Asymmetric Directional Control of Thermal Emission.

Control over the directionality of thermal emission plays a fundamental role in efficient heat transport. Although nanophotonic technologies have demonstrated the capability for angular-selective thermal emission, achieving asymmetric directional thermal emission in reciprocal systems with energy directed to a single output angle remains challenging due to symmetric band dispersion. In this work, a general strategy for achieving asymmetric directional thermal emission in reciprocal systems is presented. With periodic perturbation and broken mirror symmetry, metasurfaces behave as resonant metagratings whose resonances can be diffracted to symmetric output angles with distinct efficiency, allowing for high emissivity toward a single direction. An asymmetric directional thermal emitter is experimentally demonstrated at mid-infrared wavelengths with high emissivity (ɛ = 0.61) at the observation angle of +30°, and low emissivity (ɛ < 0.3) at other angles. This work highlights the potential for manipulating the directionality of thermal emission, which holds promise for developing ultrathin customized thermal sources and impacts on various thermal-engineering applications.