Fig. 1
Topological nanocavity design concept. a Schematic of the nanocavity formed by interfacing two photonic crystals (PhCs) with a common bandgap and distinct Zak phases. The Zak phases denoted in the bracket are respectively for the lowest and the second lowest band. b Detailed description of the design. The two PhCs are in essence the same but differ in the way that the unit cells are defined. Blue unit cell arranges the small airhole with a width of d1 at the center, while red unit cell has the large airhole with d2 at the center. The centers of the two holes respectively correspond to the inversion centers of the double periodic PhC. The sum of d1 and d2 equals to 0.5a. c Band structures and wave functions at the band edges calculated for the PhC with the blue and red unit cell. The color of the band curve expresses its Zak phase (blue = π, red = 0). A band inversion occurs when swapping the unit cells. For the case of d1 = 0.25a ( = d2), a Dirac point appears at the band edge
