A Robust Whispering-Gallery-Mode Resonator Platform for Refractive Index Sensing

Whispering-gallery-mode (WGM) resonators offer superior optical confinement and higher resolution for biochemical sensing compared to conventional fiber interferometers. However, traditional functionalization often relies on thick polymer coatings that introduce structural instability and hinder direct light-matter interaction. In this work, we propose a robust, coating-free WGM resonator platform featuring a molecular-level surface functionalization strategy. A key innovation is the 'closed-vessel' surface modification process, which protects the fragile sensing region from mechanical damage and contamination. For practical implementation, a specialized 3D-printed sensor holder is developed to enable liquid-phase measurements with minimal analyte volume. A wavelength shift of 0.15549 nm is observed when changing the liquid from deionized water to pH 0.44 hydrochloric acid. Experimental results using a bare-fiber resonator demonstrate excellent reversibility and structural integrity when cycling between deionized water and acid solutions. The proposed platform is easy to fabricate and offers a versatile template for various reliable sensing applications, ranging from environmental monitoring to real-time chemical detection.