Direct Observation of Valley-polarized Topological Edge States in Designer Surface Plasmon Crystals (1703.04570v3)

Abstract: The extensive research of two-dimensional layered materials has revealed that valleys, as energy extrema in momentum space, could offer a new degree of freedom for carrying information. Based on this concept, researchers have predicted valley-Hall topological insulators which could support valley-polarized edge states at non-trivial domain walls. Recently, several kinds of photonic or sonic crystals have been proposed as classical counterparts of valley-Hall topological insulators. However, direct experimental observation of valley-polarized edge states in photonic crystals is still difficult until now. Here, we demonstrate a designer surface plasmon crystal comprising metallic patterns deposited on a dielectric substrate, which can become a valley-Hall photonic topological insulator by exploiting the mirror-symmetry-breaking mechanism. Topological edge states with valley-dependent transport are directly visualized in the microwave regime. The observed edge states are confirmed to be fully valley-polarized through spatial Fourier transforms. Topological protection of the edge states at sharp corners is also experimentally demonstrated.

• The paper demonstrates experimental observation of valley-polarized topological edge states in DSP crystals by breaking mirror symmetry in a triangular lattice.
• It employs a Z-shaped waveguide configuration to achieve topological protection with negligible scattering, validated through numerical simulations and effective Hamiltonian modeling.
• The findings pave the way for scalable, planar photonic devices with efficient waveguiding, holding promise for advanced telecommunications applications.

Direct Observation of Valley-polarized Topological Edge States in Designer Surface Plasmon Crystals: An Expert Analysis

This paper addresses the experimental realization and observation of valley-polarized topological edge states in designer surface plasmon (DSP) crystals, a domain previously limited by indirect approaches. The research groups utilized DSPs, which are non-leaky electromagnetic modes akin to optical plasmons but operative at microwave frequencies, to construct a valley-Hall photonic topological insulator (PTI). By breaking mirror symmetry, the team demonstrated the formation of topological edge states with valley-dependent transport properties, which were directly visualized and experimentally confirmed.

Methodology and Results

The DSP crystal under investigation features a triangular lattice formed of copper patterns on a dielectric substrate. Key to its functionality is the breaking of mirror symmetry, instigated by perturbing the structure. The modification lifts degeneracies at valleys in the band structure, thereby opening bandgaps. This engenders a topological phase transition characterized by non-trivial valley Chern numbers.

The experimental setup allowed for direct detection of the valley-polarized edge states in a microwave regime. Significant attention was given to realizing topological protection at sharp corners, showcased through a Z-shaped waveguide configuration without observable scattering losses. The research capitalizes on the intrinsic properties of DSPs, which afford vertical confinement in free space, reducing experimental complexities compared to other PTIs that require covers to prevent leakage.

Numerical Insights and Theoretical Confirmation

The paper also substantiates its experimental findings with numerical simulations. An effective Hamiltonian method, based on the k∙p theory near K/K′ valleys, models the topological phase transition analytically. Berry curvature computations further authenticate the valley Chern numbers, essential in verifying the theory of bulk-boundary correspondence. The calculated band structures corroborate the emergence of valley-polarized edge states at domain walls, adhering to the predictions made by topological band theory.

Implications and Future Directions

The implications of this research are substantial for photonics and telecommunication domains. The valley-Hall PTI's ability to guide electromagnetic waves through sharp bends with negligible scattering highlights a potential for more efficient waveguides integral to communication technology. The DSP crystal's planar and ultrathin geometry, combined with ease of fabrication, could facilitate integration with existing electronic systems and support scalable manufacturing.

The theoretical underpinnings and practical demonstrations described in this paper pave the way for advancements in other frequency domains, including terahertz and optical regimes. The successful application of valleytronic principles in a photonic platform not only opens avenues for further research but sets the groundwork for innovative device applications. Continued exploration may focus on miniaturization, frequency scalability, and integration of valley-Hall PTIs with other topological phases, enhancing the capabilities of photonic devices.

Overall, this paper contributes a crucial empirical validation to the valleytronics field, demonstrating that DSP structures can effectively emulate the electronic valley-Hall effect. It provides a strong basis for future investigations aimed at optimizing and diversifying applications of topological insulators across various electromagnetic spectrum regimes.