Photothermal Tuning and Size Locking of Salt-Water Microdroplets on a Superhydrophobic Surface

Liquid microdroplets are attractive as optical microcavities with tunable resonances for applications in quantum optics and biological sensing, owing to their flexible nature and spherical shape. Salt-water microdroplets can be used in such experiments while standing on a superhydrophobic surface that preserves their spherical geometry. Here, we report how the photothermal effect enables continuous tuning or locking of the whispering gallery mode (WGM) spectrum and size of salt-water microdroplets on a superhydrophobic surface. Local heating by an infrared laser focused at the center of a 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 Å. We combine this effect with fluorescence excitation spectroscopy using a fixed wavelength laser to measure Q-factors of up to ∼105. Conversely, focusing the heating laser to the microdroplet rim reveals absorption resonances, leading to a hysteretic behavior when cycling the laser power. We show that this behavior can be used to lock the size of a microdroplet and make it exhibit optical bistability. WGM resonances of locked microdroplets are probed using a tunable laser, showing a spectral locking precision reaching <0.01 nm over tens of minutes. These results indicate that the wavelength stability and positioning challenges inherent to liquid microdroplets in air can be overcome, providing an easily tunable and lockable alternative to solid optical microcavities and making them potential candidates for studies in cavity optomechanics.