Intrinsic exciton states mixing and non-linear optical properties in   transition metal dichalcogenide monolayers
  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 develop the theory of the excited excitonic states energy spectrum fine structure. We predict that $p$- and $s$-shell excitons are mixed due to the specific $D_{3h}$ 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. The corresponding contribution to the nonlinear susceptibility is calculated. 
M. M. Glazov, L. E. Golub, G. Wang, X. Marie, T. Amand, and B.   Urbaszek
cond-mat.mes-hall cond-mat.mtrl-sci