  Metamaterials and metasurfaces are widely used to manipulate electromagnetic
waves over a broad range of wavelengths. Several recent efforts have focused on
metalenses, ultra-thin optical elements that focus light using subwavelength
structures. Compared to their refractive counterparts, metalenses offer reduced
size and weight, improved manufacturability, and new functionality such as
polarization control. However, metalenses that correct chromatic aberration
also suffer from markedly reduced focusing efficiency. Here we introduce a
Hybrid Achromatic Metalens (HAML), that overcomes this trade-off and offers
improved focusing efficiency over a broad wavelength range from 1000 - 1800 nm.
Fabricated HAMLs demonstrated diffraction limited performance for numerical
apertures (NA) of 0.27, 0.11, and 0.06 with average focusing efficiencies $>
60\%$ and maximum efficiencies ~ $ 80\%$. HAMLs can be designed by combining
recursive ray-tracing and simulated phase libraries rather than computational
intensive global search algorithms. Moreover, HAMLs can be fabricated in
low-refractive index materials using multi-photon lithography for customization
or using molding for mass production.
