Unified theory for perfect absorption in ultrathin absorptive films with constant tangential electric or magnetic fields

Abstract

The maximal absorption rate of ultrathin films is 50% under the condition that the tangential electric (or magnetic) field is almost constant across the film in a symmetrical environment. However, with certain reflectors, the absorption rate can be greatly increased, even to perfect absorption (100%). In this paper, we explicitly derive the general conditions of the ultrathin absorptive film parameters to achieve perfect absorption with general types of reflectors under the condition that the tangential electric (or magnetic) field is almost constant across the film. We find that the parameters of the film can be classified into three groups, exhibiting (1) a large permittivity (permeability), (2) a near-zero permeability (permittivity), or (3) a suitable combination of permittivity and permeability, respectively. Interestingly, the latter two cases demonstrate extraordinary absorption in ultrathin films with almost vanishing losses. Our paper serves as a guide for designing ultrathin perfect absorbers with general types of reflectors.