Far -Infrared Study of Two-Dimensional Air-Rod Photonic Crystals : Band Gap, Uncoupled Mode and Wavevector Mismatch

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Two-dimensional photonic crystals both of square and triangular lattices composed of circular air-rods with the lattice constant of 170 μm were fabricated. The photonic band structures were studied by far-infrared transmission measurements. The clear opaque regions due to the band gap and due to the non coupling between photonic mode and external wave were observed in both lattices. The uncoupled modes can not be excited by an external mode because their wave functions are anti-symmetric under the relevant mirror reflection defined on the lattice. Furthermore, some regions with very low transmittance were found, where exist only a band whose dominant plane-wave component in eigen-function has wavevector not parallel to the propagating direction of the external incident wave. These low transmittance are considered to be caused by wavevector mismatch between photonic modes and external incident waves. The observed spectra are discussed compared with the results of band structure calculated by means of a plane-wave expansion method as well as of the symmetry consideration.
信州大学の論文