We discuss the possibilities of a far-field beam profiling technique for the determination of the near-focal profiles of laser beams focused beyond the paraxial approximation. The analysis is based on the rigorous vectorial Fourier synthesis of the fields and includes polarization effects. The wave-vector distribution can be calculated either a priori using the radius of the impinging laser beam and the data of the objective, or by obtaining its parameters from profiling at different z positions far behind the objective. For such measurements we designed a profiling system suitable for in situ application in microscopy. The device consists of a fiber optic taper bonded onto the chip of a CMOS camera, allowing taking pictures in the working medium in planes sufficiently close to the focus. The near-focal profiles obtained by these two approaches show good agreement, but only if the measured effective numerical aperture NAeff is used. We have further readdressed the effect of filling the objective pupil on the near-focal fields. The small improvement in focusing by the commonly practiced overfilling is hardly worth the significant loss of power. Moreover, overfilling above an optimal beam radius aopt results in an increase of aberrations when focusing through an optical interface.
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 σ2R6g=16.0±3.0GM and σ2RB=17.9±3.0GM, respectively, were measured.