Ultra-long and high uniform optical needle generated with genetic algorithm based multifocal positions optimization

Abstract

We propose a simple and effective method to generate an ultra-long and high uniform optical needle through tightly focusing a radially polarized beam with a pure-phase filter modulation. An optical needle with strong longitudinally polarized field is obtained through creating, shifting and merging multifocal spots on the optical axis. Genetic algorithm is introduced to improve the optimizing efficiency and provide a universal approach for solving multifocal positions simultaneously. The axial full-width at half-maximum (FWHM) along the optical axis has been extended to 20.3λ. More attractively, the 18λ long central part of the needle exhibits a uniformity of 96.7%. The maximum side lobes in axial and radial directions are 2% and 23% respectively. We further explored the maximum uniform length can be achieved in the needle under various amounts of focal spots to be merged, which may provide a reference for designing a uniform optical needle with customized length. The proposed methods offer simple and effective design processes for ultra-long and high uniform optical needles, which may find wide applications in particle acceleration and optical trapping.