Robust chirality via merging accidental BICs with net zero topological charge

Chirality refers to the structural property of an object that cannot be superimposed on its mirror image. It is a widespread phenomenon in nature, found in essential building blocks of life such as amino acids and sugars, as well as in many critical pharmaceutical molecules. The precise identification of chiral optical responses in materials is of great importance for fields like drug development, life sciences, and quantum optics.

 

However, the interaction between natural chiral materials and light is inherently weak, making the detection of their chiral optical responses a significant challenge. The advent of metasurfaces has brought new hope, particularly chiral metasurfaces based on BICs, which can greatly enhance the chiral response of artificial microstructures. In the process of enhancing optical chiral responses using BIC metasurfaces, researchers have faced a significant challenge: achieving high sensitivity while maintaining wide-angle applicability—two qualities that have proven difficult to combine.

 

Recently, a joint research team from Harbin Engineering University, the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, Jinan University, Guizhou Minzu University, East China Normal University, and the National University of Singapore has successfully leveraged the physical mechanism of “merging BICs with net zero topological charge (ZTC)” on a simple planar silicon metasurface. This approach enabled the simultaneous achievement of high sensitivity (Q factor exceeding 10,000), near-perfect circular dichroism (0.99), and wide-angle robustness. This breakthrough not only resolves the long-standing trade-off between high sensitivity and wide-angle applicability but also paves the way for the practical deployment of high-performance chiral photonic devices. The findings have been published in the top-tier journal eLight under the title “Robust chirality via merging accidental BICs with net zero topological charge.” The co-first authors are Dr. Hui Hu (Ph.D. candidate) from Harbin Engineering University, Professor Chaobiao zhou from Guizhou Minzu University, Dr. Yukang Zhang (Ph.D. candidate) from the Shanghai Institute of Technical Physics, and Dr. Meng-xia Hu (Ph.D. candidate) from Jinan University. The corresponding authors are Professor Jinhui Shi, Professor Guanhai Li (Researcher), Professor Cheng-Wei Qiu, Professor Zi-lan Deng, and Professor Lujun Huang.

 

Background

The use of BICs to enhance optical field confinement has become a central theme in photonics research. By breaking the symmetry of a metasurface, a perfect BIC can be transformed into a qBIC with finite radiation loss, enabling extremely strong light–matter interactions—including the enhancement of chiral signals.

 

However, existing approaches face a fundamental limitation: the strong chiral responses enabled by BICs are typically confined to the vicinity of an isolated point in momentum space (the Γ point). This means that optimal performance occurs only within a narrow angular range around that specific point. Even a slight angular deviation of a few degrees leads to a sharp drop in both quality factor (Q factor) and chiral response.

 

Moreover, many designs aimed at generating strong chirality rely on complex three-dimensional nanostructures. These structures demand high fabrication precision and are highly sensitive to processing imperfections, which can severely affect performance or even reverse the intended chirality. As a result, achieving high Q factor, strong chirality, and wide-angle robustness simultaneously—while maintaining structural simplicity and ease of fabrication—remains a critical hurdle in transitioning the field from laboratory research to real-world applications.

 

Highlights

Principle and design: Merging BICs with net zero topological charge creates a broad momentum-space “safe zone” for chirality

The core innovation of the research team lies in a novel mechanism “merging BICs with net zero topological charge,” that transforms the originally localized chiral response at an isolated point into a broad and stable region in momentum space.

 

This process can be understood as a series of coordinated “topological charge manipulations.” Starting from a highly symmetric square silicon nanoblock array, the team introduced three carefully designed “perturbation” steps:

• Introducing in-plane offset: Offset holes (d_y) were etched into the nanostructures to break the C₄ symmetry, causing the originally degenerate bands to split and multiple “accidental BICs” to emerge.
• Creating chirality sources: Leveraging the vertical asymmetry introduced by the substrate, these accidental BICs were transformed into circular polarization (C) points carrying half-integer “topological charges”—the physical origin of the strong chiral response.
• Driving charge convergence: An additional in-plane perturbation (d_x) was applied to drive the orderly movement and charge exchange of these C points—carrying opposite “charges”—in momentum space. Ultimately, C points of the same “chirality” were attracted and converged near the center (Γ point), while those of opposite “chirality” were repelled or annihilated.

 

This series of manipulations culminates in the formation of a broad and stable “chiral safe zone” (i.e., a wide momentum-space region) around the Γ point. Within this zone, a stable, near-ideal chiral response is maintained over a wide range of incident angles, effectively overcoming the angular confinement that limits conventional designs (Fig. 1, Fig. 2).

 

Experimental validation: Achieving high Q factor and wide-angle response simultaneously

To validate the theoretical design, the research team successfully fabricated the planar chiral metasurface on a standard silicon-on-insulator platform. Experimental measurements demonstrated high-level performance across key metrics:

• Near-ideal chirality: A circular dichroism value of up to 0.99 was achieved at a wavelength of 1312 nm, approaching the theoretical maximum for distinguishing between left-handed and right-handed circularly polarized light.
• High sensitivity: The Q factor exceeded 10,000, with an absolute efficiency of 82%.
• Wide-angle robustness: Experiments confirmed that even at incident angles up to 5°, the chiral response remained stable across a full 360° azimuthal range, with the Q factor maintaining values above 5000 (Fig. 3).

 

Summary and outlook

The value of this research lies in unifying four key attributes—ultra-high sensitivity, near-ideal chirality, wide-angle robustness, and ease of fabrication—onto a single planar metasurface platform through the mechanism of merging BICs with net zero topological charge.

 

The single-layer planar structure is compatible with standard semiconductor fabrication processes, enabling large-scale manufacturing. Its operational wavelength can be tuned across the visible, terahertz, and other spectral ranges, and it can be integrated with light-emitting or two-dimensional materials. This offers a promising technological pathway for next-generation high-performance chiral photonic devices.

 

This work is expected to enable:

• Highly sensitive chiral molecular sensors for rapid detection of drugs or disease markers at low concentrations
• Integrated chiral light sources that emit pure circularly polarized light for quantum communication and display technologies
• Novel optical communication components that harness chirality as an additional degree of freedom for faster and more secure data transmission

 

These directions could help advance chiral optics from single-function devices toward multifunctional integration, and from proof-of-principle demonstrations toward real-world applications.

---

---