Broadband infrared metamaterial absorber with raindrop-shaped submicron-scale disk array

In this paper, we present results from our theoretical and experimental exploration of tailoring the absorption spectrum of a type of metamaterial absorber through manipulating the symmetricity and uniformity of the metallic submicron particle array on the top layer. The absorber under study is a metal-insulator-metal (MIM) trilayer structure made up of a top layer of engineered metallic submicron particles, a middle insulator spacer layer and an opaque ground metal reflector layer. We first studied the structure with a top layer consisting of a uniform array of raindrop-shaped gold (Au) submicron disks. We designed the raindrop shape with a reflectional symmetry on the 45° line. We compared the spectrum generated with that of a similar structure but the top layer which is filled with uniformly arranged circular submicron discs. It has been well reported that an array of circular particles each with both reflectional and rotational symmetries usually generates a spectrum with one absorption spike. By changing the circular shape to raindrop shape, the MIM absorber has been predicted to generate two absorption peaks with significantly broadened absorption bandwidth. Subsequently, we found that even wider spectra could be achieved if the top layer is built with a periodic arrangement of the unit cells containing differently sized raindrop-shaped disks. This leads to a wider bandwidth of higher than 50% absorbance ranging from 2.80 μm to 3.90 μm.