Abstract

We report lasing characteristics of 40-60-µm diam rhodamine 590:water solution droplets pumped by 532-nm Q-switched pulses. Water was employed to avoid competition from whispering mode stimulated Raman scattering normally encountered in the alcohols when using nanosecond or shorter pulse excitation. We have been able to establish the threshold and find that the low level for lasing (10⁴ W/cm² for 10⁻⁴-M solution) is consistent with cavity Q approaching 10⁴. We have also obtained stable high quality lasing spectra and have identified many of the mode features. At 30-Hz excitation rate the spectral emission from a stream of droplets is stable in real time. Consequently, the rhodamine/water solution droplets reported here provide a useful model system for theory/experiment comparison. Our data appear consistent with a physical model of the droplet blanketed by many narrow ribbonlike regions, each lying on a great circle of the sphere and each possessing a distinct orientation. Oscillation is primarily of TM mode character although along some ribbon orientations TE modes lase as well. Mode features are easily tuned in frequency by varying particle size. In the far field, the lasing particle approximates a coherent point source emitting uniformly in all directions.

© 1986 Optical Society of America