  Blackbody thermal emission is spatially diffuse. Achieving highly directional
thermal emission typically requires nanostructuring the surface of the
thermally emissive medium. The most common configuration is a subwavelength
grating that scatters surface polaritonic modes from the near-field to the
far-field and produces antenna-like lobes of thermal emission. This concept,
however, is typically limited to a particular linear polarization, and requires
sophisticated lithography. Here, we revisit the simple motif of a planar
Salisbury screen. We show analytically how the interplay between the real and
imaginary part of the dielectric permittivity of the emitter layer defines the
directional characteristics of emission, which can range from diffuse to highly
directional. We propose a realistic configuration and show that hexagonal Boron
Nitride thin films can enable grating-like thermal emission lobes in a
lithography-free platform.
