![]() ![]() Wide-angle polarization-independent chiral metamaterial absorber has been also reported 25. Polarization-independent negative refraction has been observed in “Swiss cross” structure in near infrared regime 24. Metamaterial also enables to manipulate polarizations. Polarization-independent waveguides have been designed in 2D photonic crystal with full band gaps 21, 22, and in 3D photonic crystal with “accidental degeneracy” in guided dispersion relations for both polarizations by tuning line-defect profile 23. For example, complete band gaps in both TM and TE polarizations are crucial in 2D and three dimensional (3D) photonic crystals 19, 20. Polarization-independence, on the other hand, is highly expected in many research fields. However, the cones are always dependent on polarizations of light in two-dimensional (2D) photonic crystals, which appears in either transverse magnetic (TM) or transverse electric (TE) polarization only. In addition, the photonic Dirac cones are shown to exist in metamaterials as well 17, 18. Similar phenomena have been reported in two-dimensional phononic crystal 15, 16, possessing zero-index with simultaneous effective zero reciprocal of bulk modulus and zero mass density. Combined with effective medium theory, the triple-degenerate conical dispersion has an effective zero refractive index at Dirac frequency. Different from those at zone boundary, the latter one is accidentally degenerate to form the conical dispersion with two linear bands and one flat band. On the other hand, a new kind of photonic Dirac cone at zone center has been reported in all-dielectric photonic crystals and aperiodic quasicrystals 9, 10, 11, 12, 13, 14. Such symmetry-protected cone plays an important role in the realization of nontrivial topological characteristics and non-zero Berry phase, inspiring many novel transport properties, such as photonic quantum Hall effect 7 and photonic spin-momentum locking edge states 8. The cone emerges at the corner of Brillouin zone, which is a counterpart to that in electronic solid state systems like graphene 1, 2, 3 and topological insulators 4, 5, 6. Photonic Dirac cone has been discovered in two-dimensional photonic crystals of dielectric cylinders. ![]()
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