Magnet-free broadband unidirectional thermal emission in gradient Weyl semimetal structure

Although nonreciprocal systems based on magneto-optical materials can support unidirectional thermal emission, they require bulky devices for applying external magnetic field and are practically unfavorable. Here, we present magnet-free broadband unidirectional thermal emission in a gradient Weyl semimetal structure supported by a conductor substrate coated with an isotropic dielectric film. The multilayer structure of gradient epsilon-near-zero Weyl semimetals supports radiative Berreman mode over a broad spectral range, which enables controlling the polar emission angle over a broad spectral range. Due to significantly large off-diagonal components of the dielectric tensor, Weyl semimetal displays strong anisotropy by the same mechanism as the Voigt effect, leading to strong azimuthal angular selectivity. Theoretical results show that the system displays unidirectional thermal emission with directionality characterized by a spatial angle of 0.2 Sr over the entire atmospheric transparency window, offering promising applications in directional heat transfer, radiative cooling, and thermal camouflage.