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The discovery of topological insulators was rapidly followed by the advent of their photonic analogues, motivated by the prospect of backscattering‐immune light propagation. So far, however, implementations have mainly relied on engineering bulk modes in photonic crystals and waveguide arrays in two‐dimensional (2D) systems, which closely mimic their electronic counterparts. In addition, metamaterials‐based implementations subject to electromagnetic duality and bianisotropy conditions suffer from intricate designs and narrow operating bandwidths. Here, it is shown that symmetry‐protected topological states akin to the quantum spin‐Hall effect can be realized in a straightforward manner by coupling surface modes over metasurfaces of complementary electromagnetic responses. Specifically, stacking unit cells of such metasurfaces directly results in double Dirac cones of degenerate transverse‐electric (TE) and transverse‐magnetic (TM) modes, which break into a wide nontrivial bandgap at small interlayer separation. Consequently, the ultrathin structure supports robust gapless edge states, which are confined along a one‐dimensional (1D) line rather than a surface interface, as demonstrated at microwave frequencies by near‐field imaging. The simplicity and versatility of the proposed approach proves attractive as a tabletop platform for the study of classical topological phases, as well as for applications benefiting the compactness of metasurfaces and the potential of topological insulators.

Reducing open waveguides enabled by surface waves, such as surface plasmon polaritons, to a one-dimensional line is attractive due to the potentially enhanced control over light confinement and transport. This was recently shown to be possible by simply interfacing two co-planar surfaces with complementary surface impedances, which support transverse-magnetic and transverse-electric modes, respectively. Attractively, the resultant “line wave” at the interface line features singular field enhancement and robust direction-dependent polarizations. Current implementations, however, are limited to microwave frequencies and have fixed functionality due to the lack of dynamic control. In this article, we examine the potential of using gate-tunable graphene sheets for supporting line waves in the terahertz regime and propose an adequate graphene-metasurface configuration for operation at room temperature and low voltage conditions. In addition, we show the occurrence of quasi-line wave under certain conditions of non-complementary boundaries and qualify the degradation in line wave confinement due to dissipation losses. Furthermore, we show the possibility to alter the orientation of the line wave’s spin angular momentum on demand unlike conventional surface waves. Our results on active manipulation of electromagnetic line waves in graphene could be useful for various applications including reconfigurable integrated circuits, modulation, sensing and signal processes.

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An eigenmode analysis is presented of the electromagnetic field which occurs between two complementary surface impedances. The analysis is based on the generalized reflection method which is a generalization of the Sommerfeld-Maliuzhinets technique. Numerical results are presented and validated against independent Comsol simulations. Also, the characteristic impedance and phase velocity are defined and calculated for further investigation of the structure.

Edge states protected by bulk topology of photonic crystals offer exciting means to study spintronics, and their robustness to short-range disorder makes robust information transfer possible. Here, we investigate topological transport under long-range amorphous deformation without external magnetic field. Vertices of each regular hexagon in a C3-symmetric crystalline structure are shifted randomly. Despite the existence of unpredictable scattering, topological edge modes are determined by statistic behavior of the whole structure. Photonic density of states as well as the Fourier transform can be used to distinguish the topological properties. We further designed and fabricated samples working in the microwave band. The measured transmission spectrum reveals the existence of robust topological states in the amorphous system. This work proves the robustness of bulk topology, points to the study of topological properties of structures undergoing amorphous deformation, and may open the way for exploiting topological insulators in materials with different phases.

In article number 1900126, Dia'aaldin J. Bisharat and Daniel F. Sievenpiper demonstrate a straightforward approach to realize photonic topological insulators (PTIs), which promise lossless wave‐transmission despite fabrication imperfections. The design is nothing but a patterned metallic thin sheet atop another of a complementary pattern. Remarkably, the electromagnetic duality achieved demands no parameter fine‐tuning, and despite its simplicity, the design outperforms existing PTIs to date in terms of bandwidth and energy concentration.