n this paper, we employ a fully microscopic approach to the study of interlayer excitons in layered materials. We discuss the utilization of Fowlers and Karplus method to access the dynamical polarizability of non--interacting interlayer excitons in a $mathrm{WSe}_{2}/mathrm{WS}_{2}$--based van der Waals heterostructure. Following from the calculation of the linear polarizability, we consider Svendsens 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 $mathrm{WSe}_{2}/mathrm{WS}_{2}$ hetero--bilayer, discussing the various intra--excitonic energy level transitions observed.
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