Experimental demonstration of dual-polarization nonreciprocal thermal emission at moderate magnetic fields
Conventional thermal emitters obey Kirchhoff's law of thermal radiation, which requires emissivity and absorptivity to be equal at each wavelength, angle, and polarization. This in turn places a fundamental limit on a range of radiative energy technologies as well as heat transfer. This constraint can however be overcome using magneto-optic semiconductors, which break Lorentz reciprocity under an applied magnetic field. However, in most prior experimental demonstrations, nonreciprocity has been restricted to TM polarization, since the electric field of TE-polarized light is parallel to the applied magnetic field. Here, we experimentally demonstrate nonreciprocal absorption in both polarizations at the same wavelengths and angles from a single structure, wherein a germanium Mie-resonant metasurface on a doped InAs layer scatters TE-polarized light into near fields with components perpendicular to the magnetic field, allowing the magneto-optic response of the InAs to act on them. Under a 0.48 T field from a permanent Halbach array, this structure exhibits a difference in absorptivity between $+B$ and $-B$ of 0.035 in both TE and TM polarization at 16.6 $μ$m. Our approach shows that the magneto-optic response of a planar semiconductor and the resonances of a dielectric metasurface can be designed together, offering a potential route to dual-polarization nonreciprocity at fields available from permanent magnets for emerging energy and heat transfer technologies.