Sensitive measurement of absolute two-photon absorption cross sections

We develop a method for measuring absolute two-photon absorption cross sect ions (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 relati ve excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the tran smission of Gaussian laser pulses through a two-photon absorbing medium. Th e expression is valid for arbitrary absorber thickness, at all distances fr om the focus. This generalizes the prevalent "z-scan" (translation of the s ample along the beam direction) technique for measuring two-photon absorban ce, removing the requirements of a "thin" (thickness < Rayleigh range of th e focused laser beam) sample and of placing the sample at the focus. This l eads to an improvement of the sensitivity of the technique by over two orde rs of magnitude, enabling measurement of the two-photon absorption cross se ctions of even weakly absorbing specimens at moderate intensities. The resu lts are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting. (C) 2000 American Institute of Phy sics. [S0021-9606(00)02321-7].