Two-photon absorption coefficient determination using the differential F-scan technique

In this paper we present a modification to the recently proposed transmission F-scan technique, the differential F-scan technique. In the differential F-scan technique, the programmed focal distance in the electronic-tunable lens is modulated, allowing the light detector of the setup to record a signal proportional to the derivative of the signal recorded with a F-scan. As with the differential Z-scan a background-free signal is obtained, but in this case the optical setup is simplified and the available laser power is twice that for the F-scan. We also present and validate a new fitting-procedure protocol that increases the accuracy of the technique. Finally, we show that fitting a signal from a differential F-scan or the derivate of the signal of a transmission F-scan is more accurate than simply fitting the signal from the F-scan directly. Results for two-photon absorption at 790 nm of CdS, ZnSe and CdSe are presented.