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Second harmonic generation (SHG) is a non-linear optical process, where twophotons coherently combine into one photon of twice their energy. Efficient SHGoccurs for crystals with broken inversion symmetry, such as transition metaldichalcogenide monolayers. Here we show tuning of non-linear optical processesin an inversion symmetric crystal. This tunability is based on the uniqueproperties of bilayer MoS2, that shows strong optical oscillator strength forthe intra- but also inter-layer exciton resonances. As we tune the SHG signalonto these resonances by varying the laser energy, the SHG amplitude isenhanced by several orders of magnitude. In the resonant case the bilayer SHGsignal reaches amplitudes comparable to the off-resonant signal from amonolayer. In applied electric fields the interlayer exciton energies can betuned due to their in-built electric dipole via the Stark effect. As a resultthe interlayer exciton degeneracy is lifted and the bilayer SHG response isfurther enhanced by an additional two orders of magnitude, well reproduced byour model calculations.

Interlayer exciton mediated second harmonic generation in bilayer MoS2