High sensitivity photo-thermal lens method for measuring of two-photon absorption

We present a two-beam mode-mismatched thermal lens method for pulse excitation aimed for the determination of one- and two-photon absorption coefficient with high sensitivity. In this scheme the excitation beam is focused onto the sample in the presence of a collimated CW probe light of low power. The Z-scan signature is single peaked with a width that depends on the number of photons involved in the absorption. We show that the method is at least two orders of magnitude more sensitive than the well-known open Z-scan transmission method commonly used for multi-photon absorption measurement. Using the proposed method we measure a two photon absorption coefficient for nitrobenzene of beta=1.12 10(-10) cm/W for nanosecond pulses and a wavelength of 532 nm.