Energy-transfer-assisted lasing from microdroplets seeded with fluorescent sol.

We report observations of lasing emission from liquid microdroplets of Fluorescein 548 dye in ethanol, seeded with submicrometer-sized fluorescent sol. An incident pump laser excites fluorescein dye molecules, which in turn couple energy to sol-dye molecules, generating lasing in the sol. The pump laser can also generate fluorescein lasing in the droplets, which may excite lasing in the sol. Other noteworthy findings include the absence of sol emission for larger sol and the presence of sol emission, even without any observable fluorescein emission, for smaller sol. All emissions are at wavelengths corresponding to morphology-dependent resonances of the droplet. Studies of the dependence of these emissions on pump laser intensity and sol concentration suggest that they are driven either by Förster energy transfer between fluorescein dye molecules and dye within the sol or by enhanced radiative transfer that occurs when fluorescein emission couples to morphology-dependent resonances of the droplet microcavity.