Study of two-photon absorption spectral property of a novel nonlinear optical chromophore using femtosecond continuum

The degenerate two-photon absorption (TPA) spectrum has been studied for a novel organic chromophore (AF389) that exhibits extremely high TPA cross-section values around the ∼800 nm range in both nanosecond and femtosecond domains. To measure the direct degenerate TPA spectrum of AF389 in the femtosecond regime, we have developed a new technique that is based on the use of a single continuum-generation beam as an intense coherent white-light source for TPA excitation. The different spectral components of the input continuum beam are spatially dispersed and then pass the sample through different pathways so that nondegenerate TPA processes among different spectral components can be avoided. Our results show that the TPA spectrum of AF389 is different from the linear absorption spectrum on the scale of absorbing photon energy. There is a significant blue shift of the absorption peak position for the TPA spectrum with respect to the linear absorption spectrum, indicating that the transition pathways are different for the TPA and linear absorption processes.