Study On Super-oscillation Focusing Device For Radially Polarized Light

Focusing radially polarized light plays an important role in metallic particle trapping,enhanced Raman spectroscopy,particle acceleration,high density optical data storage and optical microscopy,Due to the diffraction limit,the radially polarized light is more difficult to realize longitudinal field super-resolution focusing in the distant field by traditional method,However,the super resolution focus can be achieved using the method of optical super-oscillation,and when the radially polarized light is focused,the super-oscillation focus spot can be focused tightly because that the longitudinal polarized component of optical field is easier to compress the focal spot size.In this paper,we studied binary phase planar lens of radially polarized light.It is found that the longitudinal light field is focused on the focal plane experimentally,which is corresponded to the simulation.The main contributions of this dissertation are listed as follows:? we introduce the application prospect and research status of radially polarized beam focusing,which we propose our research objects according to.? We introduce the correlation theories and optimum design methods of focusing device for radially polarized light.And we derive the vectorial angular spectrum propagation formula of radially polarized beam.? We combine particle swarm optimization and the angular spectrum propagation theory of radially polarized beam to design the super-oscillation focusing device based binary phase modulation.The parameters of the device we design include:wavelength(?=632.8nm),focal length(fs=200?),diameter of device(d=1000?)and numerical aperture(NA=0.929).The results of the design is as following:FWHM is 0.378?(239.48nm),peak intensity is 39853.4 and sidelobe ratio is 19.88%on the focal distance.Compared with the criterion of super-oscillation,which require the FWHM is less than 0.409?(0.38?/NA),the device we designed realizes the super-oscillation focusing.Comsol Multiphysics was used to conduct the simulation to verify our design,and the results are as following:FWHM is 0.388?(245.3nm),peak intensity is 11139.1,sidelobe is 24.5%on the focusing distance(fc=200.3?),which also prove our device can achieve super-oscillation.In order to determine the processing plan,we discuss the influence of choosing different substrate material and changing thickness and width of silicon nitride to peak intensity,FWHM and sidelobe.And we conclude that the influence can be ignored in the range of lOnm machineing error.? At last,we introduce the processing technology of the super-oscillation focusing device.After set up the optical path,we finish the test of the super-oscillation focusing device,and anylize the experimental result.Near the focusing distance(z,=201.4?±10.88?),the average FWHM is 0.457?(289nm),sidelobe is 53.6%and peak intensity is 5946(counts).Compared with the diffraction limit,which is 0.539?(341.lnm),our device achieves super-resolution focusing by experiment.Meanwhile,we discuss the influence of the alignment deviation of optical path to speak intensity,FWHM and sidelobe.Then we find that on the focal plane,if incident light is not aim of the center of the device or the device is slant,the sidelobe,which is influenced most,FWHM and peak intensity will increase.We explained the possible reasons for the difference between the results of the experiment and the simulation.