Incident angle influence on ripples and grooves produced by femtosecond laser irradiation of silicon

We report an experimental investigation on laser induced periodic surface structures formed on crystalline silicon at various angle of incidence θ, for p-polarization in air. The features of ripples and grooves formed in the shallow craters were analyzed for two different laser wavelengths λ (1026 nm and 513 nm). The period of the ripples displays the expected angular dependence for p-polarized light irradiation with the two different branches Λ±=λ/Reη∓sinθ, where η is the effective refractive index of the air-target interface. Instead, the grooves period shows a decreasing angular dependence well-described by a cosθ function. In addition, very regular ripples with subwavelength periodicity Λ− are formed at some characteristic angles for which the expected period Λ+ is very close to an integer multiple of Λ−. Simulations of the process at various angles were also carried out by using a multiscale numerical model in which the enhanced contribution of hydrodynamical effects were highlighted. The simulations predict angular dependencies of the surface structures period qualitatively similar to the ones observed in the experiments clearly supporting the influence of hydrothermal waves in grooves formation process.