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.