Magnet-free nonreciprocal thermal emitters via VO2–Weyl semimetal double-sided grating metasurfaces

Abstract

We propose a magnet-free route to nonreciprocal thermal emission using a VO₂–Weyl semimetal (WSM) heterostructure patterned with double-sided one-dimensional Ge gratings. The metasurface consists of a VO₂ film of thickness d₁ on a WSM layer of thickness d₂, with identical gratings with period p = 9 μm, a fill factor of f = 0.5, and grating depth h = 100 nm on the top and bottom interfaces. Under oblique observation (θ,ϕ) for both s- and p-polarizations, simultaneous breaking of mirror and spatial-inversion symmetries by the dual gratings, together with the intrinsic gyrotropic optical response of the WSM, produces a k → −k -asymmetric photonic density of states. Full-vector rigorous coupled wave analysis shows that the grating's reciprocal-lattice vectors phase-match thermally excited evanescent fields to hybrid modes, including guided-mode resonances, Fabry-Pérot resonances in VO₂, and surface modes in the WSM, yielding narrow and high-Q emission bands with strong asymmetry between forward and backward observation. The VO₂ insulator–metal transition enables reversible thermal tuning of isolation contrast and center wavelength, while the parameters (p,f,h) control the bandwidth and angular selectivity. This platform offers a fabrication-friendly path to compact thermal diodes, one-way emitters, and on-chip mid-infrared heat routing, achieving nonreciprocity without external magnets.