Single-beam differential z-scan technique.

We report a single-beam, differential z-scan technique with improved sensitivity for the determination of nonlinear absorption and refraction of materials. A sample is scanned in the direction of beam propagation as usual, but, in addition, its longitudinal position is dithered, producing a detector signal proportional to the spatial derivative of only the nonlinear transmission and therefore giving a background-free signal; the nonlinear transmission for any spatial position of the sample can be recovered by simple integration. For both open and closed aperture scans in GaP, we find an improvement in the signal-to-noise ratio of >5 x compared with a balanced z-scan setup, but this can be improved with apparatus optimization. Nonlinear phase distortions <lambda/1500 are obtained with a 78 MHz, mode-locked Ti:sapphire laser.