Dark-field Z-scan imaging technique and application to optical nonlinear refraction measurement

We introduce an imaging technique based on a combination of Dark-field microscopy and Z-scan techniques (called Dark-field Z-scan) in order to characterize optical third-order refractive nonlinearity. The overall proprieties of this new method is presented. In particular, we show that it is possible to determine the magnitude and the sign of the induced nonlinear phase shift. Application to two porphyrin compounds showing both negative and positive NL refractive indices is performed. Advantages and inconveniences of this technique are discussed.