▎ 摘　　要

We report on Z-scan investigations of the nonlinear optical properties of thin pyrolytic carbon films and two- and three-layer graphene films, utilizing an 800 nm pulsed laser generating ultrashort 180 fs pulses. We analyze the sign and magnitude of nonlinear refraction and absorption in these materials, given contradictory results in this respect in the literature. In particular, we find that nonlinear refraction of graphene changes sign with changing light intensity with concomitant subtle changes in nonlinear absorption, a behavior not reported previously that suggests an interplay of several mechanisms that depend on intensity. Pyrolytic carbon films, on the contrary, exhibit positive refraction, whose magnitude (nonlinear refractive index n(2) congruent to 10(-7) cm(2)/W) is comparable to that of graphene, and strong negative absorption, i.e., bleaching, contrary to previously reported results. We discuss implications and possible mechanisms of the observed phenomena.