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  • Review Article
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Applications of bound states in the continuum in photonics

Nature Reviews Physics volume 5, pages 659–678 (2023)Cite this article

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Abstract

Bound states in the continuum (BICs) have attracted attention in photonics owing to their interesting properties. For example, BICs can effectively confine light in a counterintuitive way, and the far-field radiation of photonic structures that exhibit BICs has fascinating topological characteristics. Early research into photonic BICs was primarily focused on designing artificial structures to produce BICs. However, since the mid-2010s, exploring the potential applications of BICs has been a growing trend in research. In this Review, we detail the unique properties of BICs, including the ability to achieve enhanced light confinement, sharp Fano resonances and topological characteristics. We explore phenomena derived from BICs, including the generation of circularly polarized states and unidirectional guided resonances, and the impact of BICs on various applications such as lasing, nonlinear frequency conversion, waveguiding, sensing and wavefront control. We also discuss the insights provided by BICs in several emerging research frontiers, such as parity–time symmetric systems, higher-order topology, exciton–photon coupling and moiré superlattices.

Key points

  • Photonics provides a versatile platform to study and exploit the properties of bound states in the continuum (BICs), leading to a wide range of applications.

  • The ability of BICs to achieve highly efficient light confinement leads to the coherent field enhancement of both electric and magnetic fields, which can improve lasing performance, nonlinear conversion efficiency and waveguiding in photonic integrated circuits.

  • Light scattered by photonic structures exhibiting BICs manifests the features of Fano resonances, leading to advanced functionalities in refractometric sensing and the identification of molecular fingerprints.

  • BICs are characterized as topological polarization vortices in far-field radiation, in which the geometric phases in momentum space can be used to manipulate light, giving rise to polarization conversion, vortex beam generation and beam shifts.

  • BICs have been integrated to explore several emerging frontiers, including parity–time-symmetric systems, higher-order topology, exciton–photon coupling and moiré superlattices.

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Fig. 1: Lasing boosted by the field confinement of BICs.
Fig. 2: Enhanced nonlinear effects and protected waveguiding with BICs.
Fig. 3: The sensing benefits of Fano resonances of quasi-BICs.
Fig. 4: Manipulating topological charges.
Fig. 5: Applications of polarization vortices.
Fig. 6: Emerging research areas.

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Acknowledgements

The authors thank Z. Q. Zhang, W. Liu, J. Li and R.-Y. Zhang for discussions. M.K. and C.T.C acknowledge support from Research Grants Council (RGC) Hong Kong through grant AoE/P-502/20 and the Croucher Foundation (CAS20SC01). T.L. and M.X. acknowledge support from the National Natural Science Foundation of China (grant no. 12274332) and the National Key Research and Development Program of China (grant no. 2022YFA1404900).

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Authors and Affiliations

  1. Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China

    Meng Kang & C. T. Chan

  2. Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, China

    Tao Liu & Meng Xiao

  3. Wuhan Institute of Quantum Technology, Wuhan, China

    Meng Xiao

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Contributions

M.K. and T.L. researched data for the article. C.T.C., M.K. and M.X. contributed substantially to discussion of the content. M.K., T.L. and M.X. wrote the article. C.T.C., M.K. and M.X. reviewed and/or edited the manuscript before submission.

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Correspondence to C. T. Chan or Meng Xiao.

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Nature Reviews Physics thanks Cheng-Wei Qiu, Boubacar Kante, Lei Shi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Glossary

Bulk Fermi arc

In classical systems, a bulk Fermi arc refers to an isofrequency contour of bulk band structures that presents as an arc in momentum space.

C 4v point group

A symmetry group with rotations and reflections associated with a square.

C 6v point group

A symmetry group with rotations and reflections associated with a hexagon.

Fabry–Pérot resonances

Resonances supported by two parallel reflecting surfaces.

Fano interference

Interference between discrete bound states and a continuum of extended states, resulting in the generation of Fano resonance.

Goos–Hänchen shifts

Lateral shifts of the reflected and transmitted beams in the direction parallel to the incident plane.

Imbert–Fedorov shifts

Spin-dependent lateral shifts of the reflected and transmitted beams in the direction perpendicular to the incident plane.

Mie resonances

Resonances that occur in scattering by particles with sizes comparable to the wavelength of the incident light.

Nonlinear frequency conversion

A type of nonlinear optical effect, which includes processes such as second-harmonic generation, third-harmonic generation, high-harmonic generation, sum-frequency generation, difference-frequency generation and spontaneous parametric down-conversion.

Photonic crystal slabs

(PCSs). Periodic planar waveguides that possess 2D periodicity but have a finite thickness.

Zero-index materials

Materials that have zero refractive index at certain frequencies.

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Kang, M., Liu, T., Chan, C.T. et al. Applications of bound states in the continuum in photonics. Nat Rev Phys 5, 659–678 (2023). https://doi.org/10.1038/s42254-023-00642-8

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