Electrostatic Interpretation of Phase Separation Induced by Femtosecond Laser Light in Glass

Abstract

Numerous studies on the effect of the femtosecond laser pulses in oxide glasses have been achieved over the last two decades, and several specific effects pointed out. Some of them are classical with respect to a laser treatment, such as thermally related effects, and are widely taken into account for applications. Other effects are directly induced by light, caused by its intricated spatiotemporal structure and associated properties: ponderomotive and polarization effects or coherence within the focal volume. These effects enable the development of forces that can lead to orientation effects. Among the specific resulting transformations from the light-induced effects in glass, the formation of so-called nanogratings was first pointed out in 2003 in silica glass. From this date, asymmetric organization into parallel nanoplanes, perpendicular to the laser polarization, have been found in many vitreous and crystalline compounds. While it is accepted that they arise from the same origin, i.e., a plasma organization that is eventually imprinted inside the material, uncertainties remain on the formation process itself. Indeed, since it exists several categories of nanogratings based on the final structuring (nanoporous phase separation, crystallization, and nanocracks), it can be expected that several processes are at the roots of such spectacular organization. This paper describes an approach based on electrochemical potential modified by an electronic excitation. The electric field induced during this process is first calculated, with a maximum of ~4500 kV/µm and a distribution confined within the lamella period. The maximal chemical potential variation is thus calculated, in the studied conditions, to be in the kJ/mol range, corresponding to a glass-to-crystal phase transition energy release. The kinetics aspect of species mobility is subsequently described, strengthening the proposed approach.