Nonreciprocal thermal emitter for near perpendicular incident light with cascade grating involving weyl semimetal

A nonreciprocal thermal emitter, consisting of a high refractive dielectric grating atop a Weyl semimetal (WS) grating and a metallic reflector, is proposed and studied. The emitter exhibits unequal directional absorption and emission, which breaks the traditional Kirchhoff's law and leads to the realization of strong nonreciprocal radiation. What's more, near-perfect nonreciprocity is realized at an angle of only 1o, which breaks the constraint of large operating angle confronted by existing nonreciprocal emitters and significantly enlarges the angular range. The physical origin of such phenomenon is disclosed by analyzing the magnetic field amplitude distribution and further confirmed by the impedance matching theory. To further confirm the range of angles that can be used, the strong nonreciprocity at several other angles is investigated. Last, the influence of the axial vector of the WS and the structure dimensions on the nonreciprocity is also investigated. We believed that the results should be useful for improving the efficiency of energy harvesting structures and developing novel thermal devices.