  Metasurfaces are nano-structured devices composed of arrays of subwavelength
scatterers (or meta-atoms) that manipulate the wavefront, polarization, or
intensity of light. Like other diffractive optical devices, metasurfaces suffer
from significant chromatic aberrations that limit their bandwidth. Here, we
present a method for designing multiwavelength metasurfaces using unit cells
with multiple meta-atoms, or meta-molecules. Transmissive lenses with
efficiencies as high as 72% and numerical apertures as high as 0.46
simultaneously operating at 915 nm and 1550 nm are demonstrated. With proper
scaling, these devices can be used in applications where operation at distinct
known wavelengths is required, like various fluorescence microscopy techniques.
