  Metasurface lenses, namely metalenses, are ultrathin planar nanostructures
that are capable of manipulating the properties of incoming light and imparting
lens-like wavefront to the output. Although they have shown promising
potentials for the future miniaturization of optics, the chromatic aberration
inherited from their diffractive nature plagues them towards many practical
applications. Current solutions for creating achromatic metalenses usually
require searching through a large number of meta-atoms to find designs that
fulfill not only phase but phase dispersion requirements, which leads to
intensive design efforts. Besides, most designs are based on regular-shaped
antennas driven by the designers' intuition and experience, hence only cover a
limited design space. Here, we present an inverse design approach that
efficiently produces meta-atoms with unintuitive geometries required for
broadband achromatic metalenses. We restricted the generated shapes to hold
four-fold reflectional symmetry so that the resulting metalenses are
polarization insensitive. In addition, meta-atoms generated by our method
inheritably have round edges and corners, which make them
nanofabrication-friendly. Our experimental characterization shows that our
metalenses exhibit superior performance over a broad bandwidth of 465 nm in the
near-infrared regime. Our method offers a fast and efficient way of designing
high-performance achromatic metalenses and sheds new insights for unintuitive
design of other metaphotonic devices.
