Bound states in the continuum: A study on metasurface engineering and photonic maneuvering

Bound states in the continuum (BICs) define a class of non-radiative modes situated inside the radiation continuum, facilitating exceptional light confinement with minimal losses. Primarily formulated in quantum mechanics, BICs have recently emerged in photonics due to their ability to support ultrahigh quality (Q) factors and strong light–matter interactions. Initially, theoretical advances demonstrated the conditions under which BICs arise in systems with specific symmetry properties. Later, these theoretical outcomes turned into experimental realizations in a multitude of platforms. Considering the recent progress, this review explores the development of BICs in metamaterials, detailing their classification into symmetry-protected, accidental, and Fabry–Pérot-like BICs. The symmetry-protected BICs originate from a mismatch in symmetry between the incident radiation and the interacting system, which prevents radiation coupling. Accidental BICs result from the destructive interference of two or more resonant channels, effectively eliminating the far-field radiation. Whereas Fabry–Pérot-like BICs arise due to the formation of Fabry–Pérot cavities among multiple resonators. Furthermore, this article highlights recent innovations in BIC and quasi-BIC research in all-dielectric and plasmonic media, highlighting their applications in nonlinear optics, sensing, imaging, nanocavity lasing, and on-chip photonic integration. In nonlinear optics, BICs enable highly efficient second- and third-harmonic generations, frequency mixing, and other nonlinear processes. In biosensing applications, the ultranarrow resonances associated with BICs significantly improve detection precision. In imaging and spectroscopy, their subwavelength field confinement enables super-resolution techniques and label-free molecular identification. Ongoing research in BICs has the potential to reveal further sophisticated features in photonic platforms, rendering them essential for emerging optical and quantum technologies.