Ultra-broadband nearly perfect absorbers based on graphene-coated lossy metallic nanostructures

In this paper, we present a new thin-film broadband nanostructured metamaterial absorber based on a graphene-coated plasmonic lossy metal–insulator–metal. Using the finite element method, we study the absorption properties of the proposed nanostructure and demonstrate the possibility of achieving an ultrabroad absorption spectrum by combining graphene and lossy metals. We demonstrate that graphene coating of the lossy metallic nano-disks improves the average absorption of the metamaterial by more than 10% while acts as a surface protection layer for the lossy metal films. Using the optimized design parameters and owing to the resonance coupling, a broad absorption from 400 nm to 2300 nm, particularly in the near-infrared region from 700 nm to 2000 nm, with a nearly flat absorption curve and an average absorption of 95.25% and a maximum absorption of 99.94% can be achieved. Other advantages of the proposed optical nano-absorber based on lossy metals instead of gold and silver include higher melting point (1668 °C compared to 1064 °C), the lower production cost of the metamaterial by using alternative materials, polarization insensitivity of the structure, and relative insensitivity to the radiation angle. In particular, the absorber maintains nearly an average of 90% absorptivity as the light incident angle reaches 30°. The proposed graphene-coated absorber demonstrates broader and stronger absorption, and much higher structural meting point compared to the previously reported thin-film based broadband absorbers, resulting in a viable platform for broadband applications with higher stability and durability.