# High-NA Achromatic Metalenses by Inverse Design

## TL;DR

This paper employs inverse design to create broadband, achromatic metalenses with high numerical apertures, surpassing traditional unit-cell methods in efficiency and achieving the first high-NA achromatic focusing.

## Contribution

It introduces a novel inverse design approach for high-NA achromatic metalenses, overcoming limitations of standard unit-cell techniques and establishing new efficiency benchmarks.

## Key findings

- Achieves high theoretical efficiencies at low NA.
- First to demonstrate achromatic high-NA focusing at NA 0.9 and 0.99.
- Provides computational bounds on unit-cell approach efficiencies.

## Abstract

We use inverse design to discover metalens structures that exhibit broadband, achromatic focusing across low, moderate, and high numerical apertures. We show that standard unit-cell approaches cannot achieve high-efficiency high-NA focusing, even at a single frequency, due to the incompleteness of the unit-cell basis, and we provide computational upper bounds on their maximum efficiencies. At low NA, our devices exhibit the highest theoretical efficiencies to date. At high NA -- of 0.9 with translation-invariant films and of 0.99 with "freeform" structures -- our designs are the first to exhibit achromatic high-NA focusing.