Ultra-long optical needles with controllable homogeneously 3D spin-orientation produced with an annular spherical mirror

Abstract

Optical needles with controllable homogeneously 3D spin-orientation generated by an annular spherical mirror is theoretically investigated in this paper. An optical needle is a 2D super-resolved electric field which has a long longitudinal size and a sub-wavelength lateral size. Such optical fields have extensive applications in many fields such as optical data storage, optical trapping and fluorescent and so on. Controllable homogeneously 3D spin- orientation provides extra degree of freedom for the optical needles, which may further extend the applications. Spherical mirrors have large aberrations, which can be used to produce super-long optical needles. It is difficult for an aplanatic focusing system to generate an optical needle which is longer than 100λ (λ is wavelength). But a spherical mirror in this paper can produce an optical needle whose LFWHM (Longitudinal full width at half maximum) is about 1000λ while keeping mean TFWHM (transverse full width at half maximum) under 0.4λ. The spin-orientation of the optical needle can be controlled by changing incident beam. By adjusting the weight factors of the radially polarized component and azimuthally polarized components, the spin-orientation in the focal region can be changed. Using the extended Richards-Wolf vector diffraction theory, the electric field can be obtained. Further, SAM (spin angular momentum) density and SOHP (spin-orientation homogeneity purity) can be calculated. The results display that the SOHP can be beyond 0.93 as long as the weight factor of the radially polarized component is not too large.