Research On Sapphire Micro-optical Elements Fabricated By Femtosecond Laser

For its excellent optical transparency from Mid-IR to UV and high hardness,thermal and chemical stabilities,sapphire has been widely used in military,industry,medicine,aerospace and other fields.As an excellent optical material,sapphire is not exploited and utilized for applications in micro-nano optics,this is mainly because that traditional mechanical processing is no longer applicable on micro-nanometer scale and chemical etching realized by mask has anisotropic problems.Recent years,three dimensional micro-nano structures fabricated by femtosecond laser micro-nano processing technology attract people's great attentions and have been used in microelectronics,micro-optics,MEMS,biological sensors etc.The preparation of micro-nano structures of hard materials such as sapphire can be realized by using femtosecond laser ablation technology.However,for the processing of hard materials,high energy density laser is usually used to ablate the material,and the machining accuracy and scanning speed are not high.In addition,the interaction between femtosecond laser and sapphire crystal is multi-photon absorption process of producing micro coulomb explosion,the surface roughness of the laser processed sapphire surface is large,which is difficult to meet the requirements of optical devices of the high surface smoothness.In order to solve the problem of low accuracy and high surface roughness about femtosecond laser processing sapphire,near-threshold femtosecond laser processing technique is proposed for high-precision processing sapphire,and wet etching assisted femtosecond laser process technology is proposed for reducing the surface roughness of the sapphire laser processing area in this paper.On the basis of this,we first prepared sapphire based UV micro-optical elements including sapphire Fresnel zone plate and sapphire Dammann gratings,and obtained good optical performance.Secondly,we fabricate subwavelength reduction reflecting structure on sapphire using wet method etching assisted femtosecond laser processing,and improved the transmittance of sapphire in Mid-IR spectral region.In addition,an efficient diffraction lens is prepared by femtosecond laser induced refractive index change in sapphire crystal.In details:(1)Using near threshold femtosecond laser processing technology to achieve the high precision processing sapphire,using the method of wet etching assisted femtosecond laser processing technology solve the problem of large surface roughness in laser processing area.By adjusting laser processing parameters such as the pulse energy of laser,laser polarization state,high resolution processing sapphire has been acquired,processing line width can reach 150 nm.Using the mixture of sulfuric acid and phosphoric acid(volume ratio of 3:1)wet etching laser processed sapphire under the condition of high temperature,by optimizing the wet etching conditions,we have obtained the good surface quality and the surface roughness down to 11.9 nm,which can meet the requirements of the optical applications.(2)Sapphire based UV micro-optical elements are fabricated by wet etching assisted femtosecond laser processing technique.Fabricated sapphire based Fresnel zone plate not only has excellent geometric morphology and surface quality,but also show excellent UV imaging and focusing performance.Besides,we have fabricated Dammann gratings(DGs)for UV beam(266nm)splitting for the first time,diffraction efficiencies of DGs that generate 2 × 2,3 × 3,4 × 4,5 ×5 spot sources are 59.26%,39.53%,44.68%,55.37%,respectively,compared with the theory of diffraction efficiency.In addition,the fabricated DGs shows good intensity uniformity of splitting performance,so the DGs has important application value in the field of UV laser shaping and laser parallel processing.(3)Sub-wavelength anti-reflection array structures on sapphire are controllable fabricated by wet etching assisted femtosecond laser processing technique.We fabricated sub-wavelength micro-pore array structures on sapphire substrate using femtosecond laser galvanometer scanning system,the transmittance of sapphire increased from 85% to 92.5% in Mid-IR region,the sapphire with sub-wavelength structures also exhibits angle-independent transmittance characteristics up to a high ?=60°.In addition,we also proposed using femtosecond laser piezoelectric platform processing system fabricate sub-wavelength anti-reflection pyramid array structure on sapphire substrate by means of vertical cross laser scanning,to realize large area of sub-wavelength anti-reflection structure,the experimental results are in further optimization.Sapphire sub-wavelength anti-reflection array structure can overcome the mismatch problem and film damage problem resulting from coating because the same components as sapphire.(4)High efficiency diffractive Fresnel lens are fabricated using femtosecond laser direct writing technology by laser induced refractive index change inside sapphire.First,we use femtosecond laser direct writing technique processing volume phase gratings inside sapphire,and measure the parameters and diffraction characteristics of the gratings,and further obtain the quantitative value of refractive index change induced by femtosecond laser.On this basis,we design and fabricate phase type diffraction lens inside sapphire crystal by femtosecond laser induced refractive index change,and the diffractive lens exhibits good UV light focusing and imaging performance.More importantly,the lens still keep good optical performance when in the environment of different refractive index and after high temperature annealing,such Fresnel lens that we fabricated shows the potential applications in harsh environments.In summary,in order to solve the problem of low accuracy and high surface roughness about femtosecond laser process sapphire materials,wet etching assisted near threshold femtosecond laser process technology is proposed.Ultraviolet micro-optics elements and sub-wavelength anti-reflective array structures on sapphire are controllable realized,and achieved good optical performance.Besides,we use femtosecond laser direct writing technology fabricate high efficiency diffractive Fresnel lens by laser induced refractive index change inside sapphire,and verified its potential applications in harsh environment.We use wet etching technology and femtosecond laser processing technology fabricate and characterize variety of micro-optics elements on sapphire,and presented application value of sapphire crystal in the field of micro-optics,provides new methods and new ideas of sapphire crystal application in micro-optics.