Second harmonic generation spectroscopy in van der Waals homo- and heterobilayers

The twist angle in transition metal dichalcogenide bilayers is a compelling degree of freedom that determines electron correlations and the period of lateral confinement of moiré excitons. We perform polarization-resolved second harmonic generation (SHG) spectroscopy of MoS₂/WSe₂ heterostructures. We demonstrate that by choosing suitable laser energies the twist angle between two monolayers can be measured directly on the assembled heterostructure. We show that the amplitude and polarization of the SHG signal from the heterostructure are determined by the twist angle between the layers and exciton resonances at the SH energy. For heterostructures with close to zero twist angle, we observe changes of exciton resonance energies and the appearance of new resonances in the linear and nonlinear susceptibilities. In inversion symmetric MoS₂ homobilayers we demonstrate tuning of non-linear optical processes with a strong enhancement of the SHG signal in applied electric fields for pump energies resonant with interlayer exciton states.