Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique

Abstract

We demonstrate direct measurements of the absolute molecular two-photon absorption (TPA) cross-sections using a fluorescence technique. A theoretical model of the detected fluorescence signal generated by a femtosecond laser pulse was developed. It is shown that the onset of excited-state saturation depends on the TPA cross-section and the local intensity but is independent of the detection efficiency and the quantum yield. Taking advantage of this, we develop a technique to measure the TPA cross-section that only requires precise knowledge of the space-time profile and the pulse energy of the exciting femtosecond laser beam and of the spatial profile of the observation beam. The exciting beam is generated in the focus of a microscope objective using a hollow core photonic fiber as a spatial filter, whereas the observation is done confocally through a conventional single-mode fiber. An in-house built profiling tool is used for the diagnosis of the tightly focused, highly divergent beams. The method was used with Rhodamine 6G and Rhodamine B dissolved in methanol and excited at 806nm; TPA cross-sections of σ_2R6g=16.0±3.0GM and σ_2RB=17.9±3.0GM, respectively, were measured.