Directional emission from WS2 monolayer coupled to plasmonic Nanowire-on-Mirror Cavity

Influencing spectral and directional features of exciton emission characteristics from 2D transition metal dichalcogenides by coupling it to plasmonic nano-cavities has emerged as an important prospect in nanophotonics of 2D materials. In this paper we experimentally study the directional photoluminescence emission from Tungsten disulfide (WS2) monolayer sandwiched between a single-crystalline plasmonic silver nanowire (AgNW) waveguide and a gold (Au) mirror, thus forming an AgNW-WS2-Au cavity. By employing polarization-resolved Fourier plane optical microscopy, we quantify the directional emission characteristics from the distal end of the AgNW-WS2-Au cavity. Given that our geometry simultaneously facilitates local field enhancement and waveguiding capability, we envisage its utility in 2D material-based, on-chip nanophotonic signal processing, including nonlinear and quantum optical regimes.

Photoluminescence (PL) and Raman enhancement, [25,26] enhanced spin-orbit interaction, [20,27] remote excitation of SERS, [28] spectrum tailoring, [29] strong coupling [30] and trion enhancement [20] has been achieved using such configuration. For these purposes many plasmonic structures like bowtie antenna, [31] nano-disk array, [32] nano-cube, [29] nanoparticle [30] and nanowire [20,28] has 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][36][37][38] logic operation, [39] Rabi splitting [40] and plasmon-exciton interconversion [20] . Silver film -Ag nanowire cavity has been recently utilized for the Trion enhancement and enhancing the spin-orbit coupling. [20] Apart from these effects, a plasmonic Ag NW can act as a subwavelength waveguide as well as a nanoscale antenna which can be harnessed to route an excited signal and emit the signal at the distal end. [41] This utility of Ag nanowire both as a plasmonic cavity and as a waveguide for 2D materials has not been explored, which we do in this paper. Figure 1 shows the studied geometry. It contains a WS2 monolayer sandwiched between a plasmonic silver nanowire and a gold mirror (AgNW-WS2-Au cavity). A thin, Al2O3 layer is deposited on gold mirror which acts as a buffer layer. In such waveguide-cavity systems, wavevector analysis becomes an important aspect of study to quantify the emission process.
Motivated by this, our study focuses on the wavevector distribution of the PL emission in AgNW-WS2-Au cavity. Specifically, we excite one end of the cavity with a focused 532nm laser and study the PL spectrum from the other end of the cavity as a function of angle and polarization. To achieve this, we employ polarization-resolved Fourier-plane optical microscopy to study the emission direction of AgNW-WS2-Au cavity and quantify the in-plane angular distribution. By studying the angular distribution and polarization-resolved spectra, we show that AgNW-WS2-Au cavity can modify the spectral feature of WS2 monolayer by interconversion between exciton and trion.

Experimental Section:
2.1 WS2 Synthesis: The WS2 monolayers were grown using atmospheric pressure chemical vapor deposition (APCVD) on 300 nm SiO2 coated silicon wafer following the procedure mentioned in ref. [42] and [43] . The substrates were sonicated in acetone and IPA for 10 min each and blow dried.
They were further cleaned using O2 plasma at 60 W for 5 min. The substrates were loaded on to an alumina boat containing 500 mg of WO3 with the smooth side facing the powder. The boat containing the WO3 was placed inside a quartz tube of 3.5 cm inner diameter in the heating zone of the furnace. Another boat containing 500 mg of Sulphur was placed upstream inside the tube, 15 cm away from the WO3 boat. The Sulphur boat was placed outside the furnace and was heated separately using a heater coil. Initially, the tube was flushed with 500 standard cubic centimeter per minute (SCCM) of argon for 10 min, and then the flow was reduced to 30 SCCM and was maintained throughout the experiment. The furnace was ramped up to 850 0 C at a rate of 5 0 C/min.
As the furnace reached 850 0 C the heater coil was heated to 240 0 C to evaporate Sulphur. These temperatures were maintained for 10 min for growing WS2 monolayer. After the growth, the system was allowed to cool naturally.

Optical Characterization:
Chemically synthesized WS2 monolayer was further characterized by PL and Raman spectroscopy. PL and Raman spectra of the same is shown in the supplementary information S1.

