Second Harmonic Generation in 3R-MoS2 Metasurfaces Around Exitonic Wavelengths

Due to their high refractive index van der Waals materials have an unprecedented potential to confine light at the nanoscale. This ability is particularly interesting for nonlinear photonics, where nonlinear interaction scales quadratically with the optical field intensity. Utilizing this enhancement could help overcoming the inherently weak nature of nonlinear interaction. Recent discovery of nonlinear activity in 3R phase of MoS₂, suggests that this material platform can be used to design compact and efficient nonlinear devices. Here we study second harmonic generation in 3R- MoS₂ metasurfaces. We reveal a complex interplay between mode confinement, linear extinction, and nonlinear susceptibility resonances.

To probe nonlinear light-matter interaction near the exciton resonances, we pump below 3R-MoS2 bandgap in the 1000 nm to 1600 nm wavelength range. This allows probing second harmonic emission at A and B excitons, and therefore examine the role of excitons in nonlinear frequency conversion. To tailor and control this interaction we fabricate several metasurfaces which exhibit resonances at ~1500 nm. By tuning the metasurface period and dimensions the resonance position can be controllably tuned. At the resonance enhanced second harmonic generation is expected. We show >150 fold SHG enhancement as compared to unpatented films of the same thickness (~100 nm thick) at ~740 nm wavelength. Our measurements further reveal a complex spectral dispersion of the generated second harmonic signal. In particular, in addition to the enhancement around the metasurface resonance frequency, we observe strong exciton-induced features seen in the strength of the second harmonic generation. To explain the complex dynamics observed, we perform full tensor numerical simulations. The modeling reveals a complex interplay and coupling between geometric resonances, optical extinction and strong susceptibility dispersion near excitons. The overlap of these effects is the key to understanding and optimizing nonlinear conversion in systems of this kind.

Using the insights from our measurements and the numerical model, we then explore harmonic generation in 3R-Mos2 metasurfaces featuring bound state in the continuum resonance. Our simulations predict that structures with a quality factor of ~1000 can allow up to 6 orders of magnitude enhancement in a structure that is just 100 nm thick.

To conclude, our work sheds light on the nonlinear light-matter interaction in 3R- MoS₂ metasurfaces and allows harnessing it’s high index combined with strong second order nonlinearities to produce ultracompact nonlinear photonic devices.