Theoretical And Experimental Study On Light Field Manipulation For Super-diffraction Limit

Super-resolution focal spot has a very extensive and far-reaching influence on the development of super-resolution imaging and other fields.However,in 1873,German physicist Abbe proposed the diffraction limit of the focus of the objective lens.He pointed out that the spot size is restricted by the light diffraction and the limited numerical aperture(NA)of the optical system when focusing.So that the full width at half maximum(FWHM)of the focal spot is about half of the wavelength of the incident light.In order to achieve super resolution focusing and obtain the super-resolution focal spot,scientists have carried out a lot of theoretical and experimental research.Scientists have proposed and realized a variety of methods to bypass the diffraction limit in the field of super-resolution imaging,but they do not break through the diffraction limit in essence.After decades of development,by modulating the incident vector beam,currently there are three main methods to generate the focal spot beyond the diffraction limit: pupil filter method,metalenses focusing method and super-oscillatory lens(SOL)focusing method.The aim of this paper “ Theoretical and experimental study on light field manipulation for super-diffraction limit”is to deeply and systematically study the amplitude,phase and polarization modulation of the incident vector beam by pupil filter,metalenses or SOL for realizing the super-resolution focusing spot in the far field and propose some innovative methods and improve FWHM of focusing spot.The main works of this paper includes:1.To overcome the limitations of pupil filter in the field of super-resolution focusing,a three-ring zoned pupil filter is proposed base on systematically studying the theoretical basis of pupil filter,in which the central circular part and outer annular band are transparent while the middle annular band is opaque.The outer annular band has additional phase modulation for the incident light.The radially polarized beam is modulated by the pupil filter and the transmitted light field of the pupil filter is focused by an objective with NA of 0.95.The FWHM of the focal spot is reduced due to the destructive interference.The intensity distribution of the focal field is calculated based on the vector diffraction theory and by optimizing the structure of the three-ring zoned pupil filter,the superresolution focal spot with the FWHM of 0.37? is obtained.The intensity of the side lobe is less than 20% of the maximum intensity of the focal spot.2.To resolve the complexity of optical path of Stimulated Emission Depletion(STED)microscope and the difficulty of alignment between the center of vortex phase plate and the center of objective lens,and the easy misalignment of coaxial properties of dual-wavelength coaxial focusing,a kind of dual-wavelength coaxial focusing metalens was designed.In the case of two circularly polarized beams with different wavelengths,the dual-wavelength coaxial focusing metalens is used to realize spherical phase modulation of one of the incident beams,and the beam is focused to generate solid focal spots.In the same time,a spherical phase modulation and vortex phase modulation of another incident light is implemented to produce a ring-shaped spot.The design scheme and the accuracy of micro-nano processing ensure that the two beams remain strictly coaxial after focusing,and the ring-shaped spot keeps strict circular symmetry.The numerical simulation and experimental test both verified the focusing effect of dual-wavelength coaxial focusing metalens on the incident beam and are in accordance with the expected design.On the basis of discussing the basic concept of SOLs and the theory of vector angular spectrum theory,an central-occluded amplitude SOL is proposed to solve the problem that there are non-negligible side lobes around the main lobe when SOLs achieve super resolution focusing,and the outer ring part is composed of binary multi-ring amplitude oscillatory structure.The central occlusion part of the SOL blocks the middle part of the incident beam which is radially polarized beam with a wavelength of 633 nm,which increases the proportion of longitudinally polarized component in the focal spot and it is beneficial to compress the FWHM of the focal spot.The intensity distribution of the focal field of the SOL is calculated by the vector angular spectrum theory.Fast Hank transformation is introduced in order to speed up the calculation process.The structure of the SOL is optimized and the super resolution focal spot with FWHM of 0.36? is obtained,and the side lobe intensity is less than20% of the maximum intensity of the focal spot.