Planar Epsilon-Near-Zero Cavity for Nonreciprocity of Thermal Radiation Enhancement

Stacked epsilon-near-zero (ENZ)/insulator/ENZ nanocavity has recently emerged as a promising platform to study and engineer mid-infrared (MIR) absorption and emission, as they can be realized by lithography-free fabrication process with fine control on the optical and geometrical parameters. Using Weyl semimetal (WSM) thin film as nonreciprocal materials, we study the absorption, emission and nonreciprocity enhancement induced by a specifically tailored ENZ/WSM/ENZ structure. The nonreciprocity equals to 0.987 with a resonant wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9~\mu m$ </tex-math></inline-formula>, which confirms an obvious violation of Kirchhoff’s law. We also discuss the possibility of tailoring the magnitude and sign of nonreciprocity within the MIR spectrum simply by finely designing the thickness of each layer in the stack. The presented unpatterned configuration and broad tunability of high-quality resonance can work for a wide range of incidence angles, making such proposal with great potential for thermal scavenging and conversion.