Enhanced coherency of thermal emission by coupled resonant cavities supporting surface waves

Surface waves have been shown to play a key role in spontaneous thermal emission in the near-field as well as the coherence and the polarization properties of the nonradiative field. The near-field coherence of the delocalized nonradiative surface waves can be transferred into radiative fields by introducing a shallow grating on the surface. We show that the coherency of the thermal radiation can be enhanced by an order of magnitude compared with the coherency imposed by the delocalized surface waves. The enhanced coherency is due to coherent coupling between resonant cavities obtained by surface standing waves, where each cavity supports localized field that is attributed to coupled surface waves. We realized coupled resonant cavity structure on amorphous SiO₂ and crystalline SiC, both support surface phonon-polaritons, to demonstrate extraordinary coherent thermal emission with a high quality factor of 600 and a spatial coherence length of 760λ (8.8mm).