Figure 1. Tunable scheme to overcome the phase distortions in passive metasurfaces.

Typical phase responses of meta-atoms within a supercell in a (a) passive and (b) a tunable gradient metasurface, at working frequency f = f₀ and non-working frequencies f ≠ f₀. The nonlinear phase distributions at non-working frequencies in the passive device, caused by frequency dispersions of passive meta-atoms (see inset to Fig. 1a), can be rectified by active external controls in the tunable device. Typical scattering patterns of (c,e) passive or (d,f) tunable metasurfaces at (c,d) off-working frequencies and (e,f) working frequency.