High-performance, spectrometer-free sensing of refractive index with a tunable, high-Q metasurface

High-performance optical sensors often face a trade-off between achieving ultra-high sensitivity via high Q-factor resonances, like bound states in the continuum (BICs), and maintaining robustness against fabrication imperfections. Moreover, reliance on bulky, costly spectrometers hinders the development of practical, high-performance refractive index (RI) sensors. This work overcomes these limitations by introducing an electro-optically tunable lithium niobate metasurface platform featuring engineered Q-factor robustness and supporting spectrometer-free RI sensing. We developed a strategy based on weak-field perturbation of symmetry-protected BICs, producing quasi-BICs that maintain ultra-high Q-factors (2.8 × 10⁶) while exhibiting significantly improved resilience to structural variations. This high-Q resonance yields an exceptional figure of merit for RI sensing exceeding 2 × 10⁶. We proposed a novel spectrometer-free RI sensing scheme, facilitated by efficient electro-optic modulation in the metasurface, which converts RI changes into simple voltage readouts via compensation. This work presents an approach for creating high-performance, robust, actively tunable, compact, and cost-effective optical sensors through engineered electro-optic metasurfaces.