Nonreciprocal Reflection of Mid-Infrared Light from Flat InAs Interfaces at Low Magnetic Fields

Lorentz reciprocity—which, broadly speaking, states that a source and a detector of electromagnetic radiation can be interchanged—assumes that the constitutive relations in Maxwell’s equations are linear and that the permittivity and permeability tensors are time-invariant and symmetric. The latter assumption can be broken using magnetic fields to create antisymmetric off-diagonal components of the permittivity tensor, giving rise to a number of nonreciprocal, magnetooptic effects such as the Faraday effect [1]. Nonreciprocity enabled by magnetic fields is of particular interest for its applications to thermal radiation, as it can lead to highly directional emission and absorption and the breakdown of Kirchhoff’s law of thermal radiation [2]. Candidate materials for these applications include highly-doped semiconductors such as InSb and InAs and magnetic Weyl semimetals [3], as they are predicted to support nonreciprocity in the mid- to far-infrared spectral range. Despite the promise of these materials, few experiments have been done to understand their capabilities under different conditions. We experimentally observed nonreciprocal reflection of mid-infrared light (~16 μm) from flat InAs interfaces at magnetic fields lower than 0.2 T, achieved using low-cost, off-the-shelf neodymium magnets. Using spectroscopic ellipsometry, we showed that the amplitude ratio Ψ and phase shift Δ of reflected light are nonreciprocal in the Voigt configuration (magnetic field direction perpendicular to the plane of incidence). Furthermore, we showed that it is possible to fit the permittivity tensor by measuring Ψ and Δ, without the need for time-consuming Mueller matrix measurements, using symmetries present in the Voigt configuration. Fourier-transform infrared spectroscopy (FTIR) measurements confirmed that the reflectance of p-polarized light is nonreciprocal. The reflectance contrast, defined as the difference in reflectance at opposite angles of incidence, increases with the magnetic field magnitude for p-polarized light and is zero for s-polarized light, as expected. We observed this phenomenon at magnetic fields as low as 0.07 T, without the need for couplers such as gratings or prisms. Our work is a step toward practical implementation of nonreciprocal thermal emitters and absorbers and could enable other applications such as remote magnetic field sensors. This work is supported by ARO MURI (Grant No. W911NF-19-1-0279) via U. Michigan. S. P. gratefully acknowledges support from the NSF GRFP under Grant No. 2141064.<br/><br/>References<br/>[1] L. D. Landau, E. M. Lifshitz, Electrodynamics of Continuous Media, 2nd ed. (Pergamon Press Ltd., Elmsford, NY, 1984).<br/>[2] K. J. Shayegan, S. Biswas, B. Zhao, S. Fan, H. A. Atwater, Nat. Photonics 17, 891 (2023).<br/>[3] Y. Tsurimaki, X. Qian, S. Pajovic, F. Han, M. Li, G. Chen, Phys. Rev. B 101, 165426 (2020).