Dual-polarized, mid-infrared nonreciprocal absorption
The emission and absorption of thermal radiation are usually coupled via Kirchhoff's law or reciprocity, stated as the equality of spectral directional emissivity and absorptivity. Magneto-optical materials have recently been identified as a promising route to lifting the constraint of reciprocity, with multiple experimental demonstrations using doped InAs. However, these demonstrations have been limited to p-polarized light in the Voigt configuration, whereas thermal radiation from a blackbody is unpolarized. Therefore, to break reciprocity in both polarization channels, we design a nanophotonic, dual-polarized nonreciprocal absorber operating in the mid-infrared spectral range (11-20 $\unicode{x03BC}$m), consisting of an a-Si photonic crystal slab on top of a doped InAs substrate described by an antisymmetric, nonreciprocal dielectric tensor under an applied magnetic field. The photonic crystal slab supports eigenmodes that couple to both s- and p-polarized light, resulting in absorption peaks that frequency shift in opposite directions for forward- and backward-propagating light$\unicode{x2014}$a signature of nonreciprocity in planar, subwavelength systems. We fabricate our design, then measure its room-temperature absorptance using magnetic-field-integrated absorptance spectroscopy, experimentally demonstrating nonreciprocal absorption for both polarizations. Our design is a step toward the complete control of light as heat, which could improve photonic energy conversion, thermal management, and mid-infrared optical isolation and circulation.