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 interlayer 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. Since interlayer exciton transitions are highly tunable also by choosing twist angle and material combination our results open up new approaches for designing the SHG response of layered materials.
Highlights
Second-harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy
We first focus on the model considerations for SHG mediated by the interlayer excitons in bilayer MoS2
The SHG signal is very sensitive to the electric field, being one of the most striking experimental observations
Summary
Second-harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. 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- and interlayer exciton resonances. The occurrence of SHG in a crystal is directly linked to its symmetry, where crystals with broken inversion symmetry can exhibit SHG and crystals which are inversion symmetric can typically not[5] This dependence on symmetry makes nonlinear optics in atomically thin crystals such as graphene a very rich field of research[23,24,25,26,27,28,29]. Excitons are strongly bound in these materials[35] and govern optical processes at room temperature, contrary to the model semiconductor GaAs10 This is crucial for nonlinear optics as excitons resonantly enhance light-matter interaction by several orders of magnitude[35]. In the interest of both fundamental physics and applications, we need to uncover the exact origins of SHG in layered semiconducting materials to enable further tuning and amplification
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