Planar binary-phase lens for super-oscillatory optical hollow needles

Gang Chen,Kun Zhang,Jing Wu,Zhihai Zhang,Luru Dai,Anping Yu,Senlin Jiang,Zhixiang Wu,Changtao Wang,Zhongquan Wen,Xiangang Luo,Yinghu He

doi:10.1038/s41598-017-05060-2

Abstract

Optical hollow beams are suitable for materials processing, optical micromanipulation, microscopy, and optical lithography. However, conventional optical hollow beams are diffraction-limited. The generation of sub-wavelength optical hollow beams using a high numerical aperture objective lens and pupil filters has been theoretically proposed. Although sub-diffraction hollow spot has been reported, nondiffracting hollow beams of sub-diffraction transverse dimensions have not yet been experimentally demonstrated. Here, a planar lens based on binary-phase modulation is proposed to overcome these constraints. The lens has an ultra-long focal length of 300λ. An azimuthally polarized optical hollow needle is experimentally demonstrated with a super-oscillatory transverse size (less than 0.38λ/NA) of 0.34λ to 0.42λ, where λ is the working wavelength and NA is the lens numerical aperture, and a large depth of focus of 6.5λ. For a sub-diffraction transverse size of 0.34λ to 0.52λ, the nondiffracting propagation distance of the proposed optical hollow needle is greater than 10λ. Numerical simulation also reveals a good penetrability of the proposed optical hollow needle at an air-water interface, where the needle propagates through water with a doubled propagation distance and without loss of its super-oscillatory property. The proposed lens is suitable for nanofabrication, optical nanomanipulation, super-resolution imaging, and nanolithography applications.

Highlights

Nondiffracting beams are propagating waves, which can travel without divergence over a comparative long distance
Among possible nondiffracting beam candidates, hollow beams have a zero intensity along the beam center-axis, making them promising for applications in optical micromanipulation, microscopy, and lithography, and have been widely investigated[31,32,33,34,35]
A hollow Bessel beam was generated by focusing a vortex beam with a water-immersed objective lens, and was applied to improve the performance of stimulated emission depletion (STED) microscopy[36]

Summary

Introduction

Nondiffracting beams are propagating waves, which can travel without divergence over a comparative long distance. Optical super-oscillation is referred to the optical phenomenon, in which the local spatial frequency of the optical field is larger than its global spatial frequency, and theoretically, it provides a possible means of creating arbitrary small features in far-field optics with the superposition of band-limited functions[16, 17] Based on this concept, sub-diffraction focusing has been demonstrated[18,19,20,21,22,23,24]. The generation of a sub-diffraction longitudinally polarized optical needle with a propagation distance of 4λ was firstly proposed theoretically by focusing a radially polarized Bessel-Gaussian beam using a combination of a binary-phase optical element and a high-NA lens[29]. The experimental generation of such a sub-diffraction optical needle was demonstrated recently by focusing radially polarized light with a single planar binary-phase lens[30]. Numerical simulation shows that the proposed optical hollow needle has good penetrability at an air-water interface with a substantially enlarged needle length in water

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Results

Conclusion