Harmonic Resonance Enhanced Second-Harmonic Generation in the Monolayer WS2–Ag Nanocavity

The enhanced second-harmonic generation (SHG) from a monolayer WS2 coupled to a plasmonic nanocavity is experimentally and theoretically investigated. The nanocavity is comprised of monodispersed Ag nanocubes separated from an Ag film by a spacer Al2O3, namely, the nanoparticle on mirror (NPoM) system. When the surface plasmon polariton resonance (SPPR) wavelength of NPoM nanocavity overlaps well with the SHG wavelength of the monolayer WS2 (namely, harmonic resonance), a ∼300-fold SHG enhancement is achieved in experiment. For theoretical understanding, the quantum mechanical density matrix method has been used to develop a theory for SHG. It is found that the SHG intensity of nanohybrid is proportional to the square of the local-field intensity in NPoM nanocavity at SHG wavelength, which is ascribed to the dipole–quadrupole interaction between dipole, PSHG, in the monolayer WS2 and quadrupole, QAg, in Ag nanocavity. It is significantly different from that in metal nanoparticles under harmonic resonance, which is proportional to the local-field intensity. Therefore, it provides a novel mechanism for enhancing SHG signals from metal–semiconductor nanohybrids, which has potential applications in nonlinear devices and hybrid nonlinear metasurfaces.