ZnS Crystal Micro-nano Structure Written Using Ultrafast Bessel Laser

The development of technology makes infrared technology increasingly important in all walks of life.Especially in military and national defense,aerospace,infrared imaging,infrared communications,infrared guidance,infrared remote sensing and other technologies have become the core competitiveness of science and technology in various countries.Zinc sulfide crystal has high transmittance and good overall performance in a wide spectrum from visible light to mid-infrared.It is currently one of the most promising infrared materials.The focused ultrafast laser has ultra-high peak power density and ultra-short pulse time relaxation,which can achieve spatially selective local modification and three-dimensional processing of materials.It is the preferred processing tool in the micro-nano field.Compared with Gaussian beams,Bessel beams have a long non-diffraction transmission distance and good self-repairing characteristics.It has strong applicability for extending the length of microchannels,high resolution and large-area precision processing,and developing new photonic functions of micro-nano-scale feature structures.Based on this research background,this article focuses on the application of infrared optical materials.Starting from the interaction mechanism of ultrafast lasers and transparent dielectric materials,a nanovoid microchannel structure with high aspect ratio is successfully prepared on ZnS crystals.This has important guiding significance for the preparation of mid-infrared photonic crystals.Secondly,in view of the high refractive index of the ZnS crystal,the Fresnel reflection at the interface with the air is as high as 14%,which will adversely affect the optical system and even cause damage.In response to this problem,we prepared a micron-level relief photonic structure on the surface of the ZnS crystal to achieve antireflection.The main research contents of this article are as follows:The basic properties and generation methods of Bessel beam are studied,and the Gaussian pulsed laser is spatially modulated by combining axicons and 4f systems to form zero-order Bessel beams.Combining experiments with numerical simulations,select appropriate experimental component parameters.Successfully built an ultra-fast Bessel laser processing system.The physical properties of different materials are different.The large band gap(3.6e V～3.8 e V)and high nonlinear absorption coefficient of ZnS crystal make the time and space distribution of the laser pulse occur when the ultrafast laser is transmitted inside it.Severe distortion,non-linear effects such as self-focusing and self-defocusing generated by plasma will cause the beam to split in space,and the laser energy cannot be effectively concentrated in the focal area.This is very detrimental to the processing of micro/nano structures with high aspect ratio.In this research context.We used the zero-order Bessel beam to prepare a nanovoids structure with high aspect ratio on the ZnS crystal through experiments,combined with SEM,FIB,PCM,EDS characterization methods,and determined the pulse width window(12.5 ps～20 ps)and pulse energy window(36μJ～63μJ).On this basis,the formation mechanism of pores was analyzed and discussed,and the influence of laser processing parameters on the surface morphology,diameter and internal characteristics of the pore structure was mainly studied.In practical applications,the serious Fresnel reflection loss at the ZnS crystal interface will cause a series of adverse effects.Therefore,it is necessary to reduce the reflection and increase the reflection.This article takes the advantages and disadvantages of traditional coating anti-reflection technology as a starting point,and analyzes the possibility and advantages of using ultrafast lasers to prepare sub-wavelength anti-reflection microstructures on ZnS crystals.Carried out the research work on the surface anti-reflection microstructure of ZnS crystal.A micron-level relief photonic structure that can be used for surface reflection reduction of ZnS crystal is proposed.First of all,the advantages and disadvantages of Gaussian beam and Bessel beam in processing this kind of embossed ring microstructure are analyzed through experiments,and it is proved that the zero-order Bessel beam can process such large area,repetitive,and reconfigurable relief.The ring-shaped microstructure is more advantageous.Subsequently,an ultrafast Bessel laser processing system was used to directly write a 5×5 mm~2 ring-shaped array microstructure on the surface of the ZnS crystal sample,which achieved broadband and large-angle antireflection.The adjustment of laser processing parameters and array period on the antireflection performance of microstructures is studied.Experimental results show that this photonic structure can increase the transmittance of ZnS crystals by more than 10%in the wavelength range 1.5μm to 10μm.