Circumventing size-bandwidth limits in imaging with flat lenses

TL;DR

This paper demonstrates how to surpass theoretical bandwidth limits of flat lenses using multi-level diffractive lenses, enabling large, lightweight, broadband imaging systems with potential for large-scale telescopes.

Contribution

The authors show a practical method to bypass bandwidth bounds in flat lenses by combining multi-level diffractive lenses with refractive lenses, expanding the design possibilities.

Findings

Achieved broadband flat lens operation from 400 nm to 800 nm.

Demonstrated a hybrid diffractive-refractive telescope with large aperture.

Bandwidth limit is constrained by image sensor, not lens size or NA.

Abstract

Recent theoretical work suggested upper bounds on the operating bandwidths of flat lenses. Here, we show how these bounds can be circumvented via a multi-level diffractive lens (MDL) of diameter = 100 mm, focal length = 200 mm, device thickness = 2.4{\mu} m and operating bandwidth from ? = 400 nm to 800 nm. We further combine the MDL with a refractive lens to demonstrate a hybrid telescope. By appealing to coherence theory, we show that the upper bound on relative bandwidth is surprisingly independent of lens diameter or numerical aperture, but is only limited by the bandwidth of the image sensor. Since large-area achromatic flat lenses produce significant reductions in weight over their refractive counterparts, these calculations and experiments open up opportunities for very large scale diffractive and diffractive-refractive telescopes.