AgNW-WS2-Au cavity preparation:
Ag nanowires with average diameter 300 nm have been synthesized by polyol process as reported in. [44] For the preparation of the gold mirror, 160 nm gold film has been deposited on the glass coverslip using thermal vapor deposition. Monolayer WS2 flakes have been transferred onto the gold mirror. Polystyrene is used as a support film for this transfer as reported here. [45] 3nm Al2O3 spacer layer was placed using atomic layer deposition in between gold mirror and WS2 to prevent the charge screening with the aim to avoid PL  3 nm Al2O3 spacer layer. Single Ag NW was placed over the monolayer WS2 flake. One end of the nanowire was excited with 532 nm laser. WS2 PL from the excitation point couples to the nanowire plasmons, which is further out-coupled from the distal end of the nanowire. The emission from the distal end was collected and was projected to spectrometer and EMCCD for spectroscopy and Fourier plane imaging, respectively.

Experimental Setup:
One end of AgNW-WS2-Au cavity has been excited using 532 nm laser with polarization along the cavity. 100x, 0.95 numerical aperture (NA) objective lens was used in backscattered configuration for both excitation and collection. Signal from the distal end of the nanowire is collected by spatially filtering the region, and the real/Fourier plane is projected into EMCCD/spectrometer. To transfer the Fourier plane from the back aperture of the objective lens to the EMCCD, 4f configuration is used. [46] High NA excitation ensures efficient excitation of surface plasmon in the nanowire as well as high electric field in the cavity. Combination of edge and notch filter has been used to efficiently reject the elastically scattered light. It can be seen from the PL spectra that the contribution from the elastic scattering is negligible (supplementary information S1). Polarizer and half wave plate is used in input path to control the input polarization.
An analyzer is used in output path to analyze the output light for polarization resolved measurements. See supplementary information S2 and our previous reports [47] for detailed experimental setup of Fourier plane optical microscopy.  Figure 2 a is the bright-field image of the Ag nanowire placed on a gold mirror AgNW-WS2-Au cavity. Excitation of nanowire end, with a tightly focused 532 nm laser excites the propagating plasmons along the nanowire. In addition, the excitation of hybridized gap plasmons between the AgNW and metal film, creates high local electric field, which enhances the PL emission from WS2 monolayer. Because of the near field interaction, the PL emission gets coupled to the nanowire SPPs travelling along the length of the nanowire. Because of the spatial discontinuity at the nanowire end, these SPPs are out-coupled as free space photons. In figure 2b, we observe strong PL emission from AgNW-WS2-Au cavity not only from the excitation and distal ends, but also throughout the nanowire. This is because of the high electric field in the nanowire on a metal film cavity, which acts as a hot-line along the length of the cavity. Figure 2c shows the emission spectrum collected from the distal end of AgNW-WS2-Au cavity. To study the wavevector of emission from the cavity, we performed Fourier plane imaging, which maps the emission wavevectors in terms of θ and φ spreading. Radial coordinate in Fourier plane is = , and φ is tangential coordinate varies from 0 to 2π.

Directional photoluminescence from AgNW-WS2-Au cavity:
Fourier plane image Figure 2d shows that the emission from the nanowire end is directed towards higher ky/k0 and covers only a small range of radial and azimuthal angles, indicating highly In contrast, six bound modes (see supplementary information S6 (c) -(h)) exhibit high loss owing to very high imaginary component of the mode index. These higher order mode for which near field is distributed near the top of the NW contributes to the propagation. [48] An inference we draw from this is that the presence of gold mirror enhances the propagation length.

Polarization resolved angular emission from AgNW-WS2-Au cavity:
The emission at the distal end of nanowire is mediated through the SPPs of the nanowire, which generally shows rich polarization signature. In addition to this, emission from the WS2 which is confined in the AgNW-WS2-Au cavity is enhanced and out-couples through the nanowire end, after propagating through the cavity. To study the effect of nanowire and metal film cavity, we performed polarization resolved Fourier plane imaging on the PL emission from the distal end of nanowire. Figure 3 a represents the PL image of the AgNW-WS2-Au cavity after rejecting the elastically scattered light captured using EMCCD. Figure 3b is the momentum space image of the PL emission from the distal end of the nanowire. Figure 3c is the momentum space image analyzed for the polarization along the long axis of the nanowire and Figure 3d represents the momentum space image analyzed for the polarization transverse to nanowire. For the output polarization along the nanowire, we observe that light is directional and most of the emission is centered around φ = 0. While in case of the output polarization transverse to the axis of the nanowire, the φ spread is more. This means that photons which are polarized along the wire are more directional in comparison to the photon whose polarization is transverse to the nanowire. This is attributed to the polarization maintaining properties of surface plasmon polaritons in a nanowire. [49]  to the nanowire axis. Apart from the reduction in the intensity, the change in polarization also leads to redshift in the peaks.PL spectrum collected from the distal end of the nanowire, red and black curve corresponds to the input polarization along the long axis of the nanowire, and transverse to the long axis of the nanowire. Since the PL spectrum has the contribution of both the neutral exciton and the charged exciton (trion), such redshift can be attributed to the exciton to trion conversion. [50] Trions are essentially formed when exciton is bound by an excess of electron or hole, also called charged exciton.

