Laser action of Rhodamine 640 perchlorate in highly scattering media consisting of TiO2 nanoparticles in methanol pumped by a 3-ns 532-nm laser pulse is observed by use of spectral and temporal measurements. The threshold for stimulated emission is dependent on the concentration of both laser dye and scattering particles. For a 2.5 × 10−2 M dye concentration, the lasing threshold was found to be reduced by more than 2 orders of magnitude when the density of scattering particles was increased from 5 × 109 to 2.5 × 1012 cm−3. However, for a concentration of 5 × 10−4 M of Rhodamine 640 the lasing threshold was higher in scattering media than for the methanol host. The pulse duration of stimulated emission for Rhodamine 640 perchlorate in these random systems is ~50 ps.
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