Abstract

We study experimentally and theoretically the intensity-dependent off-resonant ultrafast third-order nonlinear optical response of ${\mathrm{As}}_2{\mathrm{S}}_2$ thin films. At low intensity, we observed saturable absorption with a negative (self-defocusing) nonlinear refractive index ($n_2$) which at higher intensity switched over to reverse saturable absorption with a change in the sign of $n_2$ to positive (self-focusing). Our findings constitute compelling evidence on how to dynamically tune the optical response with the intensity that has its origin in the combined effect of two-photon absorption and Pauli blocking. The results were explained with the help of time-resolved measurements and rigorous theoretical and numerical simulations.

© 2018 Optical Society of America

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