Design and characterization of MoS2-based devices for enhanced second-order nonlinear optical response

MoS2 has recently garnered significant interest for applications in optoelectronics and quantum technologies because of its strong single layer second-order nonlinear optical response, up to 104 pm/V. Although the lack of inversion symmetry vanishes for an even number of layers, recent experimental results suggest that MoS₂ resonators up to 100 nm thick can still generate significant second harmonic signals due to resonant high filed localization on the surface. Indeed, MoS₂'s high refractive index around the C exciton peaks (450 nm) induces steep field gradients at the interface, resulting in a pronounced nonlinear response due to magnetic dipole and electric quadrupole terms. However, the typical exfoliation-based deposition techniques only allow for the realization of micrometer-sized areas and limit integration with available photonics technologies, thus severely impeding the future industrial development of MoS₂-based nonlinear optical devices. Here we report experimental results on nonlinear characterization of large area, polycrystalline, MoS2 films and 1D-gratings on SiO2 substrate. We also report numerical studies on new designs for enhanced nonlinear second-order interactions in MoS2-based integrated devices. More in detail, the high nonlinear response of thin MoS2 stripes is coupled to Si3N4 ridge waveguides in a counter-propagating second harmonic generation scheme. The numerical results offer interesting perspectives for integrated nonlinear frequency converters and waveguided twin photon generators operating by tailored spontaneous parametric down conversion.