On the design of perfect metamaterial absorbers

Perfect absorbing metamaterials comprising a metallic frequency selective surface printed on a thin low loss grounded substrate are analyzed. An efficient transmission line approach is employed to derive simple formulas which describe in detail the absorption mechanism of the subwavelength structure. In particular, the real part of the input impedance as a function of the imaginary part of the substrate dielectric permittivity is analytically derived in order to show why the presence of moderate losses in the substrate is sufficient to achieve matching with free space, that is, perfect absorption. The shape of the unit cell allows tailoring the absorption bandwidth in correspondence of the chosen resonant frequency. Once fixed the substrate thickness, the choice of low capacitive elements allows obtaining perfect absorption in a small frequency region with very low substrate losses while the choice of highly coupled elements allows obtaining the largest possible bandwidth with higher substrate losses.