Photothermal phase shift interferometry: An approach for nonlinear absorption measurements

A thermal lens approach for measuring two-photon absorption cross sections (2PACSs) in fluorescent liquid samples is described and demonstrated. The method takes advantage of the sensitivity of a Sagnac interferometer, and the accumulative photothermal effect, for the detection of the induced photothermal phase shift. This leads to absolute estimations of the 2PACS of the sample. Experimental results demonstrate that this approach is suitable for obtaining the nonlinear absorbance spectrum of fluorescent molecules frequently used in two-photon fluorescence microscopy with a sensitivity of λ/1500.