Experimental measurement of the critical population inversion for the dye solution laser

The quenching of stimulated emission from organic dye solutions has previously been attributed to optical absorption by dye molecules in the metastable triplet state. By comparing the critical inversion expected for a dye laser with no triplet losses to that obtained from an analysis of the laser that includes the triplet-state loss, the magnitude of the optical loss due to the triplet state can be determined in terms of the absorption and emission spectra, fluorescence decay time, quantum yield, and laser wavelength for a given dye. Measurements of the critical inversion and triplet-state concentration for a flashlamp-excited laser are presented for the xanthene dyes acridine red, fluorescein disodium salt, rhodamine B, and rhodamine 6G. Experiments with 7-hydroxycoumarin were also performed but, due to lack of triplet-state absorption data, the triplet-state concentration could not be determined. It is possible from the measurements to conclude that triplet-state absorption is important in 7-hydroxycoumarin. The critical inversion is found to be approximately 10¹⁵cm^-3for the xanthenes. This value is, in all cases, many times larger than the critical inversion calculated in the absence of triplet losses.