Improving the fidelity of two-photon absorption reference standards

Reference standards with well-defined femtosecond two-photon absorption (2PA) properties facilitate accurate measurement of nonlinear-optical spectra by bypassing tedious characterization of the photon flux. The 2PA standards are increasingly used for developing advanced multi-photon fluorescent probes and, since recently, also for probing intra- and intermolecular electrostatic interactions. We have recently reported 2PA cross section values of a set of common organic dyes in different solvents in 680-1050 nm wavelength range with estimated accuracy of 8%. In the present work, we aim at further improving the accuracy and fidelity of the absolute 2PA cross section data by comparing in a pair-wise manner the relative 2PA efficiency of nine standards with partially overlapping absorption- and fluorescence emission spectra. We measure the relative 2PA-induced fluorescence for each pair under identical excitation conditions, which allows revealing inconsistencies potentially present in the previously published data due to errors in estimating the excitation laser beam spatial- and temporal profile, pulse energy and other critical parameters. Our current measurements confirmed and in some cases improved previously reported error margins thus improving the fidelity of the reference data. We also present refined 2PA cross section data on 9-Chloroanthracene in dichloromethane.