Surface sensitive microfluidic optomechanical ring resonator sensors

The microfluidic optomechanical ring resonator (μFOMRR) based on a thin-walled glass capillary supports high Q-factor (>103) mechanical modes in the presence of liquids. In this letter, the sensitivity of the μFOMRR to the surface change is studied by layer-by-layer removal of SiO2 molecules from the μFOMRR inner surface using various concentrations of hydrofluoric acid solutions. A frequency downshift is observed with a sensitivity for the surface density change of 1.2 Hz/(pg/mm2), which translates to a detection limit of 83 pg/mm2. This work opens a door to using the optomechanical mode for detection and characterization of molecules near the resonator surface.