Surface structures induced by ultrashort laser pulses: Formation mechanisms of ripples and grooves

We report a detailed theoretical and experimental analysis of the surface structures induced by ultrashort laser pulses on silicon. We address two issues seldom considered in the literature: i) bending and bifurcation of surface ripples and ii) the mechanism of microgrooves formation. Our experimental results illustrate the diverse morphological features of surface structures, which are explained by taking into account the variation of the dielectric constant of the target surface at different excitation levels, by means of a theoretical approach based on the combination of two-temperature model, free-carrier dynamics and Sipe model. In particular, calculated spatial distributions of the deposited laser energy on the target surface provide direct evidence of the diverse morphological characteristics of the produced structures, eventually explaining ripples bending and bifurcation phenomena, as well as grooves formation. The good agreement between model predictions and experimental findings suggests that the proposed approach ascertains the main physical mechanisms underlying both ripples and grooves formation, and influencing their fine morphological features. In addition to providing a deeper understanding of the mechanisms involved in femtosecond direct laser surface processing, our results highlight interesting ways of implementing the design of surface structures of applicative interest.