Enhancement and blueshift of high-frequency laser-induced periodic surface structures with preformed nanoscale surface roughness

Ultrafast laser irradiation on material surface can lead to grating like rearrangements of matter, the laser-induced periodic surface structures (LIPSSs). Among them, high-spatial-frequency laser-induced periodic surface structures (HSFLs) with a periodicity significantly below the wavelength of illuminating light show a great potential in nano-structuring of materials by laser light. Using metallic tungsten as a model material, the interaction between an evolved surface topography with ultrafast laser pulses is investigated. By extensive Finite-Difference Time-Domain (FDTD) simulations including light-matter interactions on the nanoscale and inter-pulse feedback mechanism, we study the effects of initial surface roughness particle size on the topography of the resulting LIPSSs. We show that a reduction in the size of the initial surface roughness particles leads to both enhancement and blueshift of HSFLs periodicity significantly below the diffraction-limit and on the same time, elimination of low-spatial-frequency laserinduced periodic surface structures (LSFLs). The underlying mechanism is attributed to enhanced near-field scattering of the illuminating light associated with reduced roughness particle size. These results indicate a potential control over the topography and periodicity of LIPSSs by preforming nano-scaled surface roughness before laser irradiation.