Measurement of nonlinear index by a relay-imaged top-hat z-scan technique

Abstract

Measurement of the nonlinear index of a number of materials of interest for the National Ignition Facility have been performed at 1064 nm and 355 nm by a modified version of the `top-hat' Z-scan technique and the results compared with the more standard gaussian-beam Z- scan technique. The top-hat technique has the advantages of higher sensitivity and smaller uncertainties introduced by beam-quality considerations. We have made what we feel to be an additional improvement by placing the defining aperture for the top hat at the front focal plane of the lens that focuses the beam into the sample and then reimaging the input aperture with a second lens onto a ccd camera. Reimaging eliminates diffraction fringes and provides a stationary image even for a wedged sample; recording the entire image permits minimization of spurious effects such as varying interference fringes.