Anisotropically Enhanced Second Harmonic Generation in a WS2 Nanoparticle Driven by Optical Resonances

Transition metal dichalcogenides (TMDCs) have attracted great interest due to their excellent electronic and optical properties. Although much attention has been paid to TMDC monolayers, the high refractive index of bulk TMDCs in the lossless region implies potential applications of TMDC nanoparticles/nanostructures in the fields of nanophotonics and nonlinear optics. In this work, the second harmonic generation (SHG) in WS2 nanoparticles with different geometrical symmetries is investigated both numerically and experimentally. It is revealed that such a WS2 nanoparticle supports electric dipole (ED) mode, anapole mode, and whispering gallery mode (WGM) in which the SHG is enhanced. It is found that the SHG from a WS2 nanoparticle with C2 symmetry exhibits a strong dependence on the polarization of the excitation laser light around the ED resonance. Interestingly, it is observed that the SHG intensities at the two polarization angles perpendicular to each other (θ = 0° and θ = 90°) become equal at the ED resonance. On the short-wavelength side of the ED resonance, the SHG intensity at θ = 0° is smaller than that at θ = 90°. The situation is reversed on the long-wavelength side. The strongest SHG is achieved at the anapole resonance for θ = 0° and at the WGM resonance for θ = 90°. The polarization-dependent ED resonance manifested in the scattering spectra of the WS2 nanoparticle is responsible for this behavior. In a WS2 nanodisk, the SHG becomes insensitive to the polarization of the excitation laser light. It is shown that the SHG intensity from a WS2 nanoparticle is much larger than that from a WS2 monolayer, with an enhancement factor of up to ∼30. Our findings suggest a way to enhance and manipulate the second-order nonlinear optical responses of TMDC nanoparticles and lay a foundation for the realization of TMDC-based photonic devices.