Valley photonic molecular crystals
A research team led by Yidong Huang at Tsinghua University, including Xue Feng and Yongzhuo Li, has developed valley photonic molecular crystals (VPMCs) on a silicon platform. Combining theoretical modeling, numerical simulations, and experimental validation, the researchers demonstrated that topological edge states remain robust under rotational disorder ranges up to 60°, opening new avenues for integrated photonic devices.
The study draws inspiration from molecular topological insulators based on metal–organic frameworks. In these materials, molecular rotations can significantly influence optical and mechanical properties, yet their impact on topology has remained largely unexplored. Because random rotations break structural periodicity, conventional topological characterization based on energy-band theory and momentum space becomes inapplicable.
To overcome this challenge, the team introduced molecular rotational freedom into valley photonic crystals. In this design, optical “molecules” occupy fixed, periodic lattice sites while retaining randomly distributed orientations. This design provides a controllable platform to study the impact of rotational disorder on valley topology.
The researchers first investigated how molecular orientation determines the topological properties of periodic VPMCs. They then constructed an effective Hamiltonian approach to characterize the valley-topological properties of disordered VPMCs via an averaged Dirac mass and an effective valley Chern number. A complementary spectral localizer analysis in real space confirmed the existence of topological edge states without relying on structural periodicity.
Devices fabricated on silicon chips provided experimental validation. Simulations showed light propagating along a Z-shaped interface under substantial rotational disorder, while measured transmission spectra retained high-transmission peaks associated with topological edge states. These features persisted for disorder ranges reaching 60°. The study also investigated competition between topological edge states and Anderson localization in the regime of strong disorder.
The findings demonstrate the robustness of VPMCs to strong, global rotational disorder and establish molecular orientation as an additional dimension to manipulate topological properties. This framework is not limited to the specific molecular structure in the paper, it is applicable to all C3-symmetric molecules. This work has the potential for programmable topological photonic systems and synthetic gauge fields, including pseudo-electric and pseudo-magnetic fields. This work could also inspire related investigations in condensed matter, acoustic, and cold atom platforms.
Reference Valley photonic molecular crystals
Zhe Li, Yiheng Fan, Ziming Chen, Deyang Kong, Kaiyu Cui, Yongzhuo Li, Xue Feng, and Yidong Huang
Chinese Society for Optical Engineering
