  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 2D materials, as it relies mainly on the small ratio
between the thickness and the wavelength.
