Coherent perfect absorption in epsilon-near-zero metamaterials

In conventional materials, strong absorption usually requires that the material have either high loss or a large thickness-to-wavelength ratio (d/lambda >> 1). We find the situation to be vastly different for bilayer structures composed of ametallic substrate and an anisotropic epsilon-near-zero (ENZ) metamaterial, where the permittivity in the direction perpendicular to its surface, epsilon(z), vanishes. Remarkably, perfect absorption can occur in situations where the metamaterial is arbitrarily thin (d/lambda << 1) and arbitrarily low loss. Our numerical and analytical solutions reveal that under the conditions epsilon(z) -> 0 and J(epsilon(z)) >> R(epsilon(z)), at perfect absorption there is a linear relationship between the thickness and the loss, which means the thickness of the absorber can be pushed to zero by reducing the material loss to zero. This counterintuitive behavior is explained in terms of coherent perfect absorption (or stimulated absorption) via critical coupling to a fast wave propagating along the ENZ layer.