Polarization-resolved PL spectral characteristics in
Photoionization and doping are the two main way to convert exciton into trion [51,52] . In this case, WS2 is slightly n doped, so the trions in the system are negatively charged. Exciton to trion conversion is reported by coupling with SPP in MIM cavity (metal insulator metal) cavity. [20] In AgNW-WS2-Au cavity there are two kind of plasmon involved. One is propagating along the wire and second is localized in the cavity. [53] The intensity component along the NW is larger as compared to the transverse to the NW.
At higher irradiation power, it has been observed that exciton converts into trion; which leads to redshift in PL spectra. [52] To further confirm this, we deconvoluted the PL spectra into two peaks by Voigt function double fit (see S5 of supplementary information). The deconvoluted spectrum is consistent with the reported exciton and Trion peak, [51] and Trion binding energy is found to be around 50 meV, which is consistent with the reported value [54,20] . However, the propagating SPP may experience reabsorption by WS2, NW or gold film during the propagation which leads to the redshift for the component having polarization along the wire. Absorption coefficient is fairly constant above 550 nm for WS2, [55] so the contribution from the reabsorption of WS2 is less probable, while reabsorption by Ag NW or metal film may be a possible reason for the above redshift. [56] Conclusion: To summarize, we have experimentally studied the polarization resolved directional PL emission nanophotonic chips and open-cavity nano-lasers. We envisage that the confined electric fields facilitated by the spatially-extended cavity can be further harnessed by engineering various parameters such as: composition, shape and size of the nanowire and the underlying gold mirror.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. Photoluminescence (PL) and Raman spectroscopy is performed using 532 nm laser excitation. We see very high PL count which shows that the WS2 on which we are performing the experiment is monolayer. In PL spectrum we have also the excitation wavelength. Small count at excitation wavelength shows that we are effectively rejecting the elastic scattered light. Further Raman measurement shows a large E2g 1 at 350 cm -1 and small A2g peak at 350 cm -1 . Intensity of E2g 1 peak is an order of magnitude higher than of A2g, which further confirms the sample is monolayer WS2.
Sample are grown on Si/SiO2 substrate we can see silicon Raman at 520 cm -1 . Figure S2. Experimental setup.

Experimental Setup
The sample was excited using a high numerical aperture 100x, 0.95 NA objective lens. The backscattered light was collected using the same lens. The 532 nm laser light was expanded using a set of two lenses L1 and L2. M1 is a mirror. The polarization of the incoming laser was controlled by a λ/2 waveplate in the path. BS1 and BS2 are beam splitters to simultaneously excite the sample with laser and its visualization using white light. Lens L3 is used to loosely focus white light on the sample plane. F1, F2, and F3 are set of two edge filters and one notch filter to reject the elastically scattered light for SERS spectroscopy and Fourier plane and energy-momentum imaging. Lenses L4 and L5 are used to project the emission to the Fourier plane onto the spectrometer or EMCCD. M2 is a flip mirror, used to project the light on the spectrometer for spectroscopy and energy-momentum imaging. Lenses L6 and L7 are flip lenses used to switch from real plane to Fourier plane.  Analyzed perpendicular to the long axis of the nano wire is de convoluted into exciton (Red dotted curve) and trion (green dotted curve).

Wire and WS2 over the glass
The output analyzed spectrum in two orthogonal polarization is deconvoluted for the trion and exciton contribution. It can be seen that the trion contribution in case of polarization along the AgNW-WS2-Au cavity trion contribution is higher in comparison to transverse to the AgNW-WS2-Au cavity. For MIM waveguide we observed eight modes. The high order SPP mode ( Fig. R2 (a) and (b) have larger propogation length as their imaginary part of effective mode index is small. In contrast, the six bound modes (Fig. R2 (c)-(h)) exhibit high loss owing to very high imaginary component of the mode index. Figure S7: FESEM image of the two typical NWs