mol), is proposed to explain the experimentally observed non-monotonic behavior of PID.