Selective Amplification of the Primary Exciton in a $\mathrm{Mo}{\mathrm{S}}_2$ Monolayer

Optoelectronics applications for transition-metal dichalcogenides are still limited by weak light absorption and their complex exciton modes are easily perturbed by varying excitation conditions because they are inherent in atomically thin layers. Here, we propose a method of selectively amplifying the primary exciton ($A^0$) among the exciton complexes in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ via cyclic reexcitation of cavity-free exciton-coupled plasmon propagation. This was implemented by partially overlapping a Ag nanowire on a $\mathrm{Mo}{\mathrm{S}}_2$ monolayer separated by a thin $\mathrm{Si}{\mathrm{O}}_2$ spacer. Exciton-coupled plasmons in the nanowire enhance the $A^0$ radiation in $\mathrm{Mo}{\mathrm{S}}_2$. The cumulative amplification of emission enhancement by cyclic plasmon traveling reaches approximately twentyfold selectively for the $A^0$, while excluding other $B$ exciton and multiexciton by significantly reduced band filling, without oscillatory spectra implying plasmonic cavity effects.

Figure 1

(a) Schematic of the experimental setup with a side view of the hybrid. PL signals were collected from the NMOR (on NW) and from the bare $\mathrm{Mo}{\mathrm{S}}_2$ (off NW) that were excited by an input laser. (b) (top) Optical micrograph showing the LIP (green arrow) and (bottom) PL image showing the collection position (white arrow) of the PL signal at the same LIP. NW length, $\sim 4\mu \mathrm{m}$. Effective NW length from the NWEP to the LIP, $\sim 3\mu \mathrm{m}$. (c) Normalized PL signals as a function of $P_{\text{ex}}$ for on NW and off NW, with examples of Lorentzian deconvolution at $P_{\text{ex}}=5\mu \mathrm{W}$. (d) PL spectra for on NW and off NW at $P_{\text{ex}}=100\mu \mathrm{W}$. (e) The log-log scale PL intensity ($I_{\mathrm{PL}}$) as a function of $P_{\text{ex}}$ derived from the PL spectra for off NW. (f) The PL spectra for off NW at $P_{\text{ex}}=5\mathrm{nW}$ and 100 nW and SLF for $P_{\text{ex}}=5\mathrm{nW}$ identified as $A^0$.

Figure 2

(a) Schematics for two different sample configurations: fully overlapping NW (FONW) and partially overlapping NW (PONW) on $\mathrm{Mo}{\mathrm{S}}_2$ flakes. $x_M$, the length of the FONW. $x$, the effective length for the PONW from the end of the bare NW to the LIP (green arrow). (b) $ϵ$ factor (maximum $I_A$ for on NW divided by that of off NW) as a function of $x$ or $x_M$. At $P_{\text{ex}}=100\mu \mathrm{W}$, the PL signals were collected at the same LIP for numerous devices with the two different sample configurations.

Figure 3

(a) Optical micrograph (left) illustrating $\mathrm{Mo}{\mathrm{S}}_2$ flake position and PL image (right). $x$, $\sim 6.5\mu \mathrm{m}$; NW length, $\sim 8\mu \mathrm{m}$; green arrow, LIP; white arrow, PL signal collection position. (b) PL spectra collected at the LIP. Log-scale PL spectra comparison between on NW and off NW with an $ϵ$ factor of $\sim 7$ at $P_{\text{ex}}=500\mu \mathrm{W}$. Inset, PL for off NW subtracted from that of on NW and its SLF identified as $A^0$ implying only $A^0$ enhancement for on NW. (c) PL spectra collected at the NWEP as a function of $P_{\text{ex}}$. Log-scale PL spectra reveal oscillatory fringes that are apparent at $\hslash \omega <\hslash {\omega }_{A0}$. Inset, linear scale.

Figure 4

(a) Complex excitons ($A^0$, $A^{\prime }$, and $B$) excited under an intense laser coupled to the SPPs at the LIP. The traveling SPPs lose their intensity via Ohmic loss along the NW and scattering loss ($S_{\text{rad}}$) at the NWEP. Reflected SPPs selectively reexcite only the $A^0$. (b) SPP cyclic traveling enhances the $A^0$ emission by restricting the band-filling effect in $\mathrm{Mo}{\mathrm{S}}_2$ excitons. (c) PL and SPP amplification flow via repeated interaction between excitons and SPPs as the traveling cycles increase. (d) Comparison of the experimental data for the PONW samples in Fig. 2 with the plots of Eq. (1) for various $\beta$ as a function of $x$.