Subwavelength focusing of cylindrical vector beams by plano-concave lens based on one dimensional metallic photonic crystal

Abstract

Cylindrical vector beams (CVB) can exhibit a unique optical field distribution and focusing characteristic, due to the cylindrical symmetry in polarization. They are widely used in optical micro-manipulation, super-resolution imaging etc. and can be extended to subwavelength scale applications rapidly. Usually, the focusing CVB in subwavelength dimensions is realized by using plasmonic lens. However, this method is restricted by the state of polarization of electromagnetic waves. Nevertheless, when the negative refraction effect of photonic crystals is utilized, subwavelength focusing or imaging can be achieved in orthogonal states of polarization simultaneously. In this paper, the one-dimensional metallic photonic crystal (1D-MPC) with stronger manipulation ability is discussed. The calculated band structure and equi-frequency surfaces show negative refraction for both orthogonal states of polarization in a specific wavelength band. A cylindrical 1D-MPC plano-concave lens is designed to simultaneously focus radially and azimuthally polarized beams to subwavelength dimensions in visible spectrum. This phenomenon is simulated using the finite element method. Furthermore, variation of the polarization components in CVB can directly modulate the spacial intensity and the polarization distribution in the focal field. In fact, subwavelength focusing of CVB with arbitrary polarization components can be achieved by using the 1D-MPC plano-concave lens. The scheme proposed in this paper can be taken as reference for other wavelength bands as well. This study is also valuable for optical micro-manipulation of small particle, super-resolution imaging, and other related areas.