Peculiarities of the nonlinear optical absorption of Methylene blue and Thionine in different solvents

The saturated and reverse saturated absorption, as well as two-photon absorption, of two thiazine cationic dyes [Methylene blue (MB+) and Thionine (TH+)] were measured at the wavelengths of 532 nm using 10 ns pulses. The concurrence of saturated absorption and two-photon absorption was analyzed in the case of aqueous and ethanol solutions of TH+ dye in the range of 10−4 - 10−3 M concentrations. The reverse saturated absorption of the ethanol and aqueous solutions of MB+ dye was observed. The nonlinear optical processes were fitted to the model of absorption allowing definition of the saturated intensities, reverse saturated, and two-photon absorption coefficients. It has been established that dimerization of MB+ and TH+ dyes plays a negative role in the processes of nonlinear absorption of 532 nm laser pulses. In this case, reverse saturation absorption for solutions containing predominantly MB+ monomers provides an effective power limitation.173