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 (${\mathrm{CaCO}}_3$) 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.

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• Light-matter interaction
• Metamaterials
• Nanophotonics
• Thermal properties
• Thermodynamics