Large spectral tuning of liquid microdroplets by local heating with a focused infrared laser

Abstract

Large deformations can easily be introduced in liquid microdroplets by applying relatively small external forces or controlling the evaporation/condensation kinetics. This makes liquid microdroplets attractive to serve as the building blocks of largely tunable optical switches or filters that are essential in optical communication systems based on wavelength division multiplexing. Solid optical microcavities have not found large use in these applications, mainly due to their rigid nature. The fact that liquid microdroplets are low-cost and disposable can also prove to be important in mass production of these photonic devices. Here, we show that local heating with an infrared laser can be used to largely tune the whispering gallery modes (WGMs) of water/glycerol or salty water microdroplets standing on a superhydrophobic surface. In the scheme presented, a liquid microdroplet kept in a humidity chamber is stabilized on a superhydrophobic surface, and an infrared laser beam is focused near tire center of the microdroplet. As a, result of the local treating, the temperature of the liquid microdroplet increases, and the water content in the liquid microdroplet evaporates until a new equilibrium is reached. At the new equilibrium state, the non-volatile component, (i.e. glycerol or salt) attains a higher concentration in the liquid microdroplet. We report tunability over large spectral ranges up to 30 run at around 590 nm. For salty water microdroplets the reported spectral timing mechanism is almost fully reversible, while for tire case of glycerol/water microdroplets the spectral timing mechanism can be made highly reversible when the chamber is saturated with glycerol vapor and the relative water humidity approaches unity.