Single-atomic-ion detection with plasmon-enhanced whispering-gallery-mode microlasers

Whispering-gallery-mode microlasers have emerged as powerful tools for label-free biosensing, yet their sensitivity has been limited to detecting nanoparticles larger than 10 nm. Here we demonstrate a plasmon-enhanced whispering-gallery-mode microlaser capable of detecting single atomic ions in solution, achieving unprecedented sensitivity. By integrating gold nanorods onto ytterbium-doped silica microspheres, we reduce the effective mode volume by approximately 1,000-fold and enhance the local electromagnetic field, amplifying the signal-to-noise ratio. The self-heterodyne detection of beatnote frequency shifts between split lasing modes enables the real-time monitoring of transient and permanent interactions of zinc (Zn²⁺) and cadmium (Cd²⁺) ions with nanorod sensing sites. We report peak sensitivities with beatnote shifts of 3.7 fm for Zn²⁺ and 7.2 fm for Cd²⁺, showcasing the potential of plasmon-enhanced whispering-gallery-mode microlasers for single-molecule and atomic-scale sensing applications, including in vivo probing.