a A closed domain wall is created by inversion symmetry breaking in the dimerized graphene lattice and shaped in the form of an equilateral triangle (reddish area). The structure is implemented in a dielectric photonic crystal slab of a finite thickness. The shaded yellow stripe illustrates a supercell with a zigzag cut. b, c Tight-binding model visualization of optical modes of the structure. d Numerically calculated bulk photonic band structure for TE-like modes. The grey shaded regions indicate the light cone. The inset shows the unit cell and dimensions of the structure: lattice period a = 460 nm, sizes of triangulated holes r1 = 174 nm, r2 = 269 nm, and slab thickness fixed to 250 nm. A degeneracy is lifted, and the bandgap opens by dimerization between the first and second bands in the vicinity of the K point. e Projected bulk bands (black curves), spectra of edge states (red and blue lines) and cavity modes (dots). The dashed level marks the triade mode frequency. Characteristic distributions of the near-field intensity |E|2 in the (f) triade and (g) edge cavity modes at resonant wavelengths of 1.49 µm and 1.51 µm, respectively. Their simulated quality factors are f 4.3 × 103 and g 1.9 × 103
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