Laser-induced periodic surface structures: Arbitrary angles of incidence and polarization states

We demonstrate that the finite-difference time-domain (FDTD) method can be used to study the formation of laser-induced periodic surface structures (LIPSSs) under oblique incidence and arbitrary polarization states. The inhomogeneous energy absorption below a rough surface at oblique incident angle is studied by the FDTD method and compared to the analytical efficacy theory developed by Sipe et al. [Phys. Rev. B 27, 1141 (1983)]. The two approaches show excellent agreement. The surface topographies of both low-spatial-frequency LIPSSs (LSFLs) and high-spatial-frequency LIPSSs (HSFLs) at oblique incidence and various polarization states (linear, circular, radial, and azimuthal) are simulated by taking into account topography-driven interpulse feedback effects. For s- and mixed s- and p-polarized beams at large incident angles, the simulated LSFLs show orientations that are neither perpendicular nor parallel to the laser polarization. Their physical origin is explained by a nonzero angle between the in-plane wave vector of the incident light and the scattered light. By using circularly-polarized beams, the simulated surface topographies are further shown to be in excellent agreement with the triangular LSFLs and the fine-dot nanoscaled structures reported in the literature. In addition, the simulation of HSFLs suggests that their periods have almost no dependence on the incident angle nor the polarization angle, while their surface topographies resemble that of blazed gratings at large incident angles for p-polarized and mixed s- and p-polarized beams. The origin is explained by the appearance of asymmetry in the scattered near-field light. The extension to oblique incidence and arbitrary polarization states demonstrates that the FDTD approach on LIPSSs is a highly versatile and powerful technique which has the potential to be one of the foundations for a complete modeling and understanding of LIPSSs under various irradiation conditions.