Plasmonic Flat‐Band Quasi‐Bound States in the Continuum for Ultrasensitive Fiber‐Tip Sensing

Abstract The physics of plasmonic bound states in the continuum (BICs) provides a powerful framework for enabling intense subwavelength light‐matter interactions. However, conventional high‐quality‐factor (high‐ Q ) quasi‐BICs typically require a complex system and precise angle‐dependent excitation, posing challenges in integrated waveguide‐coupled systems. Here, this work demonstrates an optical fiber‐tip‐integrated quasi‐BIC metasurface as a compact, alignment‐free platform for high‐ Q resonance. The metasurface consists of asymmetric gold cross‐shaped slots that tailor quasi‐BIC mode transitions from local to nonlocal regimes through controlled symmetry breaking. This strategy generates a flat‐band plasmonic high‐ Q quasi‐BIC in the semi‐local regime, exhibiting ultralow angular dispersion and reduced Ohmic losses. Crucially, the engineered flat‐band state facilitates efficient coupling with the fundamental mode of a single‐mode fiber. Experimental results reveal dual resonances directly excited by the fundamental fiber mode. Real‐time refractive index (RI) monitoring of 1‐µL low‐molecular‐weight analytes demonstrates dual‐channel sensitivities of 346.4 and 529.1 nm per RIU. This work achieves a ultrahigh Q ‐factor for fiber‐tip plasmonic metasurfaces and establishes a novel paradigm for high‐precision and measurements enabled by fiber‐integrated BIC physics.