Rapid, broadband two-photon-excited fluorescence spectroscopy and its application to red-emitting secondary reference compounds

Abstract

We present a methodology for rapidly acquiring broadband two-photon absorption (2PA) spectra by means of a compact two-channel femtosecond fluorescence excitation apparatus. This technique is insensitive to differences in excitation and collection geometries as well as detection efficiencies between the sample and reference, in addition to variations in average power, pulse duration and spatial beam profile, as it utilizes sequential measurement of the sample and reference in each of the two cell positions. Our approach eliminates the need to determine the fluorescence quantum yields of the sample and reference, as it allows measurement of emission from samples at a common specified wavelength. These attributes allow for acquisition of 2PA spectra with an estimated accuracy of ± 15% (limited almost exclusively by the uncertainty in the 2PA cross section for the reference standards) over an excitation range of 550-1600 nanometers with a typical time per spectrum of ~30-60 minutes. We have applied this technique to determine the 2PA spectra of six commercially available organic dyes over a wide range of excitation wavelengths (670-1600 nm), which can be used as secondary reference standards emitting in the red and near-infrared spectral region (600-1000 nm). We have also characterized some of these compounds using the femtosecond-pulsed Z-scan method and found very good agreement with the fluorescence-based measurements.