Interlayer exciton mediated second harmonic generation in bilayer MoS2

TL;DR

This paper demonstrates how interlayer excitons in bilayer MoS2 can be used to significantly enhance second harmonic generation through resonant tuning and electric field manipulation, enabling control over non-linear optical responses.

Contribution

It reveals the tunability of SHG in inversion symmetric bilayer MoS2 via interlayer excitons and electric fields, a novel approach for non-linear optics control.

Findings

SHG amplitude is enhanced by resonant tuning to exciton resonances.

Electric fields further increase SHG response by lifting exciton degeneracy.

Bilayer SHG can reach amplitudes comparable to monolayer off-resonance.

Abstract

Second harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also inter-layer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations.