Second harmonic generation in h-BN and MoS2 monolayers: Role of electron-hole interaction

We study second harmonic generation in h-BN and MoS2 monolayers using an ab initio approach based on many-body theory. We show that electron-hole interaction doubles the signal intensity at the excitonic resonances with respect to the contribution from independent electronic transitions. This implies that electron-hole interaction is essential to describe second harmonic generation in those materials. We argue that this finding is general for nonlinear optical properties in nanostructures and that the present methodology is the key to disclose these effects.