Bandgap structure of thermally excited surface phonon polaritons

A wide bandgap of thermally excited surface phonon polaritons (SPhPs) is experimentally observed. Formation of the bandgap and coupling to radiative waves is done by a binary biharmonic structure formed on a SiC substrate. The bandgap width is controlled by the ratio of the two harmonic magnitudes of the structure’s profile. The characteristic one-dimensional Van Hove singularity is experimentally observed in the spectral density of states of the SPhPs. Moreover, an inverse relation is found between the gap width and the squared spatial coherence length of the emitted thermal radiation, as predicted by theoretical calculations.