Intensity mediated change in the sign of ultrafast third-order nonlinear optical response in AsS thin films.

We study experimentally and theoretically the intensity-dependent off-resonant ultrafast third-order nonlinear optical response of AsS thin films. At low intensity, we observed saturable absorption with a negative (self-defocusing) nonlinear refractive index (n) which at higher intensity switched over to reverse saturable absorption with a change in the sign of n 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.