Unveiling local molecular desorption dynamics using higher-order optical resonances.

Understanding the sorption dynamics between water molecules and various solid surfaces is of great interest in diverse fundamental and industrial research. For studying such dynamics in a microsystem, existing investigations mainly focus on sorption behaviors mediated by external temperature variations. Here, we demonstrate a route to in situ sensitive detection of laser irradiation-induced localized water molecule desorption at a sub-monolayer level on an oxide surface. Harnessing a tailored set of optical whispering-gallery-mode (WGM) resonances in a nanomembrane-based microtube cavity, the desorption can be tracked by resonance mode shift in real-time, and further explained using a combination of pseudo-first-order and pseudo-second-order models. Additionally, upon adjusted laser excitation locations, the axial-mode-dependent responses enable the retrieval of corresponding profiles of desorption-induced perturbation at equilibrium. This study provides new insights into molecular desorption kinetics and introduces a spatially resolved sensing technique with applications in surface science, molecular sensing, and the study of desorption dynamics at the nanoscale.