Second harmonic generation enabled by van der Waals metasurfaces

Taming second order nonlinearities is of great importance for a range of applications from quantum and classical light sources to computing and communications. Here we study second harmonic generation (SHG) in bulk nanostructured 3R MoS2 metasurfaces. Unlike the most prevailing 2H phase, 3R MoS2 is non-centrosymmetric and possesses non-zero 2^nd order susceptibility. Prior works have studied SHG in exfoliated thin film flakes. In this work we study SHG generation enhancement in  resonant metasurface arrays. We pattern 3R MoS2 (thickness ~130 nm) into arrays of nano-disk resonators. Such disk resonators exhibit strong resonances far below the optical bandgap (0.775 – 0.886 eV), thanks to material’s intrinsically high refractive index. We design structures to be resonant at the pump wavelengths in the range of 1400nm – 1600 nm. In this case generated SHG signal occurs at and below MoS₂ A and B excitons (~1.85 eV and ~2 eV, respectively). By varying the pump wavelength and tuning the geometry of the metasurface we study the influence of second harmonic excitons and their extinction on the efficiency of the second harmonic generation. We also study theoretically and experimentally the interplay of different symmetries on the nonlinear wave-mixing. Our results show significant enhancement of SHG in patterned metasurfaces.