The competition between two lasing modes in one-dimensional active random media

Abstract

The competition on population inversion between two lasing modes in one-dimensional (1D) active random media is investigated by simultaneously solving Maxwell's equations and rate equations of electronic population. Results indicated that the increasing of concentration of the dye increase the spectral intensity of the dim er and the i ncrease of the pump energy produces a major radiative and non-radiative energy transfers from monomers to dimers. The si mulations of the new monomerdimer laser model agree very well with the experimental datas and promote the development of random lasing in the mixed dye media. IntroductionRandom lasers are based on the intimate combination between a disordered medium that confines light waves and the gain amplifying medium. Random lasing in disorder media is of significance for both fundamental research and technological applications. In the past four decades, great research efforts have been expended in the field [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . Lasing phenomenon has been found in few materials [1][2][3][4][5] , such as dye solutions; dye doped solid polymer matrices; laser crystal powders, and others. One of these systems consisting by optically pumped solutions of Rhodamine 640 in methanol with TiO 2 particles presents a peculiar feature: the appearance of bi -chromatic emission, with well defined wavelength peaks. Detailed experimental works on this subject were carried out by Sha et al. [6][7] . Based on their experiments, the spectral properties as a function of the scattering particle density, dye concentration, and pump energy were studied. In recent experiments, the radiation peaks with narrow line width can be observed in the random media that is combined with liquid crystal and dye. The multiple lasing in such random media is related to the diffusive regime of the scattering in the structures. For the moment, several models have been set up in the random laser field, such as the diffusion equation with gain, the random walk and amplification of photons, and the ring laser with non-resonant feedback. About the bi-chromatic emission phenomena, John and Pang [8] created a four-level energy model of the dye consisting of singlet and triplet fundamental (S0, T0), and first excited (S1, T1) states. Different from John's explain, Balachandran and Lawandy [5] , suggest that bi-chromatic emission is due to re-absorption and re-emission effects of the single singlet species: the primary emission peak pumps the dye, which then produces a secon d longerwavelength peak. And then, Vaveliuk brought forward "monomer -dimer" laser model in 2003 [9] , he suggested that the bi-chromatic laser emission from laser dy e solutions containing scattering random media is due to the presence of two fluorescent aggregates: monomers and dimers. The new model can explain the many experimental results perfectly. Unfortunately, the new m odel cannot analyze the te mporal property of the light-waves in the random system and the optical effect by the inner parameters of the random media.