Broadband/multiband absorption through surface plasmon engineering in graphene-wrapped nanospheres.

In this paper, a thin film constructed by a periodic assembly of graphene-wrapped particles with spherical geometry has been proposed as a polarization-insensitive reconfigurable perfect absorber. The performance of the proposed structure is based on the cooperative excitation of the quadrupole localized surface plasmons on graphene shells. By sweeping the quality of graphene shells, it is recognized that the low-quality graphene material is the best choice for the absorber design. Moreover, the effect of graphene chemical potential and periodicity of the particles on the absorptivity of the structure is investigated. The physical mechanism of performance is clarified by investigating the excited localized surface plasmon resonances. In addition, the angle-independent behavior up to around 60 degrees for both transverse electric (TE) and transverse magnetic (TM) waves is proved. Interestingly, by engineering the substrate height, our proposed absorber exhibits dynamic broadband performance due to the impedance matching and multiband absorption by enhancing the Fabry-Perot resonances of a micrometer-sized substrate. The possibility of attaining a similar static broadband response by stacking multiple layers is also proved. Our proposed sub-wavelength absorber can be suitable for novel optoelectronic devices due to its simple geometry.