Abnormal period of laser-induced periodic surface structures on thin metal films impacted by substrate optical properties

Laser-induced periodic surface structures (LIPSS) are essential for surface functionalization applications, where microstructuring with a certain submicron period is important. LIPSS on metal substrates are expected to be about the laser wavelength λ in period, but in this work we show both theoretically and experimentally that on the thin enough metal films their period changes to λ⁄n sub , effectively placing them into LSFL-II category usually attributed to LIPSS appearing on dielectrics. We present a new phenomenological model that describes the surface electromagnetic wave appearance and propagation along the film-substrate interface and explains the LIPSS formation in the wave nodes distanced at λ⁄n sub period. These findings show that it is possible to expect the average LIPSS periods to vary significantly in the range from about 1470 nm (i.e., Ho:YAG laser, Fluorspar substrate) down to 130 nm (i.e., 4th harmonics Nd:YAG laser, Kapton substrate) simply by changing the laser wavelength or the substrate. The research results could be instrumental for the advancement and development of new functional surface patterning methods.