Fig. 1. Characterization of the Ag-WS₂ heterostructure. (a) Schematic of the Ag-WS₂ heterostructure, where Ag nanodisks were fabricated on WS₂ monolayers by EBL. The resonance wavelength of the Ag disk is designed to be resonant with the A-exciton in WS₂ monolayers. (b) SEM image of the Ag-WS₂ heterostructure. The diameter of Ag disk is 90 nm. Scale bar: 150 nm. (c) Normalized reflectivity spectra of pure WS₂ monolayers (red line) and bare Ag nanodisks (blue line). The reflectivity spectrum of WS₂ monolayers displays a deep dip at ~610 nm, which is consist with the plasmon resonance of Ag nanodisk with diameter of 90 nm. (d) Reflectivity spectra of the Ag-WS₂ heterostructure with the diameter of Ag nanodisk increased from 85 to 95 nm.

Fig. 2. Strong coupling in the Ag-WS₂ heterostructure with an optical microcavity. (a) Schematic of the optical microcavity with an embedded Ag-WS₂ heterostructure. The microcavity is manufactured in a sandwich structure with a 100 nm-thick Ag layer at the bottom, a 185 nmthick MgF₂ layer in the middle, and a 20 nm-thick Ag layer on the top. (b) The reflectivity spectral mapping of the Ag-WS₂ heterostructure with optical microcavity. Here the diameter of Ag nanodisk is 95 nm. Scale bar: 10 μm. (c) Cross section view of the Ag-WS₂ heterostructure with optical microcavity at a tilted angle 52°, scale bar is 150 nm. (d) Normal-incidence reflectivity spectra of the Ag nanodisk with different sizes directly embedded in the optical microcavity. The Ag nanodisks couple with the optical microcavity, which generates two new hybrid modes with the bandwidth smaller than plasmon resonances of bare Ag nanodisks. (e) Normal-incidence reflectivity spectra of the Ag-WS₂ heterostructure with optical microcavity. Three different hybridized modes, as the upper branch, middle branch, and lower branch are observed in reflectivity spectra, which are indicated by black solid lines. Vertical gray dashed lines respectively represents the WS₂ A-exciton energy and the resonance energy of bare optical microcavity. (f, g) Expanded views of reflectivity spectral features of the heterostructure with the Ag nanodisk diameter of 110 nm (f) and 95 nm (g).

Fig. 3. FDTD simulation results. (a) Simulated reflectivity spectra of Ag nanodisk embedded in an optical microcavity. White dashed lines represent the resonance wavelength of the empty microcavity (~650 nm). (b) Normalized reflectivity spectra of Ag-WS₂ heterostructure with the optical microcavity. Three different energy branches are emerged because of the strong coupling among the optical microcavity, surface plasmons and A-exciton of WS₂ monolayers. (c) Electric field intensity distributions (E/E_in)² on the xoy plane of Ag nanodisk with and without microcavity. (d) Expanded views of the simulated reflectivity spectral feature for disk size of 110 nm (top line) and 95 nm (bottom line) in (b), respectively.

Fig. 4. Anticrossing behavior of the strong plasmon-exciton-cavity coupling. (a) The three-coupled harmonic oscillator model, which includes the surface plasmons, A-exciton, and microcavity mode as three oscillators. (b) Energies of reflectivity dips as a function of the nanodisk diameter extracted from the reflectivity spectrum. Red dots with error bars show energies obtained from the reflectivity spectrum. The horizontal black dashed lines respectively represent the A-exciton and the microcavity resonant energy. The black slanted short-dashed line represents plasmon resonance mode. Three green solid curves correspond to theoretical fits of hybrid branches based on the three-coupled oscillator model. The error bar represents the standard error of a set of measurements. (c) Hopfield coefficients for plasmon, exciton, and microcavity contributions to upper, middle, and lower hybrid states as a function of diameter, calculated using the three-coupled oscillator model, which provide the weighting of each constituent. (d) Reflectivity spectral linewidths of the upper, middle, and lower branch modes as a function of nanodisk diameter.