| Laser-induced periodic surface structure(LIPSS)is a micro-nano periodic stripe structure produced on the surface of material after laser irradiation.The morphology of typical LIPSS is similar to grating.These periodic surface structure have a very broad application in the field of surface functionalization,such as optical encryption,optical coding,metal anti-corrosion,self-cleaning of clothing and wall,resistance reduction of submarine and hull,impeller anti-corrosion,outdoor material anti-freezing,lubrication of friction structure,materials for the implantable joint replacement of human body,etc.The functional orientation of the surface of this material is determined by its morphology of micro-nano structure on the surface,namely,the morphological factors of LIPSS such as magnitude of period,pattern and aspect ratio,etc.,among which the magnitude of period has the most significant influence on the functional orientation of the surface.The paper aims at studying the evolution mechanism of transient optical properties of monocrystalline silicon surface induced by ultrashort pulse laser,as well as the mechanism and control method of LIPSS induced by ultrashort pulse laser on silicon surface,so as to provide sound theoretical references for the surface functionalization of monocrystalline silicon induced by ultrashort pulse laser.To study the mechanism of interaction between ultrashort pulse laser and monocrystalline silicon,the paper has,based on Surface Plasma Theory and Drude Hypothesis,introduced latent heat of phase transformation,and established the model for simulating the changes of physical parameters such as carrier temperature,lattice temperature,carrier density and other parameters during the interaction between ultrashort pulse laser and monocrystalline silicon.Based on the aforementioned model,the paper has studied the energy transfer process from photon to electron and from electron to phonon,calculated the dielectric constant of monocrystalline silicon,and finally obtained the transient variation of refractive index and extinction coefficient of monocrystalline silicon surface.The study will be helpful to reveal the evolution mechanism of the transient optical properties of monocrystalline silicon irradiated by ultrashort pulse laser within the pulse width range from sub-picosecond to picosecond.The results of the calculation have suggested that the influence of laser energy density and pulse width on refractive index and extinction coefficient is very limited under the circumstance that single laser pulse cannot melt the monocrystalline silicon.The change of refractive index and extinction coefficient was less than 0.5%for each change of energy density by 0.01 J/cm~2.Given that a single laser pulse can melt the monocrystalline silicon,the change of laser energy density and pulse width has different influences on the refractive index and extinction coefficient of silicon surface.Combined with the theoretical model and experimental verification,the paper has studied the physical mechanism of periodic surface structure induced by laser within different pulse width ranges of 100 picosecond,picosecond and sub-picosecond on silicon surface.The results have shown that when the pulse energy density of the incident laser exceeded the threshold energy density of LIPSS,the period of LIPSS was positively correlated with the energy density of the laser.When the energy density of the lases rests in a certain range higher than the excitation threshold of LIPSS,the magnitude of period of LIPSS will rise rapidly until it is close to the wavelength of incident laser.Therefore,this extremely unstable energy density range should be avoided in the processing.In terms of lasers that induce LIPSS on the surface of monocrystalline silicon,in addition,the influence of thermal effect should be considered when its pulse width is larger than or equal to picosecond.Moreover,the energy accumulation effect will also affect the formation of LIPSS. |
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