Leveraging in-plane anisotropic materials for lithography-free omnidirectional thermal emission control

We present a theoretical framework for controlling thermal emission in both the azimuth and zenith directions by integrating an in-plane uniaxially anisotropic spacer into a planar three-layered heterostructure. We analytically derive design conditions for directional thermal emission in planar structures with in-plane anisotropic spacer layers. We predict a regime of polarization-selective thermal emissivity along different crystal axes, as well as a regime of high emissivity over a narrow angular cone. As a realistic example, we propose structures with a calcite (CaCO3) spacer combined with a silicon carbide (SiC) emitter and a gold back-reflector, achieving high directivity in the midinfrared. These results demonstrate a lithography-free route to narrow band, polarization-dependent control of the thermal emission direction in simple multilayer structures.