The Study Of Multi-picosecond Laser Induced Periodic Surface Structures On Typical IR-window Materials Mechanisms And Control Method

Laser-induced periodic surface structure is a common phenomenon in the laser material interaction process.It can be found on the surface of metal,semiconductor and dielectric under linearly polarized laser irradiation.The pulse duration range of the applied lasers can be from continuous wave to only a few femtoseconds.Periodic structures on the surface of the material can change its wettability,optical and tribological and other properties which make the LIPSS widely used in the fields of biology,medicine,chemistry and microelectromechanical systems.In this paper,the method of multi-pulse picosecond laser-induced periodic structure on the surface of typical infrared window materials is studied from the aspects of experimental research and theoretical analysis,and the anti-reflection effect of laser-induced periodic structure is discussed using the equivalent medium theory.The existing theoretical system of laser-induced periodic surface structure was discussed,and a theoretical model of picosecond laser-induced periodic structure generation was established by analyzing part of the theory based on femtosecond laser-induced periodic structure generation.By classifying the mechanism of laser-induced periodic surface structure,this paper explains the generation of periodic surface structure from two aspects:electromagnetic theory and hydrodynamic theory.The effects of laser fluence and the pulse number on the period and depth of the periodic structure on the surface of the crystalline silicon are studied.The corresponding relationship between the laser parameters and the periodic structure parameters is obtained.Through the experiments,it can be found that the surface morphology induced by laser can be divided into three types: LIPSS melting,LIPSS,and surface oxidation.The threshold of the laser fluence and the pulse number used to form LIPSS were determined.A mechanism model for the formation of periodic structures on the surface of crystalline silicon induced by a 532 nm picosecond laser was proposed,which explained the formation of the micrometer structure in the center of the spot and the sub-micrometer structure at the edge.The formation mechanism of the micrometer structure is based on the capillary wave in fluid mechanics,and the formation mechanism of the sub-micrometer structure is based on the SPP theory.The periodic surface structure is equivalent to a grating structure by referring to the micrometer structure parameters obtained in the experiment.The refractive index of the equivalent grating structure is calculated using the effective medium theory.The reflectivity of the surface periodic structure is calculated for the typically used laser wavelength.By comparing with the initial surface,the surface with the periodic structure can reduce the reflectivity when the incident light is in some wavelength ranges.In this paper,the control method of picosecond laser-induced periodic structure on the surface of crystalline silicon is studied.The mechanism for the formation of periodic surface structure is discussed.The combination of SPP theory and capillary wave theory is innovatively used to explain the causes of the two structures on the same laser spot.The research results in this paper can be helpful for surface antireflection of picosecond laser used in infrared window material processing.