Experimental evidence of two-photon absorption and its saturation in malachite green oxalate: a femtosecond Z-scan study

Third-order nonlinear optical (NLO) properties of malachite green oxalate (MGO) dye were systematically studied at different input intensities and in a wide range of wavelengths by the Z-scan method using ∼150  fs, MHz femtosecond pulses. The sample transmittance results are explained by fitting the data with phenomenological models of linear and multi-photon absorption and their saturations. Intensity-dependent measurements demonstrated the influence of saturation on the absorption process. The data obtained suggests that by tuning the input intensity single or combination of nonlinear phenomena can be achieved in MGO. Wavelength-based measurements reveal a strong correlation between linear and NLO responses. The combinations of two- and three-photon absorption and saturation of two-photon absorption occur simultaneously in a wide wavelength range (750–900 nm). Multi-photon absorption cross sections have been calculated using the NLO coefficients and the concentration data. Two- and three-photon absorptions (including an excited state) are determined as the excitation mechanisms of nonlinear absorption in MGO.