Study Of Femtosecond Laser Micro/nano-fabrication For Hard And Brittle Materials Based On SLM

As excellent optical materials,transparent and hard materials such as silica and sapphire have wide range of applications from people's daily life to the industrial and military fields due to the high transmittance in wide wave range and thermal and chemical stability.But in micro/nano-optical element,the high hardness and low plasticity of these materials make it difficult to reach the precision and surface roughness requirement by traditional machine tools.The lithography method needs multi-step processing and can't fabricate structures inside the transparent materials.As a new type of non-contact micro/nano processing technology,femtosecond laser has the advantages of high processing accuracy,wide application range for many materials and true three-dimensional processing capability.It has broad application prospects in micro/nano optics,microfluidics and micromechanics.The common femtosecond laser direct writing method using the point-by-point strategy to fabricate structures lead to a low processing and laser energy utilization which greatly hinder its applications in many fields.Moreover,the surface roughness can't reach the requirement of micro-optical element which is ablated by femtosecond laser.And the distortion of the focus inside the transparent materials has a huge effect on the processing precision which is caused by the refractive index mismatched between interfaces.In order to solve the above problems,a new method,combining LC-SLM,femtosecond laser direct writing and Secondary processing technology such as wet etching,is proposed to fabricate micro-optical components in/on silica and sapphire with high efficiency and precision by studying the interaction between laser and materials.The details are shown as follows:(1)Control of diameter and numerical aperture of microlens array by holographic femtosecond laser ablation and wet etching technology:By studying the influence of laser ablation area in/on silica after HF solution etching for the formation of structures,a holographic is designed to modulate the phase of input beam and realize multi-focus along the optical axis after the objective.Using this multi-focus ablation and wet etching method,microlens array can be fabricated on silica with high efficiency.By fixing the laser energy,the size of microlens can be changed with holographic together.The depth and radius of single microlens can be tuned from 6 and 19?m to20 and 34?m,respectively.The numerical aperture can be adjusted from 0.22 to 0.41.(2)High aspect ratio and precision structures fabricated in silica using phase modulation and accurate control of processing parameters:Due to the non-uniform and uncontrollable size of nanogratings induced by femtosecond laser irradiated in silica,a uniform intensity distribution along the axis can be realized by phase modulation technology based on the correction of refractive index mismatched.Combined with femtosecond laser micro/nano-fabrication,a ultra-high aspect ratio and high precision refractive index changed area can be fabricated in silica when processing parameters was tuned accurately.The maximum width and depth can reach90 nm and 48?m respectively,the maximum ratio aspect can be 500.In addition,by systematic studying the influence of processing parameters for the structures,an innovative method is proposed to tune the birefringence inside the silica just by changing the interspace between the nano-lines,and the maximum retardance of one single layer is 150 nm.The resolution can down to 200 nm.In order to demonstrate the reliability,a one-dimension Dammann-grating is fabricated inside the silica with a size of 100?m~2?100?m~2.One single layer efficiency of Dammann-gratings can reach 49.8%(79%in theory),and the far-field diffractive patten has a uniform energy distribution.(3)Precisely control of birefringence induced by stress inside the sapphire using femtosecond laser microfabrication with a uniform energy distribution:By modulating the energy distribution behind the objective into the line-sharp along the optical axis,a uniform refractive index changes are realized inside the sapphire with170 nm width.The minimum interspace between lines is 300 nm.With this high precision microfabrication method,accurate stress induced birefringence distribution can be realized by designing stress accumulation and release area inside the sapphire.The maximum retardance with one layer is 1/4?,and a higher one can be reached by multi-layer method.This stress induced birefringence at no laser irradiated area can avoid the scatter of light by structures at irradiated area which decrease the efficiency of the micro-optical element.In summary,a LC-SLM and femtosecond laser direct writing method are combined to fabricated microlens array on silica with a high processing and energy utilization efficiency after post-processing of wet etching.In addition,by precisely controlling the processing parameter,a uniform refractive index changes can be realized inside silica and sapphire with nano-resolution and high ratio aspect.A detail test and analysis of optical element fabricated by this method are performed to demonstrate the practical application in the field of micro/nano-optics,and this method provides a new technology and idea for femtosecond laser micro/nano-fabrication in transparent materials.