  Formation of nanoscale laser-induced periodic surface structures on thin
metal films (of the size of the optical penetration depth) is a yet unexplored
area that is expected to open new routes for laser patterning and a wealth of
exciting applications in optics, photonics, and sensing. In contrast to the
common belief that excitation of Surface Plasmon Polaritons (SPPs) on the
air/metal interface plays the dominant role in the features of the induced
topographies, in this work, we demonstrate that the excitation of coupled SPPs
in both air/metal and metal/substrate interfaces, along with other parameters
such as the thickness of the material, the photon energy, and the substrate
refractive index, dictate the spatial modulation of the absorbed energy. A
detailed theoretical analysis of the excited plasmonic waves and a multiscale
modelling of laser-induced physical phenomena manifests that depending on the
laser conditions and thickness of the irradiated solid, topographies with
periodic features of diverse sizes (ranging from $\lambda_L/3$ to $\lambda_L$,
where $\lambda_L$ stands for the laser wavelength) and different orientation
can be realized. The capability to control and tune the characteristics of the
produced structures on thin films is expected to enable novel surface
patterning approaches.
