Ultrahigh-Sensitivity RI Sensor via Cascaded Microcavities With Direct Liquid Core Interaction

Abstract

Refractive index (RI) sensors are indispensable for label-free biochemical analysis, environmental monitoring, and food safety. However, conventional optical sensors fundamentally rely on weak evanescent fields for analyte interaction, creating a persistent sensitivity bottleneck. Here, we overcome this limitation by introducing a paradigm shift: a sensor where the analyte itself becomes an integral part of the resonant cavity medium. Our device employs an asymmetric cascade of two microdisk resonators on a low-cost polymer platform—a solid reference disk and a hollow, etched sensing disk that allows direct liquid injection. By making the analyte a core component of the whispering gallery mode (WGM) cavity, we dramatically enhance light-matter interaction. Combined with a Vernier effect amplification from the asymmetric cascade, our sensor achieves an ultrahigh sensitivity of 14,029.91 nm/RIU, a detection limit of $2.14\times 10^{-6}$ RIU —representing an order-of-magnitude improvement over conventional WGM sensors. This work directly addresses the long-standing trade-off between sensitivity and complexity, providing a scalable, cost-effective platform poised to transform point-of-care diagnostics, real-time environmental surveillance, and industrial process control.