Experiment and Theory of the Broadband Absorption by a Tapered Hyperbolic Metamaterial Array

A broadband absorber based on a tapered alternating metal-dielectric multilayered structure was realized in the visible and IR range. The structure is fabricated by using a versatile method easily scalable to large areas by taking advantage of the line width reduction that occurs naturally in masked evaporation processes. The multilayered structure can be treated as a hyperbolic metamaterial (HMM), and the tapered structure can be regarded as a HMM waveguide with varying width. Light couples into the tapered structure most strongly at the mode cutoff position in the waveguide due to hyperbolic dispersion and the phase matching condition. Resonant cavities are formed between the top of the tapered structure and the cutoff level, producing strong absorption peaks. These resonances are closely spaced spectrally due to the high k modes in HMM. Finally a broad absorption is formed due to the broadening of the resonances from an array of coupled HMM tapered structures.