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Directional Emission from Tungsten Disulfide Monolayer Coupled To RESEARCH ARTICLE www.adpr-journal.com Directional Emission from Tungsten Disulfide Monolayer Coupled to Plasmonic Nanowire-on-Mirror Cavity Shailendra K. Chaubey, Gokul M. A, Diptabrata Paul, Sunny Tiwari, Atikur Rahman, and G. V. Pavan Kumar* Plasmonic nanocavities are important Influencing spectral and directional features of exciton emission characteristics architectures to study because of the very from 2D transition metal dichalcogenides by coupling it to plasmonic nano- high electric field inside the cavity caused [11] cavities has emerged as an important prospect in nanophotonics of 2D materials. by hybridization of gap plasmon. Ultrasmall volume inside the cavity provides Herein, the directional photoluminescence emission from a tungsten disulfide a large Purcell enhancement factor.[12,13] To (WS2) monolayer sandwiched between a single-crystalline plasmonic silver this end, nanocavity formed by a nanoparti- nanowire (AgNW) waveguide and a gold (Au) mirror is experimentally studied, cle on a gold film has been utilized for thus forming a AgNW–WS2–Au cavity. Using polarization-resolved Fourier-plane strong coupling at the single-molecule optical microscopy, the directional emission characteristics from the distal end of level,[14] single-molecule surface-enhanced Raman scattering (SERS),[15,16] controlled the AgNW–WS2–Au cavity are quantified. Given that the geometry simulta- reflectance properties,[17] and enhanced neously facilitates local field enhancement and waveguiding capability, its utility spontaneous emission.[13,18] in 2D material-based, on-chip nanophotonic signal processing is envisaged, Of relevance to this study is the AgNW including nonlinear and quantum optical regimes. on a gold mirror cavity. Nanocavity formed by a AgNW on a gold film, apart from the abovementioned advantages, can also work as an optoplasmonic waveguide of cavity’s 1. Introduction emission, as shown recently by our group.[19] Such nanocavities are also studied in the context of Purcell enhancement,[18] remote In the last few years, 2D transition metal dichalcogenides excitation of the molecules,[19] spontaneous emission enhance- (TMDs) have attracted significant attention because of their novel ment,[18] wavevector distribution,[19] and trion enhancement.[20] – optical and electronic properties.[1 4] Direct bandgap in their Coupling surface plasmon polaritons to TMDs can facilitate monolayer makes them a suitable candidate for electronics appli- interesting optical properties. To this end, studying the optical – cations.[3 5] Very high oscillator strength leads to a narrow peak transition characteristics of TMDs in vicinity of a plasmonic – with a pronounced optical transition.[6,7] In addition, TMDs nanostructure near resonance has gained relevance.[21 24] A vari- exhibit valley polarization effects, making them suitable candi- ety of prospects, such as PL and Raman enhancement,[25,26] – date for spin–orbit interaction studies.[8 10] Coupling 2D materi- enhanced spin–orbit interaction,[20,27] remote excitation of fi [28] [29] [30] als such as a tungsten disul de (WS2) monolayer to plasmonic SERS, spectrum tailoring, strong coupling, and trion nanocavities can enhance and influence light–matter coupling. enhancement,[20] have been achieved using such configuration. In this article, we report on our experimental studies of direc- For these purposes, many plasmonic structures, such as bowtie [31] [32] [29] [30] tional photoluminescence (PL) from WS2 monolayer sandwiched antenna, nanodisk array, nanocube, nanoparticle, and in a unique kind of cavity: silver nanowire (AgNW) on a gold NW,[20,28] have been either fabricated over TMDs[33] or TMDs is mirror. 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] S. K. Chaubey, G. M. A, D. Paul, S. Tiwari, Dr. A. Rahman, logic operation,[39] Rabi splitting,[40] and plasmon–exciton inter- Prof. G. V. P. Kumar conversion.[20] Silver film–AgNW cavity has been recently uti- Department of Physics – Indian Institute of Science Education and Research lized for the trion enhancement and enhancing the spin orbit Pune 411008, India coupling.