Compact mid-infrared broadband absorber based on hBN/metal metasurface

We propose a route to obtain broadband absorption in the mid-infrared region by coupling plasmon and phonon polaritons in compact hBN/metal metasurface. Here we show, with the assistance of the dispersion relation contours, that hybrid phonon-plasmon polaritons modes can exist in the hBN/metal heterostructures. By exciting these modes, i.e., hyperbolic phonon-plasmon polaritons in the Reststrahlen band and surface phonon-plasmon polaritons out of the Reststrahlen band of hBN, broadband near-perfect absorption can be achieved when carving the hBN/metal mutilayer into sawtooth gratings. The thickness of the gratings is about λ0/13, which is an ultra-thin metasurface compared with previous works. The electric field and power dissipation density distributions are plotted to elucidate the mechanisms of the near-perfect absorption in different regimes. Furthermore, we present a detail analysis about the influence of changing the shape of gratings on the absorption performance and quantitatively evaluate the role hBN played in the absorber. The proposed route of designing broadband absorbers will benefit many practical applications, especially in the mid-infrared range.