A 3D printed fiber-tip whispering-gallery-mode resonator for sensing in complex liquid environments

WGM resonator integrated on a seven-core fiber tip via two-photon lithography. • Exhibits reusability and robustness in complex liquid conditions environments. • High-Q micro-rings enhanced optical resonance for sensitive in-situ detection. • Sensitive detection of organic matter was achieved. • Achieving a NEP of 6.6 mPa ⋅Hz −1/2 with bandwidth exceeding 200 MHz. Stable operation of whispering-gallery-mode (WGM) microcavity sensors in complex liquid environments remains challenging due to surface fouling, optical coupling instability, and environmental interference. Although fiber-integrated WGM devices enable miniaturized and in-situ sensing, their performance is often limited by fragile coupling configurations and poor long-term stability in liquids. Here, a fiber-tip WGM resonator is fabricated via two-photon lithography (TPL) by directly printing a microcavity onto the facet of a seven-core optical fiber (SCF) and subsequently encapsulating it with a low-refractive-index glue (LRIG) layer. This integrated architecture stabilizes optical coupling and suppresses resonance degradation in complex liquid environments, enabling high-contrast WGM spectra with quality factors on the order of 5.39 × 10 4 in liquid. Benefiting from enhanced light-matter interaction, structural robustness, and operational repeatability, the device exhibits sensitive responses to the non-specific adsorption of representative organic molecules, including ethanol, acetone, and isopropanol. Moreover, the same fiber-tip resonator enables ultra-sensitive ultrasound detection via acousto-optic interaction, achieving a noise-equivalent pressure (NEP) of 6.6 mPa ⋅Hz −1/2 with a full bandwidth exceeding 200 MHz. These results demonstrate a compact, robust, reusable, and scalable fiber-tip WGM device capable of optical and acoustic sensing in liquid environments, providing a versatile building block for integrated fiber-optic sensing platforms. Download: Download high-res image (163KB) Download: Download full-size image Herein, we report a seven-core fiber-tip WGM resonator integrated via two-photon lithography and low-index encapsulation. This robust architecture ensures high-Q (5.39 × 10 4 ) resonance in liquids, enabling high-contrast organic molecule sensing. Furthermore, the device achieves ultra-sensitive ultrasound detection with a record-low noise-equivalent pressure (NEP) of 6.6 mPa ⋅Hz −1/2 and bandwidth exceeding 200 MHz.