Bound States in the Continuum across the Electromagnetic Spectrum toward Nanophotonic Applications

Abstract Radiative loss to the surrounding continuum fundamentally limits the quality factor (Q) of the resonant modes in open systems. This constrains the light–matter coupling strength, emission coherence, and efficiency of nonlinear interactions, all of which critically depend on high-Q confinement. Bound states in the continuum (BICs) offer an elegant solution to this challenge by delivering high-Q resonances and strong field localization, even when the resonant frequency lies within the radiation continuum. In this Review, we trace the physics of BICs from their formation mechanisms to platform-dependent implementations and device-level functionalities, highlighting what they enable across different frequency regimes. We identify two broad categories, symmetry-protected and accidental BICs, each realized across metasurfaces, gratings, photonic crystal slabs, integrated waveguides, and hybrid resonant structures. Platform-dependent radiation channels and coupling conditions, in turn, determine three core physical effects, which are enhanced light–matter interaction, topological field modulation, and strong wave confinement. These effects drive applications across the entire electromagnetic spectrum from ultraviolet to radio frequencies. By establishing radiation coupling suppression as a unifying paradigm, BICs have unlocked performance regimes, broad cross platform, and research directions that extend far beyond conventional resonance-engineering applications.