Cavity optomechanical mass sensor in water with sub-femtogram resolution

Sub-femtogram resolution of an in-liquid cavity optomechanical mass sensor based on the twin-microbottle glass resonator is demonstrated. An evaluation of the frequency stability using an optomechanical phase-locked loop reveals that this cavity optomechanical sensor has the highest mass resolution of ( 7.0 ± 2.0 ) × 10 − 16 g in water, which is four orders of magnitude better than that in our first-generation setup [Sci. Adv. 8, eabq2502 (2022)]. This highly sensitive mass sensor provides a free-access optomechanical probe in liquid and could thus be extended to a wide variety of in situ chemical and biological metrology applications.