Conventional thermal sources usually have a broad spectrum and isotropic emittance. However, recent works have shown that coherent thermal emission can be obtained by electromagnetic surface waves excitation. Surface waves are confined waves due to collective oscillations of the free electrons in metals (surface plasmon polaritons) or resonant collective vibrations in polar materials (surface phonon polaritons).The emission properties, such as coherence length or energy density, can be engineered by modifying the dynamics of the surface waves, for instance, by localization [1] . One of the ways to achieve this localization is by introducing a Bragg grating on the surface of the polaritonic material which results in a bandgap in the dispersion curve. A convenient method of observing these energy gaps is by coupling the polaritons to propagating waves. For this purpose, an additional grating component is essential. By composing the Bragg and the coupler components a biharmonic structure is created such that the radiative waves carry information about the bandgap.

         A wide bandgap of thermally excited surface phonon polaritons was observed in our experiments [2]. 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. In addition, an inverse relation was found between the gap width and the squared spatial coherence length of the emitted thermal radiation at the band extrema frequencies, whereas the spectral quality factor remained constant. This feature can be utilized to design thermal emitters, polaritonic bandgap microcavities for sensing applications, and thermophotovoltaic systems.

References:

1. N. Dahan, Y. Gorodetski, K. Frischwasser, V. Kleiner and E. Hasman, Phys. Rev. Lett. 105, 136402 (2010).
2. Balin, N. Dahan, V. Kleiner, and E. Hasman , Appl. Phys. Lett. 96, 071911 (2010).