Multilayer Huygens' metasurfaces as a platform for large area multiwavelength metalenses

Large lens apertures are crucial for many emerging metalens applications. For example, it is particularly important in achieving adequate signal to noise ratio in atmospheric monitoring and long-distance laser communications, which often require lens diameters on the order of centimetres in size. Achromatic and multiwavelength large-area metalenses pose unique open research questions. Central to this is the issue of scale. Centimetre scale metalenses must be engineered on both the macro-scale size of the entire lens as well as that of the subwavelength scatterers. The mismatch in scale leads to two main challenges. First, it limits the applicability of freeform inverse design procedures due to the computational domain size. Second, under a traditional design approach individual meta-atoms have constraints on their maximum phase-dispersion values which in turn significantly restrict the diameter of an achromatic single layer lens phase profile for a given numerical aperture (NA). Here we demonstrate non-interacting multilayer Huygens' metasurfaces as a platform to create centimeter scale multiwavelength metalenses. In this configuration each layer modulates a specific wavelength while achieving high transmittance and low phase disturbance at alternative wavelengths. This significantly eases alignment tolerances and allows each layer to be fabricated as a separate metalens. The operating mechanism, main design considerations and limitations are discussed. An example two-wavelength 0.12 NA metalens operating at 2 and 2.34 μm is then developed using an evolutionary algorithm based inverse design scheme within the locally periodic approximation (LPA). It is simulated using FDTD and numerically characterized.