A Metalens with Near-Unity Numerical Aperture

TL;DR

This paper introduces a flat metalens with a near-unity numerical aperture (>0.99) that is ultra-thin, efficient, and capable of high-resolution imaging, surpassing traditional and previous flat lens designs.

Contribution

The work demonstrates a novel, experimentally validated flat lens with near-unity NA, operating at 715 nm, using a new diffractive design that overcomes efficiency limitations of prior metasurfaces.

Findings

Achieved a flat lens with NA > 0.99

Demonstrated sub-wavelength thickness (~λ/3)

Validated imaging capability with diamond nanocrystals

Abstract

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high NA lenses in an ultra-flat fashion. However, so far, these have been limited to numerical apertures on the same order of traditional optical components, with experimentally reported values of NA <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction limited flat lens with a near-unity numerical aperture (NA>0.99) and sub-wavelength thickness (~{\lambda}/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in sub-diffractive diamond nanocrystals. This work, based on diffractive elements able to efficiently bend light at angles as large as 82{\deg}, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated to the standard, phase mapping approach.