Broadband Nonreciprocal Thermal Radiation With Weyl Semimetal-Based Pattern-Free Heterostructure

Violation of Kirchhoff's law is of fundamental importance to improve the efficiency of energy harvesting. However, experimental confirmation of breaking the balance between emissivity and absorptivity with complex designs are difficult, and previous theoretical proposals are restricted to narrow band nonreciprocal radiation. Here, we offer a lithography-free avenue to create structure for the realization of pronounced nonreciprocity without an external magnetic field. The proposal consists of a dielectric spacer separating a Weyl semimetal (WSM) film from a back-reflector. This dielectric layer introduces an additional degree of freedom in optimizing the nonreciprocal thermal emitter, via imposing a tunable phase to the reflected fields. The difference between absorption and emission at wavelength of $10.99 similar to\mu m$ can reach 0.932 at the incident angle of 0 degrees. By engineering the structural parameters, we reveal a surprising result of breaking Kirchhoff's law without requiring any surface patterning within broad wavelength range, which identifies unique fundamental and technological prospects of WSMs for engineering thermal radiation with various requirements.