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

Abstract

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, P SHG, in the monolayer WS2 and quadrupole, Q Ag, 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.

Enhanced Trion Emission and Carrier Dynamics in Monolayer WS2 Coupled with Plasmonic Nanocavity

“…Plasmonic nanocavity, composed of metal structures, can realize the localization of electromagnetic field energy, improve the electric field strength, and enhance the linear and nonlinear optical effects. [20][21][22] By coupling TMD materials to plasmonic nanocavity, certain specific optical properties can be obtained through light-matter interaction, such as strong enhancement of Raman scattering, [23] photoluminescence (PL), [24] second harmonic generation (SHG), [25,26] and plasmon-exciton coupling. [27,28] Flexible control of light-matter interaction in TMD materials is a key issue in the research and development of next-generation optoelectronic devices.…”

Second-harmonic generation enhancement in monolayer transition-metal dichalcogenides by using an epsilon-near-zero substrate

“…Several strategies have been designed to enhance the nonlinear optical SHG process in TMDCs and other 2D materials, such as excitation near excitonic resonances [25][26][27][28] , including experiments carried out at low temperature 29 and with electrostatic doping 30 . Another promising class of methods is the combination of the 2D materials with different fieldenhancement platforms, including plasmonic nanostructures for localized surface plasmon excitation [31][32][33][34][35] , hybrid dielectric structures 36,37 , metallic and dielectric metasurfaces governed by bound states in the continuum [38][39][40][41][42] , photonic crystal nanocavities 43,44 , optical microcavities 45,46 and waveguides 47 .…”

Directional Emission from Tungsten Disulfide Monolayer Coupled to Plasmonic Nanowire‐on‐Mirror Cavity

“…For these purposes, many plasmonic structures, such as bowtie antenna, [ 31 ] nanodisk array, [ 32 ] nanocube, [ 29 ] nanoparticle, [ 30 ] and NW, [ 20,28 ] have been either fabricated over TMDs [ 33 ] or TMDs is transferred onto the structures. [ 34 ] Specifically, in the context of 2D materials, TMDs have been coupled to a single AgNW for studying remote SERS, [ 27 ] second‐harmonic generation, [ 35–38 ] logic operation, [ 39 ] Rabi splitting, [ 40 ] and plasmon–exciton interconversion. [ 20 ] Silver film–AgNW cavity has been recently utilized for the trion enhancement and enhancing the spin–orbit coupling.…”