Modified Z-scan technique using focus-tunable lens

In this work we demonstrate a modification of the well-known Z-scan technique. In classical Z-scan the sample is moved along the direction of propagation, z. In our experimental setup the position of sample is fixed, but the focal length f is changed by electrically focus-tunable lens. We show theoretically that measurements performed with our method (so called 'f-scan') are equivalent to those obtained using classical Z-scan in both closed and open aperture configuration. We also present example experimental data revealing nonlinear refraction and two-photon absorption phenomena. Although the modification that we propose is very simple, it provides important practical advantages over Z-scan, such as significant increase of the speed of measurements and no need for sample displacement.