Theory of third-order polarizability of interlayer excitons due to intra-excitonic energy level transitions

In this paper, we employ a fully microscopic approach to the study of interlayer excitons in layered materials. We discuss the utilization of Fowler’s and Karplus’ method to access the dynamical polarizability of non–interacting interlayer excitons in a WSe2/WS2–based van der Waals heterostructure. Following from the calculation of the linear polarizability, we consider Svendsen’s variational method to the dynamic third–order polarizability. With this variational method, we study both two–photon absorption and third–harmonic generation processes for interlayer excitons in a WSe2/WS2 hetero–bilayer, discussing the various intra–excitonic energy level transitions observed.