  Long-range spatial coherence can be induced in thermal emitters by embedding
a periodic grating into a material supporting propagating polaritons or
dielectric modes. However, the emission angle and frequency cannot be defined
simultaneously and uniquely, resulting in emission at unusable angles or
frequencies. Here, we explore superstructure gratings (SSGs) to control the
spatial and spectral properties of thermal emitters. SSGs have long-range
periodicity, but a unit cell that provides tailorable Bragg components to
interact with light. These Bragg components allow simultaneous launching of
polaritons with different frequencies/wavevectors in a single grating,
manifesting as additional spatial and spectral bands upon the emission profile.
As the unit cell period approaches the spatial coherence length, the coherence
properties of the superstructure will be lost. Whilst the 1D k-space
representation of the grating provides insights into the emission, the etch
depth of the grating can result in strong polariton-polariton interactions. An
emergent effect of these interactions is the creation of polaritonic band gaps,
and defect states that can have a well-defined frequency and emission angle. In
all, our results show experimentally how even in simple 1D gratings there is
significant design flexibility for engineering the profile of thermal emitters,
bound by finite coherence length.
