Going beyond perfect absorption: Superdirective absorbers

In the context of electromagnetic absorption, it is obvious that for an infinite planar periodic structure illuminated by a plane wave the maximum attainable absorptance, i.e., perfect absorption, is theoretically limited to 100% of the incident power. Here we show that an intriguing possibility of overcoming this limit arises in finite-sized resonant absorbing arrays. We present a comprehensive analysis of a simple two-dimensional strip array over an infinite perfectly conducting plane, where the strips are loaded with bulk-impedance loads. The absorptance is defined as the ratio of the dissipated power per unit length of the strips to the incident power on a unit length of the array width. The results show that even regular subwavelength arrays of impedance strips can slightly overcome the limit of 100% absorptance, while with use of aperiodic arrays with optimized loads, absorptance can be significantly increased as compared with the scenario where the strips are identical. In principle, by tuning of the bulk loads, high superunity absorptance can be realized for all angles of illumination.