Hybrid coupling and coherent perfect absorption in ultra-thin bilayer metamaterials

Subwavelength interference in ultra-thin bilayer metamaterials is investigated, where both near- and far-field interactions play important roles. A hybrid coupling model is established, which clearly shows the different contributions of near- and far-field couplings to electromagnetic responses: the former is only responsible for the energy level splitting, while the latter mainly reshapes the profile of the resonance spectra. Accordingly, the reflection spectrum exhibits a very sharp subradiant resonance within the envelope of a superradiant resonance and interference between them leads to a sharp Fano resonance. Under two-antisymmetric-beam illumination, the subradiant mode can be selectively excited, while the superradiant mode is highly suppressed, attributing to their different mode symmetry. A sharp coherent perfect absorption (CPA) peak with well-defined Lorentzian lineshape is achieved under critical conditions, relating to the completely excitation of subradiant mode. The peak frequency can be tuned by tailoring the near-field coupling, which alters the energy-level splitting. Ultrathin bilayer terahertz metamaterials with flexible polyimide substrate and interlayer are fabricated and tested by THz time domain spectroscopy, showing very good agreement with the theoretical and numerical results. This work provides a method of how to extract narrow subradiant resonance from an asymmetric Fano lineshape, which may enlighten the way for selective mode excitation via coherent illumination.