Lithography-free directional control of thermal emission.

Blackbody radiation is incoherent and omnidirectional, whereas various novel applications in renewable energy require a degree of directional control of a thermally emitted beam. So far, such directional control has required nano-structuring the surface of a thermally emitting material, typically by forming diffraction gratings. This, however, necessitates lithography and usually results in polarization-dependent properties. Here, we derive analytical conditions for highly directional thermal emission using a planar 3-layer structure analogous to a Salisbury screen. We present design rules for maximizing the directionality of such structures. Notably, these design rules pertain to both linear polarizations, thus generalizing the principles of a grating for unpolarized light. We show that the key requirement to achieve such performance is ultra-high quality factor resonances in materials supporting phonon polaritonic modes, as those found in low-dimensional materials. We propose a realistic device based on hexagonal Boron Nitride and predict performances comparable to lithography-based nano-structures.