Design And Optimization Of Grating Microlens For Specific Focal Field Focusing Of Cylindrical Vector Beams

The spatially non-uniform cylindrical symmetric polarization state of cylindrical vector beams have a wide range of applications in the fields of specific focal field focusing,super resolution microscopy,optical capture,laser micromachining and so on.The design of compact focusing optical lens have attracted much attention from researchers.Traditional lens and mirrors with specific surface curvature are hard to break through the diffraction limit,and their sizes are large.In addition,the focal spot of the reflector is on the same side as the light source,which is not conducive to practical application.Binary optical devices or 4-f systems can generate specific focal fields,but the design and implementation of these devices and systems are complicated.Surface plasmon microlens have polarization limitation and focal length cannot be adjusted flexibly.One-dimensional photonic crystal lens consists of two or more materials arranged alternately,which is difficult to prepare.In conclusion,in order to achieve flexible focal field control,microlens should have the characteristics of specific focal field focusing,specific focal length,no polarization limitation,and easy preparation.This paper proposes a grating plano-concave microlens which can achieve the specific focal length focusing of cylindrical vector beams and overcomes the defects of common optical devices.The structure of the plano-concave microlens is designed based on the equivalent negative refraction effect of all-dielectric grating-1 diffraction and Fermat’s principle.The area division of the exit plane corresponding to different preset focal lengths forms a dual focal field satisfying a specific focal length.The influence of the structure’s high secondary diffraction effect on the focusing effect was analyzed,and the coupling of secondary focus and double focus,migration and other high secondary diffraction due to the large structural period were found.A two-step optimization scheme of hollowing out at the bottom of the plano-concave microlens and regional connecting part was formed,which effectively suppressed the secondary focus,corrected the double focus position,and optimized the quality of the focal field.The adjustment of the polarization components and parameters of the incident light satisfies the demand of flexible regulation of the transverse and longitudinal optical fields.Further,this paper proposes a grating plano-conical microlens that can achieve specific focal field focusing.Based on the equivalent negative refraction effect of all-dielectric grating diffraction of order 1 and Fermat’s principle,the design is completed.Under different cone angles,it has the ability to obtain focal field of light needle and optical chain.The special cone-angle plano-conical microlens designed in this paper can make the light deflect at 90°,so that the focal field of the needle is more compact,and the light source is located on the other side of the lens,which overcomes the disadvantages of the traditional mirror.With the reasonable setting of the cone Angle,the-1 order diffracted light of the all-dielectric grating can obtain the special phase relation to satisfy the focal field focusing of optical chain on the axis of the grating plano-conical microlens,and the generated compact focusing optical chain has the advantages of controllable number of focal points and spacing.The number of focal points in the focal field of the optical chain can be changed by adjusting the structural parameters of the plano-conical microlens and the incident light parameters.The work in this paper focuses on the design and optimization of grating microlens which can achieve the specific focal field focusing of cylindrical vector beams.Grating microlens has the advantages of miniaturization,simple structure design and single refractive index.This research provides new ideas for super resolution microscopy,photolithography,multi-particle optical micromanipulation and other fields.