Temperature-induced tuning of emission spectra of liquid-crystal optical microcavities

Emulsion droplets of liquid crystals (LC) suspended in water and labeled with a suitable fluorescent dye can serve as active optofluidic microcavities, since the contrast of refractive index between the LC droplets and the surrounding aqueous medium allows excitation of whispering gallery modes (WGMs) in the droplets. In addition, such emulsion droplets can be also stably trapped in three-dimensions using optical tweezers which stabilizes the droplets while investigating their spectral characteristics. We explore various combinations of fluorescently dyed LC droplets and host liquid - surfactant systems and show that the WGM emission spectrum of an optically trapped LC droplet-based cavity can be largely and (almost) reversibly tuned by controlled changes of the ambient temperature that induce phase transitions in the LC droplets. Our results indicate feasibility of this approach for creating miniature tunable sources of coherent light.