Mechanism Of Surface Two-plasmon Resonance For Femtosecond Laser- Nanostructuring On Solid Surface

Solid surface structures in mirco/nano scales characterized by non-classical effects,such as prominent small size effect,surface effect,mesoscopic effect and quantum effect,allow super surface performance or functions,such as light,sound,thermal,electricity,magnetism,mechanics,and so on,analogy to super surface functions induced by skins of many biological creatures in nature.These mi-cro/nano structures on material surfaces fabricated by controllable preparation according to the expected requirements will give rise to a revolutionary impact on many applied fields,such as biological sensing,microfluidic devices,solar cells and fuel cells,aviation and military,et al.Therefore,development of various fab-rication and characterization methods for micro/nano structures is an important scientific strategy for the future.Understanding the formation mechanism of surface structures is of critical importance to realize the surface characteristics of micro/nano structures accord-ing to the expected requirements of the controllable fabrication.In this paper,we investigate the underlying mechanism for the self-formation of periodic sur-face topological structures on subwavelength scales on solid surfaces induced by fs-laser pulses from the ultra-fast physical interaction.The related nonlinear physical effects which play the key role in the formation of surface micro/nano structures are revealed,the physical model of fs-laser induced surface periodic nano-structures are provided,which agree well with the present and previous ob-servations in surface-structuring experiments.Some prospective experiments are carried out based on the model,which further prove the new formation mecha-nism are self-consistency in physics and validity in theory.The main innovative results are as follows:1.New physical model:surface two-plasmon resonance(STPR)set upBased on the fluid mechanics equation,for the surface electron heat flux under the fs-laser,we prove that the laser-matter interaction will result in the opposite sister Langmuir waves phase-locked resonance,i.e.surface two-plasmon resonance(STPR),when the three waves meet the phase matching condition and the plasma characteristic parameters are in the dynamic range(i.e.k?_D<0.29),during the three waves of the laser wave and the surface two plasmon resonance waves coupling caused by a small perturbation of electron density gradient near n_c/4(n_cis critical density).The dispersion relation and the STPR wavelength are derived.It can be concluded that the electrostatic field force between the electronegative centers formed the peaks of STPR wave to the bound target-ion exceeds its binding energy in the lattice,are the necessary and sufficient condi-tion leading to Coulomb ablation(i.e.Coulomb explosion),and producing sub-wavelength surface structures.The phase-locked STPR is the physics mechanism of the subwavelength surface gratings self-formation.The wavelength of STPR is the spatial structure periods(SSPs)of grating structures.The STPR mechanism especially can self-consistently explain the puzzling case of SSP<?/2,and ex-plain the formation of grating structures perpendicular to the laser polarization for the first time.2.Key physical parameters of surface nano-structuring induced by STPR ablation are provided and provedFor the self-formation process of fs-laser induced surface nano-structures,in theory,through the interface electrostatic field express created between negative electricity centers and target ions,the acceleration time of ions driven by the in-terface electrostatic field(which is the characteristic time of completing coulomb explosion process),and the temperature of the surface plasmon electronic scal-ing relations,it has been found that the bulk lifetime of surface plasmon is much longer than the acceleration time,which is essential for STPR ablation.In exper-iment,the evolution relaxation of surface plasmon wave(i.e.lifetime)is obtained by applying time-resolved spectrum method.The acceleration time of ions dur-ing the Coulomb explosion process is less than the lifetime of surface plasmaon are proved by both theoretical calculation and time-resolved spectrum.There-fore,the self-consistent physics of STPR ablation mechanism driven by fs-laser is further confirmed.3.New concepts about surface nano-structures of“layer-carving”effect pro-posed based on STPR ablation modelBased on STPR ablation model,the time-delayed fs-pulse splitting beam exposure method are used to ablation the same position on the W-target surface by changing the polarization direction and the delay time of the two pulses,the results show that the SSP and the orientation of the two surface grating structures induced by the successive two pulses are independent.This is similar to the digital“layer-carving”process of the traditional carving technology sculpture.It is further proved that the“layer-carving”notion is applied to not only static exposure,but also pulse scanning for surface nano-structuring,and both are decided by the STPR ablation effect driven by fs-laser.4.Surface subwavelength structures fabricated by fs-laser fast scanning are realizedBased on the“layer-carving”effect,large area ripple gratings,square grids and parallelogram grids are realized by fs-laser fast scanning fabrication on both plane and non-plane surfaces,and the grating ripples formed between two scan-ning areas are well jointed in subwavelength grating structures experiments in-duced by fs-laser scanning process.Further experiments show that,when the height(h)of the cambered surface cap irradiated by the laser pulse is less than a Rayleigh length(Rayleigh length,i.e.,half the confocal parameter),the unifor-mity of the structured period is maintained on non-plane surface,as the same as the results on plane surface.