Tunable and broadband coherent perfect absorption by ultrathin black phosphorus metasurfaces
Atomically-thin materials have limited interaction with electromagnetic radiation. By utilizing clever structuring, however, physicists from the USA proposed that such thin materials can absorb nearly 100% of radiation in a wide frequency range. Remarkably, a three-layer structure with an overall thickness of about 20 nm can absorb long infrared waves with the wavelength around 30 um. Since discovery of graphene, a two-dimensional sheet of carbon atoms, there is a strong interest towards the studies of fundamental properties and potential uses of various atomically thin materials. Here, authors consider two-dimensional black phosphorus, and show that it is superior to graphene in terms of the achievable bandwidth of absorption. They study the structure made of a thin dielectric layer covered by rectangular patches of black phosphorus from either side, and theoretically analyze its properties. In order to achieve large absorption, authors consider the so-called coherent absorption regime, when the incident wave is split into two parts, which are then sent on the structure from opposite directions. As it was predicted before, in such a scheme the broadband operation can be achieved when the absorber is very thin, and the structure proposed in this work confirms this.