Evolution of surface relief gratings in As₂₀Se₈₀ amorphous chalcogenide films under laser illumination

Variation of grating amplitudes on a surface of amorphous chalcogenide films (ACF) As₂₀Se₈₀ has been studied under illumination by a band-gap light with the purpose to understand mechanism of photo-induced (PI) mass transfer. After holographic recording of surface relief gratings (SRGs) of various periods Λ (from 3 to 15 µm) they were illuminated by a diode laser (λ = 660 nm) and their profile variation was analyzed using optical microscopy, atomic force microscopy, light scattering, and optical profilometry. The SRGs with Λ < 8 µm exponentially flattened with time of illumination, whereas amplitudes of the SRGs with Λ > 8 µm exponentially grew. Theoretical analysis of the kinetics of PI mass transfer shows that the SRG profile variation occurs by bulk diffusion of As and Se atoms as a result of competition between capillary forces and electrostatic forces created by redistribution of electrons and holes generated by light. Our theory predicts both flattening and growth of the SRG amplitudes under uniform light illumination. The PI diffusion coefficients have been estimated both from the flattening kinetics of the SRG amplitude and from its growth and found to be in good agreement with the theory. It is also observed that the profile evolution is accompanied by dewetting of ACFs and roughening of the initial SRGs.