Graphene Photonics || Nonlinear Optical Properties

摘要： In the preceding two chapters, we consider only the linear optical response of graphene; fields of different frequencies do not mix, and the amplitude of the response is linearly proportional to the strength of the excitation optical field. In this chapter, the nonlinear optical response of graphene is discussed. In general, the nonlinear optical response of a material is described by a polarization that is a nonlinear function of the optical field; such a nonlinear polarization is characterized by a nonlinear susceptibility. Because graphene can be treated as a conductor, its nonlinear optical response can alternatively be described by an optical current that is a nonlinear function of the optical field; such a nonlinear current is characterized by a nonlinear conductivity. As in the case discussed in the preceding two chapters for the relation between linear susceptibility and linear conductivity, these two alternative descriptions of nonlinear optical properties of graphene are equivalent. In this section, we first define the nonlinear susceptibility and then make the connection between the nonlinear susceptibility and the nonlinear conductivity.