Bandwidth and size limits of achromatic printed-circuit metasurfaces

Metasurfaces can implement a wide variety of wave-manipulation functions with sub-wavelength layers. They are typically created from resonant elements, thus their refraction properties depend strongly on frequency. The resulting chromatic aberration is undesirable for most applications, motivating recent efforts in the development of achromatic metasurfaces. However, it remains unclear whether there are any physical limits on the achievable operating bandwidth of achromatic metasurfaces. Here we address this question, considering a common microwave metasurface geometry based on three metallic layers, separated by dielectric substrates. Since each of these metallic layers is modeled as an impedance, we apply Foster's reactance theorem to determine the bandwidth over which they are physically realizable using passive, causal and lossless structures. We derive limits for the bandwidth and total size of the metasurface, showing that there is a trade-off between these two parameters. A higher angle of refraction, corresponding to a larger numerical aperture for a lens, further limits the realizable bandwidth. We consider both Huygens' and Omega-bianisotropic metasurface types, and show that the limit is more severe for bianisotropic metasurfaces, making them less suitable for broadband achromatic designs.