Metasurface Enabled Wide‐Angle Fourier Lens

TLDR

Fourier optics underpins modern optical processing, but conventional thin lenses are limited to near‑normal incidence, causing loss of high‑order Fourier components and leading to bulky, costly high‑NA Fourier lenses. A dielectric metasurface of high‑aspect‑ratio silicon waveguides experimentally achieves 1‑D Fourier transformation over a wide incident‑angle range and broad bandwidth, expanding the Fourier space with large numerical aperture, negligible angular dispersion, and compact form suitable for micro‑optical integration.

Abstract

Abstract Fourier optics, the principle of using Fourier transformation to understand the functionalities of optical elements, lies at the heart of modern optics, and it has been widely applied to optical information processing, imaging, holography, etc. While a simple thin lens is capable of resolving Fourier components of an arbitrary optical wavefront, its operation is limited to near normal light incidence, i.e., the paraxial approximation, which puts a severe constraint on the resolvable Fourier domain. As a result, high‐order Fourier components are lost, resulting in extinction of high‐resolution information of an image. Other high numerical aperture Fourier lenses usually suffer from the bulky size and costly designs. Here, a dielectric metasurface consisting of high‐aspect‐ratio silicon waveguide array is demonstrated experimentally, which is capable of performing 1D Fourier transform for a large incident angle range and a broad operating bandwidth. Thus, the device significantly expands the operational Fourier space, benefitting from the large numerical aperture and negligible angular dispersion at large incident angles. The Fourier metasurface will not only facilitate efficient manipulation of spatial spectrum of free‐space optical wavefront, but also be readily integrated into micro‐optical platforms due to its compact size.