Ultrathin perfect absorber with graphene. Reflectance (a) and relative phase of the reflected field (b) as a function of cavity thickness, d. The solid white lines point at the plasmon resonance of the corresponding graphene metasurface without the cavity. (c) Reflectance, absorbance and phase for the case of cavity thickness d = 1 μm. The lines correspond to analytics and the dots to full electrodynamic simulations. The conductivity modulation strength of graphene was w ₀ = 3 in all cases.