Influence of multiphoton events in measurement of two-photon absorption cross-sections and optical nonlinear parameters under femtosecond pumping

Absolute values of two-photon absorption cross-sections of some laser dyes in methanol solution have been measured by using the transmission method. From the fluorescence yield measurements under multiphoton excitation, the number of involved photons in the process has also been estimated. It is found that the presence of higher photon events may lead to erroneous values of measured two-photon absorption cross-sections. It is shown here that the Z-scan technique is capable of differentiating between the two- and three-photon absorption processes. Nonlinear optical parameters under femtosecond pumping by multiphoton resonant excitation at 800 nm are also given here.