Broadband achromatic dielectric metalenses

TLDR: This work developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses.

Abstract: Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length over λ = 1200–1650 nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses. Small, high-performance imaging systems could be built using flat lenses made from specially arranged nanoscale pillars. Traditional lenses rely on the curvature and thickness of glass to focus light, but metalenses, which can be smaller, thinner, and more flexible, have surfaces comprised of thousands of nanoscale pillars whose geometries are carefully designed to control optical phase. However, problems still arise in maintaining the same focal length across a wide wavelength range, leading to image blurring. Now, Nanfang Yu at Columbia University in New York, USA, and co-workers have designed a library of meta-units—the nano-pillars used to create metalenses—with several different cross-sectional geometries. They have combined these meta-units in various patterns to build broadband metalenses, which exhibit consistent focal length across a broad near-infrared wavelength range, significantly improving the final image quality. Furthermore, such metalenses work in the transmission mode and can focus light of any arbitrary polarization state.