Leveraging Strong Electric Field Gradients at Anapole Resonances for Enhanced Second Harmonic Generation from...

Van der Waals layered materials are emerging as promising nanophotonic platforms for realizing linear and nonlinear optical devices. In particular, the high refractive indices of transition metal dichalcogenides, combined with their strong layer dependent optical properties offer interesting possibilities for designing photonic functionalities. In this study, the resonant enhancement of second harmonic generation (SHG) using molybdenum disulfide (MoS₂) disks on a silicon substrate with an intermediate silicon dioxide (SiO₂) layer is demonstrated. The resonant enhancement observed here is attributed to the combined interaction of the anapole scattering modes supported within the disk and Fabry-Perot (FP) modes within the multilayer stack. For a fixed MoS₂ disk dimensions, an optimized SiO₂ layer thickness results in constructive interference between the anapole and FP modes, thereby enhancing the fundamental field within the disk. Utilizing this resonance mechanism, SHG enhancements as high as 160 and 85-times are experimentally demonstrated when compared with unpatterned multilayer and monolayer MoS₂ flakes respectively. These are the highest reported enhancement values for the widely used 2H-polytype MoS₂. A significant enhancement in SHG is observed here for 2H-MoS₂ despite the alternating sheet dipoles canceling the far-field emission owing to the strong electric field gradient setup within the disk at the anapole resonance.