Ripples in amorphous chalcogenide films under homogeneous laser illumination

• Formation of ripples occurs due to photo-induced coarsening of surface roughness.
• Mass transfer occurs under competition of electrostatic and capillary driving forces.
• Spectral harmonics are smoothed or enhanced depending on their wavenumber.
• Ripple period corresponds to wavenumber of harmonics growing with maximum rate.

Under homogeneous illumination of thin chalcogenide glass films by polarized light at a wavelength near the band gap ripples form, with a period of the order of 10–15 µm, directed normal to the light polarization. The formation of the ripples cannot be explained by interference phenomena, which predict the ripple periods of the order of light wavelength. Our experimental and theoretical studies of the ripple formation in 1 µm thick As10Se90 chalcogenide films show that the profile variation occurs due to lateral mass transport accelerated by light. The ripple formation is caused by competition between capillary forces and steady state electrostatic forces induced by redistribution of electrons and holes generated by light. Under these driving forces, each harmonic of the film roughness spectrum should exponentially grow or flatten, depending on its frequency. The average period of the ripples corresponds to those harmonics in the roughness spectra, which grow with maximum rate. Light-induced diffusion coefficients have been estimated from the kinetics of the ripple formation.