The generation of ultra-long pure longitudinal magnetization needle and magnetization chain with multi-belt filter

In this paper, we optimized a multi-belt filter with ternary phase amplitude. Single and dual channel longitudinal magnetization needles were obtained by tight focusing azimuthally polarized sinh-Gaussian beam using by the multi-belt filter and spiral phase plate in single-lens system. And longitudinal magnetization chains were obtained using by the same incident beam and multi-belt filter in 4π focusing system. The depth of focus and transverse size was 64λ and 0.36λ in the obtained magnetization needles, respectively. For magnetization chains, the DOF was 60λ when the normalized intensity was within 0.8, and there were 43 magnetization spots in the range. The depth of focus and transverse size were 0.566λ and 0.36λ in each magnetization spots, respectively. In addition, the longitudinal magnetization chains can move along z-axis linearly by changing phase difference of the two counter-propagating incident beams.