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Sci Rep. 2017 Aug 7;7(1):7380. doi: 10.1038/s41598-017-07810-8.

A subwavelength spot and a three-dimensional optical trap formed by a single planar element with azimuthal light.

Author information

1
Institute of Ultra-Precision Optoelectronic Instrument Engineering, Harbin Institute of Technology, Harbin, 150080, China.
2
Institute of Ultra-Precision Optoelectronic Instrument Engineering, Harbin Institute of Technology, Harbin, 150080, China. linjie@hit.edu.cn.
3
Department of Electrical and Computer Engineering, Dalhousie University, Halifax, Nova Scotia, B3J 1Z1, Canada.
4
Institute of Ultra-Precision Optoelectronic Instrument Engineering, Harbin Institute of Technology, Harbin, 150080, China. P.Jin@hit.edu.cn.

Abstract

The generation of subwavelength spots smaller than the Abbe diffraction limit has attracted great interest due to the various applications in many fields, such as high-density optical data storage and particle manipulation. Planar optics that can miniaturize conventional refractive optics have become increasingly attractive. In this work, we first formed a subwavelength bright spot and a three-dimensional optical trap under the illumination of an azimuthally polarized (AP) beam by only a single planar element, a spiral zone plate (SZP). Initially, the SZP was proposed as a computer-generated hologram to generate optical phase singularities. However, the SZP in this work was used to focus and modulate the incident AP beam with a vortex phase simultaneously. Therefore, no additional vortex phase modulating element was introduced in our method. The SZP has an ultra-long focal length of 250λ for a numerical aperture (NA) of 0.95 and an incident wavelength of 632.8 nm. The generated spot is purely transversely polarized with a lateral full width at half maximum (FWHM) of 0.43λ beyond the diffraction limit of 0.54λ. The generated focal field formed a stable optical trap for a Rayleigh dielectric particle in three dimensions.

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