Laser-induced deep-subwavelength periodic nanostructures with large-scale uniformity

Femtosecond lasers are capable of fabricating uniform periodic nanostructures with a near-wavelength periodicity; however, it is challenging to produce subwavelength nanostructures with large-scale uniformity. Here, we investigate femtosecond laser-induced self-assembly of periodic nanostructures on Si-on-Pt hybrid ultrathin films via photothermal-induced oxidation. The coexistence of scattering light and surface plasmon polaritons on the hybrid films gives rise to a diversity of surface morphologies. Depending on the laser power and sample scanning velocity, beyond the traditional one-dimensional nanogratings that exhibit a near-wavelength periodicity, two types of nanostructures with subdiffraction-limit periodicity while large-scale uniformity are also observed. The first type, occurring at high laser energy and low scanning velocity, is generated by the spatial frequency doubling of the traditional laser-plasmon-interfering nanogratings. It exhibits a periodicity of <λ2. The second type, deep-subwavelength nanostructures, takes place at low pulse energy or low scanning velocity. It is in the form of two-dimensional nanoparticles and has a periodicity of <λ4. The far-field laser-plasmon interference associated with near-field scattering is attributed to the formation of such deep-subwavelength nanostructures, as confirmed by finite-difference time-domain numerical simulations. Our work provides a route toward high-throughput laser fabrication of large-scale deep-subwavelength periodic nanostructures.