Thermal Vertical Emitter of Ultra‐High Directionality Achieved Through Nonreciprocal Magneto‐Optical Lattice Resonances

Abstract Kirchhoff's law shows that reciprocal materials have equal spectral emissivity at two symmetric polar angles, which is a fundamental limit for a thermal emitter to achieve a small angular divergence in the normal direction. Nonreciprocal materials allow violation of Kirchhoff's law as the emissivity at the two symmetric polar angles can be different. However, achieving strong nonreciprocal thermal radiation near zero angle is challenging. In this work, to reduce the power consumption of a light source for e.g. gas sensing, an ultra‐high‐directional nonreciprocal thermal vertical emitter is proposed, with a periodic structure of magneto‐optical material. When B = 3 T or 1.5 T, magneto‐optical lattice resonances enable the near‐perfect emissivity at 22.36 µm or 22.99 µm at zero angle. The strong nonreciprocity contributed by the collective modes allows for a near‐complete violation of Kirchhoff's law at small angles of ±1°. The nonreciprocal emitters have a very small angular divergence (≈1°), which is better than that of the state‐of‐the‐art thermal emitters. The highly directional nonreciprocal thermal emission is robust despite ±25% change in material loss and ±5% fluctuation in structural parameters. This work should inspire the design of high‐directional nonreciprocal thermal emitters and their applications in high‐resolution thermal imaging, infrared gas sensing, biomedical breath monitoring, and so on.