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

摘要

Kirchhoff’s law shows that reciprocal materials have equal spectral emissivityat two symmetric polar angles, which is a fundamental limit for athermal emitter to achieve a small angular divergence in the normal direction.Nonreciprocal materials allow violation of Kirchhoff’s law as the emissivity atthe two symmetric polar angles can be different. However, achieving strongnonreciprocal thermal radiation near zero angle is challenging. In this work,to reduce the power consumption of a light source for e.g. gas sensing, anultra-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 at22.36 μm or 22.99 μm at zero angle. The strong nonreciprocity contributed bythe collective modes allows for a near-complete violation of Kirchhoff’s lawat small angles of ±1°. The nonreciprocal emitters have a very small angulardivergence (≈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 thermalemitters and their applications in high-resolution thermal imaging, infraredgas sensing, biomedical breath monitoring, and so on.