Linear and nonlinear optical properties of transition metal dichalcogenides monolayers are controlled by the Wannier-Mott excitons forming a series of 1s, 2s, 2p, . . . hydrogen-like states. We demonstrate that p- and s-shell excitons are mixed due to the specific D3h point symmetry of the transition metal dichalcogenide monolayers. Hence, both s- and p-shell excitons are active in both single- and two-photon processes providing an efficient mechanism of second harmonic generation. In addition, s-excitons are active in two-photon absorption due mixing with p-excitons in remote bands. The corresponding contributions to the nonlinear susceptibility are calculated . We also study the Auger exciton-exciton annihilation which results in the photoluminescence upconversion.
 Intrinsic exciton-state mixing and nonlinear optical properties in transition metal dichalcogenide monolayers, M. M. Glazov, L. E. Golub, G. Wang, X. Marie, T. Amand, and B. Urbaszek, Phys. Rev. B 95, 035311 (2017).