Abstract
We report on the first experimental observation of two-dimensional isotropic double Dirac cones in the electromagnetic dispersion relation, which were materialized by the accidental degeneracy of E1- and E2-symmetric eigenmodes on the Γ point of photonic crystals. We fabricated triangular-lattice photonic crystal slabs of the C6v symmetry on silicon-on-insulator wafers by electron beam lithography and examined their dispersion relation by high-resolution angle-resolved reflection spectroscopy in the mid-infrared range. We observed the characteristic linear dispersion of the double Dirac cones together with the polarization selection rules derived from the mode symmetries. All these observations agreed with numerical calculation of the dispersion relation and reflection spectra by the finite element method.
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