Inverse-designed achromatic flat lens enabling imaging across the visible and near-infrared with diameter > 3 mm and NA = 0.3

Abstract

It is generally thought that correcting chromatic aberrations in imaging requires multiple surfaces. Here, we show that by allowing the phase in the image plane of a flat lens to be a free parameter, it is possible to correct chromatic aberrations over a large continuous bandwidth with a single diffractive surface. In contrast to conventional lens design, we utilize inverse design, where the phase in the focal plane is treated as a free parameter. This approach attains a phase-only (lossless) pupil function, which can be implemented as a multi-level diffractive flat lens that achieves achromatic focusing and imaging. In particular, we experimentally demonstrate imaging using a single flat lens of diameter > 3 mm and focal length = 5 mm (NA = 0.3, f/1.59) that is achromatic from λ = 450 nm (blue) to 1 μm (NIR). This simultaneous achievement of large size, NA, and broad operating bandwidth has not been demonstrated in a flat lens before. We experimentally characterized the point-spread functions, off-axis aberrations, and broadband imaging performance of the lens.