High precision size tuning and stabilization of single salt-water microdroplets on a superhydrophobic surface

While their spherical geometry is mostly preserved, salt-water microdroplets can be studied in stable experimental conditions when standing on a superhydrophobic surface. Here, we report how the photothermal effect can be used to continuously tune or lock the whispering gallery mode (WGM) spectrum (therefore the size) of salt-water microdroplets on a superhydrophobic surface. The microdroplets are kept in the controlled atmosphere of a humidity chamber. Local heating by an infrared laser focused at the center of the microdroplet causes it to depart from its equilibrium size, shifting the WGM spectrum. This photothermal tuning effect is fully reversible and can be used to tune the microdroplet radius with a precision reaching 1 Aring by finely controlling the heating laser power. We demonstrate a new spectroscopy method based on this effect, and use it to measure Q-factors of WGM resonances of up to ~10⁵. Conversely, focusing the heating laser to the microdroplet rim causes it to experience absorption resonances, leading to a hysteretic behavior when increasing and decreasing the laser power. We show that this behavior can be used to lock the size of a microdroplet and make it function as an optically bistable element. WGM resonances of microdroplets locked in such a way are probed using a tunable laser, showing a locking precision reaching <0.01 nm over tens of minutes. These results indicate that the challenges in terms of position and wavelength stability inherent to liquid microdroplets surrounded by air can be overcome, and that they provide an easily tunable and lockable alternative to solid optical microcavities.