[20] E-mail: [email protected] Apart from these effects, a plasmonic AgNW can act as a sub- The ORCID identification number(s) for the author(s) of this article wavelength waveguide as well as a nanoscale antenna, which can can be found under https://doi.org/10.1002/adpr.202100002. be harnessed to route an excited signal and emit the signal at the [41] © 2021 The Authors. Advanced Photonics Research published by Wiley- distal end. This utility of AgNW both as a plasmonic cavity and VCH GmbH. This is an open access article under the terms of the Creative as a waveguide for 2D materials has not been explored, which we Commons Attribution License, which permits use, distribution and do in this article. reproduction in any medium, provided the original work is properly cited. Figure 1 shows the studied geometry. It contains a WS2 mono- DOI: 10.1002/adpr.202100002 layer sandwiched between a plasmonic AgNW and a gold mirror Adv. Photonics Res. 2021, 2100002 2100002 (1 of 6) © 2021 The Authors. Advanced Photonics Research published by Wiley-VCH GmbH www.advancedsciencenews.com www.adpr-journal.com from the other end of the cavity as a function of angle and polari- zation. To achieve this, we use polarization-resolved Fourier- plane optical microscopy to study the emission direction of – – the AgNW WS2 Au cavity and quantify the in-plane angular dis- tribution. By studying the angular distribution and polarization- – – resolved spectra, we show that the AgNW WS2 Au cavity can modify the spectral feature of the WS2 monolayer by interconver- sion between exciton and trion. 2. Result and Discussion 2.1. Directional PL from AgNW–WS2–Au Cavity Figure 1. Schematic of the experimental setup. The WS2 monolayer was Figure 2a is the bright-field image of the AgNW placed on a gold placed over a gold film separated by a 3 nm Al O spacer layer. Single 2 3 mirror AgNW–WS –Au cavity. Excitation of NW end, with a AgNW was placed over the monolayer WS flake. One end of the NW 2 2 tightly focused 532 nm laser, excites the propagating plasmons was excited with 532 nm laser. WS2 PL from the excitation point couples to the NW plasmons, which is further out-coupled from the distal end of along the NW. In addition, the excitation of hybridized gap plas- the NW. The emission from the distal end was collected and was projected mons between the AgNW and the metal film creates high local fi to spectrometer and EMCCD for spectroscopy and Fourier-plane imaging, electric eld, which enhances the PL emission from WS2 mono- respectively. layer. Because of the near field interaction, the PL emission gets coupled to the NW surface plasmon polaritons (SPPs) travelling along the length of the NW. Because of the spatial discontinuity at – – (AgNW WS2 Au cavity). A thin, Al2O3 layer is deposited on gold the NW end, these SPPs are out-coupled as free space photons. In – – mirror, which acts as a buffer layer. In such waveguide-cavity sys- Figure 2b, we observe strong PL emission from the AgNW WS2 tems, a wavevector analysis becomes an important aspect of Au cavity not only from the excitation and distal ends, but also study to quantify the emission process. Motivated by this, our throughout the NW. This is because of the high electric field study focuses on the wavevector distribution of the PL emission in the NW on a metal film cavity, which acts as a hotline along – – fi in the AgNW WS2 Au cavity. Speci cally, we excite one end of the length of the cavity. Figure 2c shows the emission spectrum – – the cavity with a focused 532 nm laser and study the PL spectrum collected from the distal end of the AgNW WS2 Au cavity. To fi Figure 2. Directional emission from the distal end of AgNW. a) Bright- eld image of NW over WS2, which is placed over a gold mirror with a 3 nm spacer layer of Al2O3.WS2 monolayer boundaries are shown with red dotted lines. One end of the NW was excited with 532 nm laser with polarization along the long axis of the NW. b) PL image of the NW after rejecting the elastically scattered light using combination of edge and notch filters. Dotted box shows the fi distal end collection of the NW. c) WS2 PL spectrum collected from the distal end of the NW as per the con guration in (b). d) Fourier space intensity distribution of the PL out-coupled from the distal end of the NW showing directional emission in narrow range of wavevectors. Adv. Photonics Res. 2021, 2100002 2100002 (2 of 6) © 2021 The Authors. Advanced Photonics Research published by Wiley-VCH GmbH www.advancedsciencenews.com www.adpr-journal.com study the wavevector of emission from the cavity, we performed near field is distributed near the top of the NW contribute to the Fourier-plane imaging, which maps the emission wavevectors in propagation.[42] An inference we draw from this is that the pres- terms of θ and φ spreading. Radial coordinate in Fourier plane is ence of gold mirror enhances the propagation length. NA ¼ n sin θ,andφ is the tangential coordinate that varies from 0 to 2π. Fourier-plane image in Figure 2d shows that the emission 2.2. Polarization-Resolved Angular Emission from from the NW end is directed toward higher ky/k and covers only a 0 – – small range of radial and azimuthal angles, indicating highly AgNW WS2 Au Cavity directional emission. Multiple measurements for the momentum space image are shown in Figure S4, Supporting Information. The emission at the distal end of NW is mediated through the A relevant question to ask is how does the presence of gold SPPs of the NW, which generally shows rich polarization signa- mirror influence the performance of the cavity on and off the ture.
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