Published by: School of Sciences Edited by: Zhou Tongjiang
WUST News - Recently, Yan Bei, a young faculty member from the Department of Applied Physics at the School of Sciences, published a paper titled Topological Dirac-vortex modes in a three-dimensional photonic topological insulator in Nature Communications. Yan Bei is the first author of this paper, and Wuhan University of Science and Technology (WUST) is the second contributing institution.
The interaction between topological lattice defects in real space and band topology in reciprocal space has led to numerous novel topological physical phenomena and wide-ranging applications, such as topological optical cavities, lasers, waveguides, optical fibers, and three-dimensional photonic topological insulators in synthetic dimensions. In particular, the topological Dirac vortex modes, originating from the Jackiw-Rossi zero modes of the massive vortex Dirac equation, have attracted significant attention in fields such as high-energy physics, condensed matter physics, and topological physics. This is due to their unique physical properties, including tunable mode area, arbitrary mode degeneracy, vector beam emission, and a large free spectral range. However, until now, research on optical topological Dirac vortex modes has been limited to two-dimensional optical systems that support zero-dimensional localized states, as illustrated in Figure 1a. Despite the realization of one-dimensional Dirac vortex transmission modes in three-dimensional phononic crystals and zero-dimensional monopole modes localized within three-dimensional Dirac vortex defects, extending topological Dirac vortex modes from two-dimensional acoustic systems to three-dimensional acoustic systems has been achieved. However, due to the vector nature of electromagnetic waves in three-dimensional space, whether topological Dirac vortex modes can be realized in three-dimensional topological optical structures (even theoretically) remains an unresolved question.
To extend topological Dirac vortex modes from two-dimensional to three-dimensional optical systems, the research team introduced Kekulé distortion in a three-dimensional tight-binding-like metallic cage photonic crystal, as shown in Figures 2a-2b. Each dielectric rod is surrounded by metallic rods to confine Mie resonances, simplifying the vector electromagnetic waves in the three-dimensional photonic crystal to scalar waves. The bulk band dispersion is similar to that of scalar waves (as shown in Figure 2c) and matches the dispersion of the three-dimensional tight-binding model (as shown in Figure 1d). By applying a position-dependent aperiodic Kekulé distortion to the three-dimensional tight-binding-like metallic cage photonic crystal, a position-dependent mass term is introduced, as illustrated in Figure 2e. Consequently, a one-dimensional topological Dirac vortex transmission mode (red line) emerges within the bulk bandgap, as shown in Figure 2f. The topological Dirac vortex mode is confined to the one-dimensional Dirac vortex line defect and propagates bidirectionally along it, as shown in Figures 2g-2h.
Using microwave near-field imaging measurements, the research team directly observed the topological Dirac vortex mode confined within the one-dimensional Dirac vortex line defect in the three-dimensional photonic topological insulator and propagating along it, as shown in Figure 3. Additionally, the team experimentally demonstrated that optical topological Dirac vortex modes exhibit strong robustness against various defects and obstacles, as shown in Figure 4, making them highly suitable for robust electromagnetic wave manipulation in three-dimensional space. The work of the research team not only extends optical topological Dirac vortex states from two dimensions to three dimensions for the first time but also provides an ideal platform for exploring novel physical phenomena and practical applications arising from topological lattice defects in three-dimensional topological photonic crystals. (School of Sciences)
Paper link:
https://www.nature.com/articles/s41467-025-61238-7
