Inverse design method for metalens with optical vortices towards light focusing through localized phase retardation

TL;DR

This paper introduces an inverse design method for ultrathin metalenses with concentric nanoring structures that generate optical vortices, enabling diffraction-limited focusing and improved manufacturability.

Contribution

It proposes a novel inverse design approach for metalenses with nanoring topology, simplifying manufacturing and enabling precise control of focusing properties.

Findings

Design of convex-like metalenses with specified numerical aperture

Generation of optical vortices in nanorings to match phase and impedance

Extension to axicon-like metalenses with focal beams

Abstract

Metalenses can achieve diffraction-limited focusing through localized phase manipulation of the incoming light beam. Because these structures are ultrathin, less than a wavelength, this has the potential of achieving ultrathin optical elements, with a thickness limited mainly by the mechanical strength of the transparent substrate. Recently proposed metalenses are based on either dielectric nanofin arrays, or nanoparticles of large number, which leads to severe manufacturing challenges. To overcome these challenges, this paper predicts a new type of metalens with concentric-nanoring topology, where the number and size of the nanorings are determined using an inverse design method. By focusing the electrical field energy at a specified position, the convex-like metalens is inversely predicted with desired numerical aperture and a diffraction-limited focal spot. The Poynting vector distribution found demonstrates the mechanism of the lensing function, in which optical vortices are generated in the nanorings to achieve a matching of the phase and impedance between the substrate and free space, and further, to form a spherical wavefront and enhance the transmission of the optical energy. The inverse design method can also be extended to predict an axicon-like metalens with focal beam. The improved manufacturability is concluded from the geometry of the concentric-nanoring configurations.