Random laser action in aqueous solutions Emerson de Lima, 1,2 C. Tolentino Dominguez, 3 and Paulo C. de Oliveira 1,* 1 Departamento de F´ ısica, Universidade Federal da Para´ ıba, Jo˜ao Pessoa, Para´ ıba 58051-970, Brazil 2 Universidade Federal de Alagoas - Campus Arapiraca, Arapiraca, Alagoas 57309-005, Brazil 3 Departamento de F´ ısica, Universidade Federal de Pernambuco, Recife, Pernambuco 50670-901, Brazil * Corresponding author: pco@fisica.ufpb.br Abstract: We show experimental results on random laser action of rhodamine 6G in aqueous solutions containing alumina particles and SDS with high efficient energy conversion. The threshold is inversely proportional to the SDS concentration. c  2010 Optical Society of America OCIS codes: 140.2050, 290.4210, 290.5850, 240.6670. 1. Introduction Laser emission from random media occurs due to propagation of light in an amplifying medium containing scattering particles [1] or in systems consisted only of particles that work simultaneously as amplifiers and scatterers [2]. In the first case the system is usually made by dissolving laser dyes in non-polar or weak-polar solvents, such as methanol or ethanol, and by adding dielectric nanoparticle scatterers, such as TiO 2 . Many experimental works show that laser dye solutions using polar solvents like water do not show fluorescence due to the fact that this kind of solvent favors the formation of aggregates, mainly the H-type dimer, which drastically quenches the quantum efficiency of the solution. These aggregates are present even at very low laser dye concentrations. The rhodamine 6G (R6G), a xanthene dye, aggregates at concentrations around 10 -5 M in water, forming stable ground state dimers via π −π mixing of the xanthene ring orbitals. At 10 -4 M R6G in water no longer lases because of the dimerization [3]. In ethyl alcohol solutions aggregation occurs at much higher dye concentrations and the dimers, of the J-type, are fluorescent. The fluorescence spectrum of the J-type dimers is red shifted in relation to the fluorescence spectrum of the monomers. As a consequence random laser bichromactic emission occurs at concentrations around 10 -3 M [4]. Water has a lot of advantages over other solvents like ethanol or methanol [5,6]. In this work we show that adding sodium dodecyl sulfate (SDS) to the water solves the problem of aggregation of R6G molecules and rises the random laser efficiency to the same level of ethanolic solutions. 2. The experiments In the experiments we have used alumina, with an average diameter of 300 nm, as the scattering particles, and R6G as the amplifying medium. The samples were pumped by a pulsed Nd:YAG laser, operating at 532 nm, with pulse durations of 5 ns. In the first experiment we prepared a solution of R6G dissolved in ethanol and another one in distilled water plus SDS, both with the same dye concentration and density of particles, in order to make a comparison between them. All the measurements presented in this Letter were taken with a dye concentration of 1 × 10 -4 M and the density of alumina of 4 × 10 12 particles/cm 3 . The spectra, shown in Fig. 1, taken at a pumping energy of 10 mJ, are practically identical except by a red shift presented by the aqueous solution sample. 1 a253_1.pdf OSA / LAOP 2010 MB17.pdf