Large Numerical Aperture Metalens with High Modulation Transfer Function

Abstract

Large numerical aperture (NA) lenses with high modulation transfer functions (MTFs) promise high image resolution for advanced optical imaging. However, it is challenging to achieve a high MTF using traditional large-NA lenses, which are fundamentally limited by the amplitude mismatch. In contrast, metasurfaces are promising for realizing amplitude and phase matching for ideal lenses. However, current metalenses are mostly based on a phase-only (PO) profile because the strong coupling among the metaatoms in large-NA lenses makes perfect amplitude matching quite challenging to realize. Here, we derive a phase-and-amplitude (PA) profile that approaches the theoretical MTF limit for large-NA lenses and use interferometric unit cells combined with a segmented sampling approach to achieve the desired amplitude and phase control. For the first time, we show that the amplitude does not require a perfect match; realizing the trend of the required amplitude is sufficient to significantly increase the MTF of a large-NA lens. We demonstrated a 0.9 NA cylindrical metalens at 940 nm with a Struve ratio (SR), which describes how close the MTF is to the upper limit, increasing from 0.68 to 0.90 compared with the PO metalens. Experimentally, we achieved an SR of 0.77 for the 0.9 NA lens, which is even 0.09 higher than the simulated SR of the PO metalens. Our investigation provides new insights for large-NA lenses and has potential applications in high-image-resolution optical systems.