Design Of Diffractive Superresolution Elements For Radially Polarized Beam

Radially polarized beam has many unique properties. The research on the generation, propagation, focusing and applications of radially polarized beam is still paid close attention to. Direct focusing radially polarized beam with high numerical aperture (NA) lens can realize optical superresolution. In this dissertation, diffractive superresolution elements (DSEs) are designed to further improve the superresolution performances of radially polarized beam. The aim is to minimize the size of the main lobe and maximize the central intensity, as well as control the intensity of the side lobe. The combination of radially polarized beam with DSEs is one path to realize smaller focused spot, and has better performance than the combination of linearly polarized beam with DSEs.Based on the analysis of focusing properties of radially polarized beam with high NA lens, the optimization method of 2-dimensional or 3-dimensional DSEs is proposed by considering only longitudinal component of the focused field of radially polarized beam. The global optimal results are 0,Ï€two-phase distributed DSEs for the longitudinal component of the focused field of radially polarized beam. The 2-dimensional or 3-dimensional superresolution performances are realized with linear programming. The exhaust algorithm is applied to improve the superresolution performances of designed DSEs with variable position and value of phase change. However, no improvement is observed, which shows the validity of the optimization method only considering the longitudinal component of the focused field.The limits of the transverse and axial superresolution of DSEs for radially polarized beam are calculated with numerical method and compared with the limits of DSEs for linearly polarized beam. Although the DSEs are globally optimal for linearly polarized beam and cannot be assured to be globally optimal for radially polarized beam, better superresolution performances can be obtained with radially polarized beam than linearly polarized beam. Moreover, the intensities of radial and axial side lobes are controlled by additional restrictions. The effects of different beams on superresolution performances of DSEs with radially polarized beam are compared.Radially polarized beam is generated by liquid crystal polarization converter, and the polarization purity of such beam is 95 percent measured by Stokes parameter method. A DSE for the radially polarized beam is fabricated. The high NA focusing fields of linearly, azimuthally and radially polarized beam as well as the DSE with radially polarized beam are measured by the near-field scanning optical microscopy (NSOM) system. The measured results of linearly and azimuthally polarized beam are basically consistent with simulation results. The measured result of radially polarized beam is basically consistent with simulation result of the transverse component of the focused field of radially polarized beam. Due to the limit of experiment conditions, the superresolution of DSE has not been observed.