Generation of a sub-wavelength optical needle by wavelength shifting and center masking of a Fresnel zone plate

Abstract

In this paper, a method to generate a sub-wavelength optical needle is proposed and demonstrated by wavelength shifting and center masking in the Fresnel zone plate (FZP). In theory, the vectorial angular spectrum (VAS) theory combined with genetic algorithm (GA) is used for the design of the center-masking FZP for generating optical needle, and finite-difference time-domain (FDTD) method is used for theoretical validation. In experiment, an amplitude-type center-masking FZP with a processing error of 5 nm is fabricated by focused ion beam etching (FIB), and the focusing intensity distribution of the optical needle is measured based on a self-made device. Finally, a sub-wavelength optical needle in far field is obtained, featuring a depth of focus of 7.16 µm, a central focal length of 26.87 µm, and a minimum full width at half maximum of 500 nm and 467 nm in x and y directions, respectively. This study provides both theoretical and experimental foundations for the practical application of FZPs.