image: Figure 1. (A, B) Structural design of the topological polycrystal and its frequency supercurvature distribution in the synthetic hybrid space. (C, D) Supercell projected band structure at a specific orientation angle θ = 135° and Δθ = 45°.
Credit: ©Science China Press
Sculpting the flow of light, whether confined to localized regions or propagating in free space, is essential to modern integrated photonics. The development of multi-channel, programmable optical waveguide and coupler arrays empowers photonic integrated circuitries (PICs) emerged as a potential alternative to electronic integrated circuitries, surpassing limitations in processing speed, operational bandwidth, and efficiency across the optical-to-microwave spectrum. However, as on-chip complexity increases, PICs face significant challenges related to long-term stability and fabrication-induced defects. Ensuring the operational stability of PICs has thus become a critical issue for their practical deployment.
The growing demand for extensive information processing has driven the requiring for more complex PICs with increased multiple channels. The emergence of topological photonics, owing to its inherent robustness against defects and scattering, holds promise for breakthroughs in this field. Nevertheless, existing PICs that exploit topologically protected domain-wall modes predominantly rely on two-dimensional microstructures and adhere to the bulk-edge correspondence principle. This approach typically supports only a limited number of edge states, thereby constraining the channel capacity and operational bandwidth of PICs. To overcome this limitation, researchers have recently focused on hybrid topological effects, aiming to integrate multiple topological phenomena within a single system. For instance, hybrid topological photonic crystals based on the quantum anomalous Hall effect and the valley Hall effect leverage the breaking of time-reversal and spatial-inversion symmetries to generate multi-band edge channels. However, these structures generally require patterning dielectric materials on metallic surfaces and remain constrained by the weak magneto-optical response, posing significant challenges for further development.
Recently, a research collaboration led by Prof. Shuming Wang and Prof. Shining Zhu at the National Laboratory of Solid-State Microstructures, Nanjing University, in conjunction with Prof. Jianhua Jiang from the Suzhou Institute of the University of Science and Technology of China, has investigated the topological effects of hybrid artificial microstructured systems and their applications. By constructing a synthetic dimension comprising one-dimensional momentum space and two-dimensional unit cell orientation space, they have proposed a configurable topological photonic polycrystal based on a synthetic hybrid dimension. Unlike the aforementioned hybrid topological photonic crystals, this dielectric metasurface, composed of orientation-tunable elliptical elements, exploits a coupling mechanism between pseudo-spin and the valley Hall effect. This concept provides a versatile platform for simultaneously manipulating multiple frequency bands, spin states, and group velocities, enabling multidimensional control over photonic states. This work has been published in National Science Review, 2025, Issue 6, under the title “Configurable Topological Photonic Polycrystal Based on Synthetic Hybrid Dimension.” Profs. Shuming Wang, Jianhua Jiang, and Shining Zhu are listed as corresponding authors.
This team proposed a perturbation theory related to the structural orientation angle for the rapid calculation and adjustment of photonic band gaps. As both pseudo-spin and valley lattices are closely associated with the orientation angle, variations in the orientation angle led to subtle shifts in the energy bands, effectively tuning the "mass term." To comprehensively capture this phenomenon, they combined the proposed scalar perturbation theory and used the orientation angle as a parameter for constructing the synthetic dimension. For a given photonic lattice, they created a three-dimensional synthetic dimension by combining the two-dimensional Bloch momentum with the one-dimensional orientation angle, allowing the calculation of topological invariants as a function of the orientation angle. This result provides a profound theoretical tool for understanding the system's topological properties.
However, after coupling the two topological effects, this hybrid-dimensional space depends on two orientation angles and the one-dimensional momentum space along the domain walls. By observing within the hybrid dimension, they demonstrated how bulk states, edge states, and corner states vary with changes in the orientation angle. This refined analysis clarifies how the hybrid topological polycrystal framework can achieve robust, on-demand controllable photonic functionalities.
Additionally, the researchers defined the local quality factor, QL—specifically addressing the hybrid eigenmodes within the band gap. By integrating the local density of states at the target frequency, this quantifiable description method evaluates the performance of such hybrid topological devices.
As a validation, they designed a hybrid topological photonic integrated circuit and successfully demonstrated high-contrast multi-band edge states and higher-order corner states distributed along the interface in experiments. This method utilizes multi-band chiral edge channels to enable on-chip logic gates, couplers, and high-density optical communication, while also supporting the development of ultra-small mode volume and diverse corner states in multi-band lasers. Furthermore, based on the orientation-dependent photonic lattice concept, it paves the way for the reconfigurable and programmable operation of active topological devices. Looking ahead, we anticipate exploring the nonlinear effects of hybrid topological polycrystals to enhance their multimodal and multi-band capabilities, laying a foundation for applications in both classical and quantum photonics.