## Abstract

We present a powerful but simple technique based on a $4f$ coherent imager system with top-hat beams to characterize nonlinear optical properties. We describe the theoretical model and the experimental details of the measurement for materials having nonlinear refraction with or without nonlinear absorption. We show that it is possible to characterize the nonlinearities by analyzing the intensity profile of the image after nonlinear filtering through the material placed in the Fourier plane of the setup. We will show that, as in the *Z*-scan technique, the use of top-hat beams instead of Gaussian beams increases the sensitivity of the measurement. Intensity-dependent nonlinearities can be studied by use of this single laser-shot technique. We validate this nonlinear imaging technique by measuring the absolute value of the ${n}_{2}$ coefficient for ${\mathrm{CS}}_{2}$ and some well-known chalcogenide glasses $({\mathrm{As}}_{2}{\mathrm{S}}_{3},$
${\mathrm{As}}_{2}{\mathrm{Se}}_{3},$
${\mathrm{GeSe}}_{4},$ and ${\mathrm{Ge}}_{10}{\mathrm{As}}_{10}{\mathrm{Se}}_{80}).$ Our values are in good agreement with those obtained by other techniques.

© 2004 Optical Society of America

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