Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling

The strong coupling between excitons and single plasmonic nanocavities enables plexcitonic states in nanoscale systems at room temperature. Here we demonstrate the strong coupling of surface plasmon modes of metal nanowires and excitons in monolayer semiconductors, with Rabi splitting manifested in both scattering and photoluminescence (PL) spectra. By utilizing the propagation properties of surface plasmons on the nanowires, the PL emitted through the scattering of plasmon-exciton hybrid modes is extracted. The analytically calculated scattering and PL spectra well reproduce the experimental results. These findings unify the scattering and PL spectra in the plexcitonic system and eliminate the ambiguities of PL emission, shedding new light on understanding the rich spectral phenomena in the plasmon-exciton strong coupling regime.

Figure 1

(a) Schematic and optical microscopy image of a single Ag NW on monolayer ${\mathrm{WSe}}_2$. The scale bar is $2\mu \mathrm{m}$. (b) Transmission and PL spectra of a ${\mathrm{WSe}}_2$ monolayer on glass substrate. (c) Scattering spectra of Ag NWs without (I, III) and with (II, IV) ${\mathrm{WSe}}_2$. The plexciton peaks are marked with red arrows.

Figure 2

(a) Scattering spectra of Ag NWs on monolayer ${\mathrm{WSe}}_2$. The redshift is marked with the red arrow. (b) Magnified view of the spectra in (a) showing peak splits (black lines). The red dashed lines are the fitting results by Eq. (3). The red dot line indicates the transmission dip energy of exciton. (c) The energies of UPB (orange), MPB (green), and LPB (wine) as a function of the energy of LEP mode $E_1$. The black lines are calculated by Eq. (1) with coupling strength of $g_1=g_2=35\mathrm{meV}$. The blue dashed lines show the energy of LEP ($E_1$) and HEP ($E_2$) extracted from Eq. (3). The red dot line indicates the transmission dip energy of exciton. (d) Hopfield coefficients for LEP, HEP, and exciton contributions to UPB, MPB, and LPB as a function of $E_1$.

Figure 3

(a) PL spectrum for a hybrid system of Ag NW and monolayer ${\mathrm{WSe}}_2$ when LEP energy is close to exciton energy ($E_1=1.6456\mathrm{eV}$ and $E_2=1.7996\mathrm{eV}$). The red dashed line is obtained by fitting the spectrum using three Lorentzian peaks (orange, green, and blue lines). (b) Normalized PL spectrum resulting from dividing the spectrum in (a) by the PL spectrum of the bare ${\mathrm{WSe}}_2$. (c) Scattering spectrum for the same NW in (a). (d)–(f) The same as (a)–(c) but for a hybrid system with HEP energy close to exciton energy ($E_1=1.545\mathrm{eV}$, $E_2=1.67\mathrm{eV}$).

Figure 4

(a) The energies of fitting peaks in the experimental (solid dots) and calculated (hollow square dots) PL spectra as a function of $E_1$. The orange, green, and blue solid dots correspond to three fitting peaks for experimental PL spectra. The orange, green, blue, and purple hollow dots correspond to four fitting peaks for calculated PL spectra. (b) Experimental results of the integrated intensity ratio between the plexcitons corresponding to solid green dots in panel (a) and excitons as a function of $E_1$.