A new method to produce optical nano-needle

A method is described to design a microstructure comprised of multi-Fresnel zone plate (FZP) fragments for shaping an optical needle with arbitrary length. Thus, a microstructure comprised of three planar FZP fragments with different focal lengths f₁, f₂, and f₃ is designed to form a long optical needle by delicate interference of coherent light beams diffracted from these three FZP fragments. For a 74.34-μm-diameter microstructure illuminated with a linearly x-polarized beam, a 7.87-λ-long optical needle is produced at a distance of 12.31 λ away from the mask surface. The sizes of transverse beam are 0.97 and 0.4 λ in x and y directions, respectively. For this work, the vectorial angular spectrum (VAS) theory is employed to describe the electric field of light behind the microstructure, as well as the three-dimensional finite-difference time-domain (3D FDTD) method is adopted to further verify the results obtained.