Free-form broadband flat lenses for visible imaging

TL;DR

This paper introduces free-form broadband-diffractive-optical elements (BDOEs) that create flat lenses capable of independently controlling resolution and light-collection ability, enabling advanced visible imaging with customizable focal properties.

Contribution

The authors design, fabricate, and demonstrate broadband flat lenses with decoupled numerical aperture and f-number, allowing independent control of resolution and light collection in visible imaging.

Findings

Achromatic focusing and broadband imaging confirmed experimentally.

Lenses exhibit high NA with large depth-of-focus.

Decoupling NA from f/# is feasible even at higher NAs.

Abstract

In the absence of separate apertures, the size and focal length of a lens determines both its resolution and light-collection ability defined by numerical-aperture (NA) and f/#, respectively. We utilize free-form broadband-diffractive-optical elements (BDOEs) to create flat lenses that decouple NA from f#, whereby the resolution of the lens may be chosen independently from its light-collection ability. Specifically, we designed, fabricated and characterized three BDOE lenses operating in the visible band (450nm-750nm), each with f/11.25, but with NAs of 0.00075, 0.0067 and 0.054, respectively. Simulations confirm that such decoupling is possible even at much higher NAs. Experiments confirm achromatic focusing and broadband imaging. One of the lenses exhibited a depth-of-focus almost 2 orders of magnitude larger than the diffraction limit. Such BDOE lenses could be very useful in focal-plane arrays with large pixel sizes, where light collection efficiency needs to be maintained. Furthermore, by abandoning rotational symmetry, one can achieve free-form geometries in the focal spot, such as a square that can more closely match the geometry of the sensor pixel.