Based on the tight-binding model calculations and photonic experimental visualization on graphene, we report the domain-wall-induced gapped topological kink states and topological corner states. In graphene, domain walls with gapless topological kink states could be induced either by sublattice symmetry breaking or by lattice deformation. We find that the coexistence of these two mechanisms will induce domain walls with gapped topological kink states. Significantly, the intersection of these two types of domain wall gives rise to topological corner state localized at the crossing point. Through the manipulation of domain walls, we show graphene not only a versatile platform supporting multiple topological corner modes in a controlled manner, but also possessing promising applications such as fabricating topological quantum dots composed of gapped topological kink states and topological corner states.

We propose an anisotropic photonic crystal (PC) structure with close-to-zero effective parameters. The anisotropic zero-index property of PCs provides complex iso-frequency contours, where their shapes can change from ellipses or hyperbolae to a linear crossing. A mechanism of light beam splitting can be achieved by utilizing the linear crossing shaped iso-frequency curve. The manipulation of light beam propagations is verified by both numerical simulations and microwave experiments. By using pure dielectrics and scaling down to optical wavelengths, we also propose a design of nonperiodic PC structures to achieve optical unidirectional cloaking without size or shape limitation. Our design provides a promising platform for cloaking applications in the optical regime.

By exploiting the scaling invariance of photonic band diagrams, a complementary photonic crystal slab structure is realized by stacking two uniformly scaled double-zero-index dielectric photonic crystal slabs together. The space cancellation effect in complementary photonic crystals is demonstrated in both numerical simulations and microwave experiments. The refractive index dispersion of double-zero-index dielectric photonic crystal is experimentally measured. Using pure dielectrics, our photonic crystal structure will be an ideal platform to explore various intriguing properties related to a complementary medium.

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Topological insulators have aroused much research interest in condensed matter physics in recent years. Topological protected edge states can propagate unidirectionally and backscattering free along the boundaries of the topological insulators' which will be important for future electronic devices for its immunity to defects. Topology is dependent only on the symmetry of lattice of the system rather than its specific wave form. Thus, based on the analogy between electronics and photons, photonic topological insulator has also been demonstrated both theoretically and experimentally. Graphene, composed of a monolayer of carbon atoms in honeycomb lattice, exhibits unusual properties due to its intriguing band diagram. Many types of graphene allotropes have been proposed theoretically. However, due to fabrication difficulties, most of graphene allotropes are unavailable. Here, we propose to study two dimensional (2D) photonic crystal (PC) with complex lattices, similar to that of graphene allotrope. The complex PC structure provides more degrees of freedom in manipulating its symmetry.Interface states can also exist in the interface region between two PCs, if they have different topological properties. Without any surface decoration, deterministic interface states can be created when bulk photonic band inversion can be induced and are demonstrated theoretically and experimentally in 2D PCs with square lattice. By controlling the parameters of PCs, their bulk photonic band properties are engineered and topological phase transition occurs. By inverting the bulk photonic band properties, interface states exist in the common band gaps for two PC systems in the gapped region. Similarly, we proceed to complex honeycomb lattice of PCs. By lowering its original C6v symmetry to C3v, C3, C2v and even C2 symmetry, the degeneracies of valley Dirac dispersion at the corners of Brillouin zone are lifted. Photonic band inversion occurs in all four symmetries and the deterministic interface states are numerically realized in the interface region between two PCs. Unidirectional propagation of interface state immune to backscattering along the interface channels is demonstrated if a source with proper optical vortex index is utilized. Due to its easy fabrication, PC is a perfect platform to explore the topological properties of complex lattice and these acquired topological optical states can be of benefit to the control the propagation of light in the photonic waveguide.