Surface Micro/nano-Structures Induced By Temporal/Spatial Shaped Femtosecond Laser Based On Electron Dynamics Control

As the higher requirement on the surface performance in practical application,surface micro/nano-structure has been extensively developed for its improvement in optical,mechanical,and biological etc properties.The fabrication and modulation on surface micro/nano-structure have been developed as one of the main researches in many frontier fields.Many fabrication methods have been developed for the surface micro/nano-structure.With the development of laser technique,especially the generation of femtosecond?fs?laser,the surface micro/nano-structure fabrication technique has been rapidly promoted.With ultrashort irradiation periods and ultrahigh power intensities,fs laser pulses in some aspects fundamentally change the laser–material interaction mechanisms comparing with long laser pulses.The ultrashort irradiation period makes fs laser-material interaction a strong nonequilibrium process.The duration of a fs laser pulse is much shorter than the electron-lattice relaxation time(10^-10-10^-12 s),so that the laser energy absorption is completed before the lattice changes,resulting in the significant nonequilibrium state between electrons and lattices.Thus,heat conduction and hydrodynamic motion are negligible during the pulse duration,leads to the minimized recast,thermal damage?microcracks?,and reduced heat-affected-zone?HAZ?.The ultrahigh power intensity also makes fs laser-material interaction a strong nonlinear process,which is quite different from the traditional continuous and long pulse lasers.This makes fs laser can fabricate almost any materials.These unique properties of fs laser enable it one of the most potential tools in the fabrication of surface micro/nano-structures.However,the lack of systematic mechnism support for the laser interaction mechanism and the fabricated material evolution property limits the controllable fabrication of surface micro/nano-structures and the development of its practical application.The overcome of these difficulties relies on the further research on the modulation and interaction mechanism of locallized transient materials properties during fs laser irradiation.Our group proposed the core idea of electrons dynamics control?EDC?based on the temporally and spatially pulse shaping.By shaping the fs laser pulse in temporal and spatial domain,we are able to control the localized transient electron dynamics,and then to modify localized transient materials properties,and finally to finish the optimization of the fabrication structures.The laser and material are the vital factors during the fabrication process.Thus,based on the core idea of EDC,researches on the fs laser-induced micro/nano-structures are developed from two perspectives in laser parameter and material property:1)The formation principle of ripple structures is intensively investigated,which reveals the fabrication rule and modulation mechanism.And on this basis,we propose the temporal fs laser pulse shaping method to control the morphology of ripples;2)Two novel fs laser spatial pulse shaping method are proposed based on the Fresnel diffraction effect of squared hole and dual color dispersion effect,by which the morphology and arrangement of surface micro/nano-structures and the plasmonic nano-structures can be effectively modulated;3)The material property of crystal structures are first considered to modulate the surface micro/nano-structures ablation properties.The main research contents and innovations are shown as follows:Based on the surface plasmon polariton?SPP?excitation,propagation mechanism and property,a grating aided coupling and directional scattering mechanism is proposed.Continuous modulation of the polarization-dependent fs laser induced periodic surface structures?also termed as ripples?can be achieved based on the control of SPP dynamics.The geometrical evolution of surface micro/nano-structures,which determined by the irradiated pulse energy is investigated.And it demonstrates the different role of SPP excitation and scattering effect with different surface micro/nano-structures.Furthermore,based on the EDC,double fs laser pulse train is adoped to modulate the morphology of ripple structures.By controlling the pulse delay between the double pulse train,we can control the free electrons generation and recombination,and then to control the SPP excitation and scattering,and finally achieve the arbitrary modulation on the anisotropic geometrical morphologies of ripple structures.Based on the geometrical morphology modulation of ripple structures,we can further modulate the scanned ripple line widths and the anisotropy of its formation and continuity,which provide technical support for the large-area uniform fabrication of ripple structures,and improve the fabrication efficiency.Based on the Fresnel diffraction effect of squared hole,a novel spatial fs laser pulse shaping method is proposed aided by squared grids,the novel method enbles the programmable modulation of the surface micro/nano-structure's geometrical morphology and its arrangement.The morphology can be modulated based on the threshold effect of the surface micro/nano-structure formation.And the formation mechanism of striped ripple lines induced by the squared grids-aided fs laser is investigated based on the SPP excitation and scattering effect.For the dynamic scanning situation,morphology control of the surface micro/nano-structures based on the scanning speed is deeply investigated experimentally.Especially,the abnormal abltion enhancement effect and polarization-sensitive ablation effect is also investigated theoretically and experimentally.Based on the focusing aberration and dispersion effect with different wavelength,another novel spatial fs laser pulse shaping method via the square grids is proposed for the modulation of the surface micro/nano-structures fabrication.And the implementation principle and property are investigated theoretically and experimentally.Based on this novel method,we can fabricated a concentric micro-structure on metallic films,on which the SPP excitation,propogation,and focusing are investigated experimentally and theoretically.Furthermore,we promote this method for the fabrication of semiconductor/metallic plasmonic nano-structures.The control of the irradiated parameter and film thickness enables the morphology modulation of the plasmonic nano-structures,and their optical properties are also investigated.Based on the crystal orientation-dependent of the electron effective mass,we propose the crystal orientation-sensitive photo ionization principle.Based on this effect,the ablation property modulation of non-metallic material with cubic crystal structures is theoretically and experimentally investigated by controlling the relative angle between the polarization direction and crystal orientation.The researches demonstrate a polarization-dependent ablation property,and the crystal symmetry determines the oscillation period.Furthermore,the role of photo ionization during the formation of the surface micro/nano-structures is investigated as the variation of the pulse energy.Finnaly,the SPP excitation and scattering effect in the formation of ripple structures and the crystal structure properties are integrated considered to modulate the formation and continuity of the ripple structures.