Experimental observation of sub-Bragg frequency gaps in photonic crystals

Photonic crystals control light propagation at a fundamental level. Photonic crystals are ordered composite materials with a spatially varying dielectric constant that has a periodicity of the order of the wavelength of light. Frequency gaps emerge for which light cannot propagate inside such structure due to Bragg diffraction. These frequency gaps appear in experiments as stopbands. This paper investigates stopbands in directions of high symmetry for two-dimensional photonic crystals. Surprisingly, a stopband below the first order Bragg condition was observed. This result is valid not only for photonic crystals, but for wave propagation in periodic media in general. This has implications for crystallography, scattering of phonons and band gap formation.