A Research Of Laser-induced Damage Performance For Subwavelength Structure On Fused Silica

Since the subwavelength structure has a characteristic size smaller than the wavelength of light,it can not only show good diffraction characteristics,but also have excellent antireflection performance.For the subwavelength structure growing on the surface of the substrate,the gradient of the refractive index from air to the substrate can be realized,and then the refractive index of the substrate surface can be adjusted.Fused silica is a commonly used transmission optical element in high-power optical systems.The energy carried by the reflected light on the surface is harmful to the optical system,so Fresnel reflection on the surface must be suppressed as much as possible.Compared with the traditional antireflective film coating,the subwavelength antireflection structure directly processed on the surface of the fused silica,due to the single material system,avoids the degradation of the stability of the optical element caused by the mismatch of the physical and chemical properties between the film layers under the laser irradiation.Based on this,the subwavelength antireflection structure was prepared on the surface of fused silica by reactive ion etching technology,and the laser damage resistance of the subwavelength antireflection structure was studied.Firstly,a periodic subwavelength structure model is established based on the finitedifference time-domain method,and the effects of the duty cycle,period and height of the structure on the antireflection performance are theoretically analyzed.It is found that as the duty cycle increases,the overall transmittance increases first and then decreases.The optimal duty cycle is 0.53.The period does not change the effective wavelength of the antireflection performance.When the period is increased to be close to the structure height,abnormal transmittance may occur in the short wavelength region due to the generation of high-order diffraction.As the height of the structure increases,the optimal antireflection band shifts red,which has similar characteristics to a single-layer film.Experimentally,subwavelength structures were prepared on the surface of fused silica by reactive ion etching based on self-masking technology.The influence of RF power on the subwavelength structures was discussed in detail,including the size of the structure,surface element distribution,optical properties,and surface functions.At the same time,in order to verify the universality of the technology,antireflection subwavelength structures have also been successfully prepared on the surface of plexiglass.Subsequently,the effects of various process parameters,including gas composition,working pressure,RF power,and etching time,on the antireflection performance of the subwavelength structure were analyzed in detail.It was also found that the etched plexiglass samples exhibited a certain degree of hydrophobicity.In the part of laser damage performance research,firstly,based on the self-masking technology,the subwavelength structure of fused silica with the antireflection band of 532 nm and 1064 nm was fabricated respectively,and the transmittance after double-sided etching were all over 99%.Corresponding dielectric antireflection films were also prepared for comparison.Before the damage test,surface defects were detected on the subwavelength structures and the dielectric antireflection films,including photothermal weak absorption test and fluorescence confocal microscope test.From the comprehensive results,the surface of the dielectric antireflection film has more defects than the subwavelength structure.The laser damage test results show that the 1-on-1 zero probability damage threshold of the 1064 nm subwavelength structure is 52% higher than that of the corresponding dielectric antireflection film.The 1-on-1 zero probability damage threshold of the 532 nm subwavelength structure is 246% higher than that of the corresponding dielectric antireflection film.Overall,it is shown that the subwavelength structure has a higher laser damage threshold compared to the dielectric antireflection film while maintaining high antireflection performance.Finally,the damage morphology of the sample was specifically observed and analyzed.