Bi-directional high-performance metamaterial perfect absorber for solar harvesting and refractive index sensing

Metamaterial perfect absorbers have been widely studied due to their potential applications in fields such as sensors, saltwater purification, switching devices, solar energy harvesting and so on. However, it is difficult for absorbers to have both narrowband absorption and broadband absorption. Up to till now, there are few reports on such absorbers. In this paper, we present an absorber that achieves ultra-high performance in an ultra-simple structure, which can attain bifunctional perfect absorption of broadband and narrowband. It consists of a multilayer planar structure with top layer periodically arranged particles. When the light is incident vertically above the surface of the absorber, the proposed absorber exhibits narrow-band absorption with three absorption peaks, among which two absorption peaks have an absorption rate of 99.3 % and 99.9 %, and the full widths at half maximum (FWHM) are 3 nm and 11 nm, respectively. When the light is incident under the surface of the absorber, then the absorber exhibits broad-band absorption, with an average absorption of 97.3 % over a 1500 nm bandwidth (600 nm-2100nm). In addition, the proposed multifunctional perfect absorber has a narrow FWHM and a large broadband absorption range in the same band range. Moreover, the absorber is both polarization-independent and large incident angle-insensitive for both TE and TM-polarized waves. This absorber is of great value in areas such as bio-optical detection and solar energy harvesting.