  The formation of a new type of laser-induced periodic surface structures
using a femtosecond pulsed laser is studied on the basis of the Sipe-Drude
theory solved with a FDTD scheme. Our numerical results indicate the
possibility of coexisting structures parallel and perpendicular to the
polarization of the incident light for low reduced collision frequency
($\gamma/\omega \lesssim 1/4$, where $\omega$ is the laser frequency).
Moreover, these structures have a periodicity of $\Lambda \sim \lambda$ in both
orientations. To explain this behavior, light-matter interaction around a
single surface inhomogeneity is also studied and confirms the simultaneous
presence of surface plasmon polaritons and radiation remnants in orthogonal
orientations at low $\gamma/\omega$ values.
