Measurement of the two-photon absorption cross section by means of femtosecond thermal lensing.

We present a variation of the single-beam thermal lensing experiment to determine the two-photon absorption cross sections of classical fluorophores. The approach is based on comparison of two thermal lensing signals simultaneously induced by a one- and two-photon absorption process from a high-repetition-rate femtosecond laser system. As a consequence of this comparison, a simplified expression independent of the several experimental parameters is obtained. Additionally, because of the low incident power levels required, undesirable optical effects such as Kerr or Raman scattering are avoided. Our experimental results agree well with those recently published for luminescent methods, validating the approach.