Measuring evaporation rates of laser-trapped droplets by use of fluorescent morphology-dependent resonances

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Morphology-dependent resonances (MDRs) are used to measure accurately the evaporation rates of laser-trapped 1– to 2–μm droplets of ethylene glycol. Droplets containing 3 × 10–5 M Rhodamine–590 laser dye are optically trapped in a 20-μm hollow fiber by two counterpropagating 150–mW, 800–nm laser beams. A weaker 532–nm laser excites the dye, and fluorescence emission is observed near 560 nm as the droplet evaporates. A complete series of first-order TE and TM MDRs dominates the fluorescent output. MDR mode identification sizes the droplets and provides accurate evaporation rates. We verify the automatedMDRmode identification by counting fringes in a videotape of the experiment. The longitudinal spring constant of the trap, measured by analysis of the videotaped motion of droplets perturbed from the trap center, provides independent verification of the laser’s intensity within the trap. © 2001 Optical Society of America.

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Applied Optics