Study On Quasi-non-diffracting Super-oscillation Optical Lens

Non-diffracting beams can travel an infinite distance without altering their transverse intensity pattern.And they have the properties of high local central intensity and self-healing.However,the realization of ideal non-diffracting beams requires infinite energy.Only quasi-non-diffracting(QND)beams can be obtained in the experiment.QND beams have great potential in many applications,including optical trapping,optical tweezing,super-resolution imaging,and materials processing.However,it is a great challenge to generate super-long QND beams with sub-diffraction and sub-wavelength transversal size.In this work,the concept of normalized angular spectrum compression(NASC)was developed and used to generate QND beams.Based on the concepts of NASC and super-oscillation,a QND super-oscillation optical lens with working wavelengthλ=632.8nm was designed and fabricated.A QND beam with102λtraveling distance was experimentally demonstrated.The beam has a sub-diffraction and sub-wavelength transverse size within the range of 80λ.This thesis was organized as following:(1)The mathematical description and physical characteristics of angular polarized beams were discussed.The propagation formulas of vector angular spectrum of angular polarized beam were derived based on vector angular spectrum method.(2)The method of NASC was developed and used to generate QND beams.It is found that the QND beam can be achieved by reducing the effective working wavelength to increasing the cutoff spatial frequency.NASC concept provides a practical and simplified tool for quasi-non-diffracting sub-diffraction optical lens designing.(3)Based on the NASC method,the vector angular spectrum method and particle swarm optimization(PSO),a binary phase lens consisting of a concentric dielectric ring structure was designed to generate super-oscillatory QND beams.COMSOL Multiphysics~?software package was adopted to conduct the numerical simulation.Simulation results indicated that the length of the QND beam was 103λ,the length of the super-oscillatory region was 86λ,the side lobe ratio of the QND beam was 28%-43%,and the FWHM of the QND beam was 0.32λ-0.4λ.(4)A quasi-non-diffracting super-oscillation optical lens was fabricated by chemical vapor deposition,magnetron sputtering,electron beam lithography(EBL)and plasma etching.The processing technology and process exploration were discussed in detail.(5)Based on a wide-field large numerical aperture microscope,the measurement system of super-oscillatory light field was builtto characterize the quasi-non-diffracting super-oscillation beam.The experimental results were agreed with the numerical simulations.According to the experiment results,the length of the quasi-non-diffractive beam was 102λ,the FWHM of the beam was between 240λand 320λ,smaller than the diffraction limit 0.5λ/NA,and the FWHM was less than or very close to the super-oscillatory criteria 0.38λ/NA.