  Nonreciprocal thermal emission is a cutting-edge technology that enables
fundamental control over thermal radiation and has exciting applications in
thermal energy harvesting. However, so far one of the foremost challenges is
making nonreciprocal emission to operate over a broad wavelength range and for
multiple angles. In this work, we solve this outstanding problem by proposing
three different types of structures always utilizing only one Weyl semimetal
(WSM) thin film combined with one or two additional dielectric or metallic
layers and terminated by a metallic substrate. First a tradeoff relationship
between the magnitude and bandwidth of the thermal nonreciprocity contrast is
established based on the thickness of the WSM film. Then, the bandwidth
broadening effect is demonstrated via the insertion of a dielectric spacer
layer that can also be fine-tuned by varying its thickness. Finally, further
control on the resulting strong nonreciprocal thermal radiation is demonstrated
by the addition of a thin metallic layer in the proposed few layer designs. The
presented composite structures work for a broad frequency range and multiple
emission angles, consisting highly advantageous properties to various
nonreciprocal thermal radiation applications. Moreover, the proposed designs do
not require any patterning and can be experimentally realized by simple
deposition fabrication methods. They are expected to aid in the creation of
broadband nonreciprocal thermal emitters that can find applications in new
energy harvesting devices.
