Shaping a far-field optical needle by a regular nanostructured metasurface

A new method is proposed for shaping a far-field uniform optical needle based on a regular nanostructured metasurface, viz. high-NA (numerical aperture) micro-Fresnel zone plate (FZP). The designed microstructure is comprised of two planar FZP-fragments with different focal lengths. Delicate interference of diffracted beams results in an optical needle at a required working distance in the post-evanescent field. For a 44.88 µm-diameter microstructure illuminated with a linearly x-polarized beam, a 5.77λ-long uniform optical needle is produced at a distance of 12.88λ away from the mask surface. The transverse beam sizes are 0.97λ and 0.39λ in x and y directions, respectively. The designed result calculated by the vectorial angular spectrum theory is in good agreement with the rigorous electromagnetic calculation using the three-dimensional finite-difference time-domain (3D FDTD) method. The designed microstructure is easy-to-fabricated with required NA and working distance. The proposed method can be readily modified for other polarized beams. These far-field uniform optical needles potentially suit the fields of nanolithography, optical trapping, and microscopy.