- We demonstrate an extraordinary quasimonochromatic thermal emission with high spatial coherence length ( l c >2400λ) ( and a quality factor Q=2320 Q at radiation frequencies that are much smaller than the plasma frequency of metal (ω≪ ω p ) ( . This emission is achieved by forming a plasmonic bandgap, which is obtained by a periodic structure on a metallic surface. Such a structure modifies the dynamics of the surface wave and results in a van Hove singularity [Van Hove, Phys. Rev. 89, 1189 (1953)] in the spectral density of states while maintaining a large coherence length.