Wide-angle camouflage detectors by manipulating emissivity using a non-reciprocal metasurface array

Camouflage detectors that can detect incoming radiation from any angle without being detected are extremely important in stealth, guided missile, and heat-seeking missile industries. In order to accomplish this, the absorption and emission processes must be manipulated simultaneously. However, Kirchhoff's fundamental law suggests that absorption and emission are always in the same direction α(θ) = ε(θ), i.e., absorption and emission are reciprocal. This means that the emission from the detector always points back to the source, for example towards a laser source in a guided missile. Thus, detector emission serves as a complementary measure to hide an object. Here, we present a novel camouflage detector that uses a nonreciprocal metasurface array to independently detect the direction of the incoming radiation as well as manipulate its emissivity response. This is accomplished by using a magneto-optical material called indium arsenide (InAs), which breaks Lorentz reciprocity and Kirchhoff's fundamental law such that α(θ) ≠ ε(θ). This design results in the following absorption and emission: α(θ) = ε(-θ). Nine metasurfaces were designed, optimized, and operated at different incident angles from +50° to -50° at a wavelength of 13 μm. Furthermore, by keeping all metasurfaces in a pixilated array form, one could make a device that works over the full ±50° range. Potentially, this array of nonreciprocal metasurfaces can be used to fabricate thermal emitters or solar-harvesting systems.