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

Abstract

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. • The nonlinear absorption of two thiazine cationic dyes MB + and TH + were defined. • Dimerization of MB + and TH + dyes plays a negative role in the processes of nonlinear absorption. • M-forms TH + participate in the processes of SA and predominantly TPA and M-forms MB + predominantly participate in RSA.