This review article provides a comprehensive overview of bound states in the continuum (BICs), a class of physical states that remain perfectly localized despite existing within the radiative spectrum. Initially a quantum mechanical concept, BICs have become a central framework in photonics for designing high-Q resonances without relying on fine-tuning. Their formation mechanisms include symmetry protection, destructive interference, and momentum-space topology, which confer intrinsic robustness and unique modal characteristics.
The authors discuss the fundamental theory of BICs using band theory, temporal coupled-mode theory (TCMT), and multipole moment superposition, providing tools to analyze, predict, and design BICs in photonic crystal slabs, metasurfaces, waveguides, and fiber systems. BICs can be categorized as symmetry-protected, accidental (parameter-tuned), single-resonance, Fabry-Pérot, Friedrich-Wintgen, and momentum-mismatch-driven, each with distinct formation principles and advantages, such as tolerance to fabrication imperfections or tunable radiative coupling. Figures 1–5 in the article illustrate examples across multiple platforms, including optical, acoustic, and metasurface systems.
The review emphasizes the practical implications of BICs in photonics: quasi-BICs allow controlled radiative coupling, enabling enhanced light emission, nonlinear optics, sensing, narrowband filtering, and wavefront manipulation. The integration of BIC concepts with topology-assisted designs and reconfigurable architectures offers opportunities for scalable, multifunctional, and intelligent photonic devices. Challenges remain in fabrication precision, multi-band control, and dynamic tunability, but ongoing research points toward their broad applicability in integrated optics, topological photonics, and advanced light-matter interaction systems.
In summary, BICs provide a unifying framework that bridges theory and practical photonic design, offering a pathway to robust, high-performance, and reconfigurable photonic technologies that extend far beyond conventional high-Q resonators
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