Femtosecond Laser-induced Micro/nano-scaled Periodic Surface Structures On Solids

Femtosecond laser-induced periodic surface structures(Fs-LIPSS)attract the scientific and technical attention due to the ability to produce nanostructures below the optical wavelength.These are essential for surface engineering and treatment,notably in tribology,wettability,mechanics,marking and counterfeiting.Depending on the regime of laser interaction,particularly on the laser fluence,pulse number and material type,ultrashort pulses can induce the low-and high-spatial-frequency-LIPPS(LSFL and HSFL),with the orientation perpendicular(?E)or parallel(?E)to the laser polarization.Considering their potential in the nano-manufacturing,this work focuses on potential mechanisms for LIPSS formation,especially HSFL formation on the metallic alloys.In order to investigate the transient optical indices of excited materials in fs-LIPSS formation,we first developed time-resolved ellipsometry to measure dynamic optical indices of excited materials.Thus we gain insights in the dynamics of the dielectric function where this is intrinsically related to the electronic configuration and lattice structure.First principle simulations are then used to reveal how the electronic configuration changes during the excitation,responsible for the transient optical indices.The effects of transient optical indices are considered in the LIPSS formation mechanisms.Based on the experiments of fs-LIPSS formations on six different materials,involving metal tungsten,semiconductor silicon,dielectric fused silica,single-crystal superalloy CMSX-4,amorphous alloy Zr-BMG and its corresponding crystal alloy Zr-CA,we investigate the LIPSS formation mechanisms in the electromagnetic domain by finite-difference time-domain(FDTD)simulations,related to the electromagnetic energy distribution followed by the dynamics of optical excitation,evolving topologies with pulse number and materials.We focus on the electromagnetic origin of LIPSS formation and reveal a potential primary factor for LIPSS formation.LIPSS formation can be explained by deposited energy modulation on surface via electromagnetic effects.The energy modulation mainly comes from the interference between incident laser and scattered surface wave(for LSFL(?E)),being complemented by the interference between scattered surface waves(for HSFL(?E)).Specially,for HSFL(?E)on Zr-CA,we proposed that the formation scenarios rely on individual anisotropic field-enhancement processes.The evolving surface topology with laser pulse number leads to a feedback-driven energy modulation deposited on surface.