SPECTRAL PROPERTIES OF LASING MICRODROPLETS

The spectral behavior of 15.3-mu-m-diameter Rhodamine 6G in water solution droplets was studied. Micro-droplet lasing is known to occur simultaneously at many discrete wavelengths, each corresponding to one of many possible spherical cavity resonances. We show that lasing takes place on several mode orders at once. Modes of a given order were found to form a bell-shaped spectral cluster of typically 4-6 resonance lines having consecutive principal mode numbers. Clusters of different mode orders appear somewhat displaced spectrally from one another, with lowest-order clusters shifted to the red. This multiplicity of lasing modes is accounted for by spatial hole-burning effects. The relative lasing intensities of the differing mode orders are explained by an output coupling theory that considers the gain enhancement that is due to cavity quantum electrodynamic effects. An upper limit of 10(8) for the Q of a nondegenerate cavity mode was estimated from the data.