Influence of nonradiative excitation-energy transfer on the dye laser active-medium gain-coefficient dichroism

Abstract

The excitation power dependences of the luminescence polarization degree and of the anisotropy of the stimulated emission from glycerin solutions of rhodamine 6G at different dye concentrations have been obtained experimentally. It is shown that the anisotropy of the stimulated emission is determined by the dichroism of induced absorption in the range of low concentrations and by the development of excitation-energy migration into the generation channel. The complex character of the behavior of the fluorescence polarization degree is explained by the competition between several processes of excitation-energy transformation -nonradiative energy transfer and directed energy transfer to excimers with an increase in the dye concentration and in the excitation power.Introduction. In a number of theoretical and experimental works [1][2][3][4], the anisotropy of emission from liquid dye lasers depending on the active substance concentration was investigated. The dependences of laser-radiation polarization for a wide range of concentrations were obtained. It was found that in polarized radiation pumping the concentration of solutions exerted an appreciable effect on the polarization degree and effectiveness of generation. The greatest generation effectiveness, to which the smallest value of the polarization degree corresponded, was reached at a certain concentration. Upon increase or decrease of concentration, along with a decrease in the effectiveness of transformation of the excitation energy into the energy of generation, one observes an increase in the degree of polarization of generation radiation up to unity, which in [1] is attributed to competition between gain and losses in the cavity. In [4], these dependences were interpreted by the different contribution of mutually perpendicular polarization modes due to nonradiative excitation-energy transfer (NEET). Therefore, to obtain complete clarity in the explanation of this effect additional experimental investigations of the dependence of the laser-radiation anisotropy on the concentration of an active substance in a solution were required. In the present work, we investigate the dependence of the luminescence anisotropy and generation of concentrated solutions of rhodamine 6G in glycerin on excitation power. Measurement Procedure. Excitation of luminescence and of solution generation was made by the second harmonic of a YAG-Nd laser (LF-116, "Solar", Belarus) with a wavelength of λ = 532 nm into the first absorption band of the rhodamine 6G dye in an orthogonal variant of pumping.The polarized radiation of the second harmonic of pulsed laser 1 (Fig. 1) with a pulse repetition rate of 10 Hz and duration of a single pulse of 10 nsec was transformed with the aid of a half-wave plate 2 into polarized radiation with a prescribed polarization vector and was used to excite the luminescence of glycerin solutions of rhodamine 6G (3). The luminescence emitted by the dye was spatially divided by a Wallaston prism 4 into two components with mutually perpendicular polarizations...