We report a direct observation of a quasi-coherent thermal emission from a heated three-dimensional photonic-crystal sample. While the sample was under Joule heating, we observed multiple oscillations in its emission interferogram and deduced a coherent length of L_(coh) approx= (20-40) μm, 5-10 times longer than that of a blackbody at comparable wavelengths. The observed, relatively long coherent length is attributed to coupling of thermal emission into lossy Bloch modes that oscillate coherently over a distance determined by decay length and the slow light nature of Bloch modes at the band-edges.
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