▎ 摘　　要

Using an enhanced optically heterodyned optical Kerr effect method and a theoretical description of the interactions between an optical beam, a single layer of graphene, and its substrate, we provide experimental answers to questions raised by theoretical models of graphene third-order nonlinear optical response. In particular, we measure separately the time response of the two main tensor components of the nonlinear susceptibility, we validate the assumption that the out-of-plane tensor components are small, and we quantify the optical impact of the substrate on the measured coefficients. Our method can be applied to other two-dimensional materials, as it relies mainly on the small ratio between the thickness and the wavelength.