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.

Wide-angle and Fano-shape contrast between emission and absorption in hybrid polariton-involved planar structure

“…The most exciting effect is that the wavelength-specific radiative emission spectrum is not necessarily equal to its directional absorption [24,25]. Emission and absorption in magnetooptical (MO) nanophotonic devices has been actively studied, the generated nonreciprocal effect can lead to a new way for handling heat radiance by breaking the widely-accepted Kirchhoff's law [26][27][28][29][30][31].…”

Broadband and wide angle nonreciprocal thermal emission from Weyl semimetal structures

“…In addition, a relevant design theoretically proposed tunable nonreciprocity by introducing a graphene monolayer between a metallic grating and InAs layer [15]. Highly directional emission was also achieved by using an InAs grating embedded inside an aluminum (Al) cavity producing strong nonreciprocity at angles extremely close to the normal direction [16]. An alternative magneto-optical material is indium antimonide (InSb) that was used to achieve broadband nonreciprocal thermal emission in a recent work [17].…”