A hot-press molding method was used to fabricate dye-doped poly(methyl methacrylate) (PMMA) slabs. Three rhodamine dyes, Rh640 (ClO4), Rh6G(ClO4), and Rh6G (Cl), were impregnated into the PMMA matrix first by dissolving the dye and granular PMMA in a solvent mixture of chloroform and methanol and then heating the mixture in vacuo at 175 °C to obtain a spongy preform. The powdered preform was molded into slabs at 175 °C and at <1 mbar, to eliminate the formation of bubbles in the slabs. We annealed the slabs for several hours to improve its optical homogeneity and hence its lasing efficiency. When pumped by a 1.5-mJ nitrogen laser, we obtained peak lasing efficiencies of 8% and 7.8%, respectively, for Rh6G (ClO4) and Rh640 (ClO4) in PMMA matrices. The lasing efficiency of Rh6G (ClO4)-doped PMMA suffered a reduction rate of 0.012%/shot compared with 0.15%/shot for Rh640 (ClO4)-doped PMMA. In contrast, Rh6G (Cl) in a hot-press molded PMMA slab suffered thermal bleaching that resulted in a low lasing efficiency of <1%; this can be explained by its absorption and fluorescence characteristics.
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