Sensitive measurement of absolute two-photon absorption cross sections

We develop a method for measuring absolute two-photon absorption cross sections (sigma(2)) and employ it to determine the sigma(2) of Rhodamine-6G in methanol (16.2 +/- 2.4 GM at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent "z-scan" (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a "thin" (thickness < Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting. (C) 2000 American Institute of Physics. [S0021-9606(00)02321-7].