Perfect mid-infrared absorption by hybrid phonon-plasmon polaritons in hBN/metal-grating anisotropic structures

A nanostructure based on periodic metal gratings and hexagonal boron nitride (hBN) films is proposed here. As a phononic two-dimensional (2D) material, hBN has two Reststrahlen bands with natural hyperbolic responses. Magnetic polaritons (MPs) in metal gratings can couple with hyperbolic phonon polaritons (HPPs) in hBN to create hybrid hyperbolic phonon-plasmon polaritons, resulting in perfect or near-perfect absorption. It is found that the majority of the power absorbed by the hybrid polaritons is dissipated in the hBN film with a unique tunable location-dependent absorption profile. Meanwhile, out of the hyperbolic region, optical phonons in hBN can couple with MPs in gratings to yield resonance absorption. The radiative properties and dissipation profiles are calculated based on rigorous coupled-wave analysis, which has been extended to include anisotropic materials, to elucidate the mechanisms of the perfect and near-perfect absorption. This work reveals the possibility of using 2D materials and nanostructures to achieve unique radiative properties for applications such as energy harvesting, radiative cooling, and photodetection.