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Spontaneous emission enhancement in strain-induced WSe2 monolayer based quantum light sources on metallic surfaces

Atomic monolayers of transition metal dichalcogenides represent an emerging material platform for the implementation of ultra compact quantum light emitters via strain engineering. In this framework, we discuss experimental results on creation of strain induced single photon sources using a WSe2 monolayer on a silver substrate, coated with a very thin dielectric layer. We identify quantum emitters which are formed at various locations in the sample. The emission is highly linearly polarized, stable in linewidth and decay times down to 300 ps are observed. We provide numerical calculations of our monolayer-metal device platform to assess the strength of the radiative decay rate enhancement by the presence of the plasmonic structure. We believe, that our results represent a crucial step towards the ultra-compact integration of high performance single photon sources in nanoplasmonic devices and circuits.

or colloidal CdSe quantum dots in the closest vicinity of metallic surfaces has also been identified to act significantly on the emitters' decay dynamics. 28,29 Hence it is important to separate the emitters from the metallic layers via a non-conducting spacer (For example, Al 2 O 3 ) of few nanometers thickness. Furthermore, plasmonic nanoparticles close to emitters do not only enhance the emission rate via the Purcell effect but also can yield enhanced excitation rates. In fact the over-all PL enhancement of emitters very close (few nm) to plasmonic nanoparticles is a result of both excitation and emission rates enhancements. 30 Recently, the deterministic coupling of single quantum emitters in thin hBN layers to plasmonic nanoparticle arrays has been shown, 31 and even in comparatively simple schemes, near unity absorption of light has been demonstrated by coupling of optical modes at an interface between a WSe 2 layer and silver film. This indicates a strongly enhanced local density of states in such architectures. 32 Enhancement of photoluminescence (PL) has also been observed from atomic monolayer materials on metallic layers. [33][34][35] Metallic substrates have the advantage of reflecting almost all of the photons towards the collecting objective, while the transmission or absorption of single photons inside the substrate is the main source of loss of the single photons. In addition, metallic nano-particles on a metal surface act as native antennas, 21,36,38,47 which in principle allow to engineer the spontaneous emission rates of surface-near quantum emitters. [39][40][41][47][48][49][50][51] In this report, we demonstrate strain induced creation of single photon sources in a monolayer of WSe 2 in the close proximity of a nano-modulated metallic substrate. Our source is temporarily stable and exhibits robust emission features comparable to monolayer single photon sources on dielectric substrates. Our joint experimental-theoretical investigation suggests, that the spontaneous emission of our single photon emitters is enhanced by coupling to plasmonic modes in the metal. The base structure of our study consists of a 600 µm sapphire substrate, which is capped by 10 nm of chromium and 200 nm of silver. Fig. 1(a-d) depicts scanning electron microscope images of a mechanically exfoliated WSe 2 monolayer transferred onto the silver substrate. In particular from the close-up images in Fig. 1 Photoluminiscence counts (a.u) Red dotted circles show strained monolayer around such nanoparticles. d) A monolayer is seen to bend over a silver nanoparticle (dotted red circle and inset: zoomed in picture of the strained monolayer due to the nanoparticle). (e) Spatially resolved and wavelength integrated (750-850 nm) PL intensity map of the structure, depicting bright emission spots (encircled) occurring on the monolayer. f) A series of representative photoluminescence spectra picked from the bright spots on the sample, corresponding with positions S1-S7., U; homogeneous background (due to unstrained monolayer), B; blank area devoid of monolayer.
it becomes evident that the monolayer bends around exposed regions of the metal surface, and fully adapts its morphology. In addition, we observe the formation of nano-wrinkles, which possible yield nanoscopic strain fields in the monolayer.
In order to test the influence of the macroscopic roughness of our prepared surface on the emergence of strongly localized excitons in the TMDC monolayers, we studied our sample via position resolved scanning PL with a step size of 250 nm. The wavelength-integrated corresponding PL intensity map is depicted in Fig. 1e. While, surprisingly, we did not see any significant signal from the attributed free exciton or trion resonance in WSe 2 , we observe a strongly inhomogeneous emission pattern giving rise to distinct high intensity spots in a wavelength range between 780 nm and 840 nm. In order to shed more light into the origin of these spots, we plot the selected spectra corresponding to positions S1-S7 in Fig. 1f. Here, it becomes evident, that a large fraction of these sites are affiliated with the emergence of sharp emission lines, which are prototypical for strongly localized quantum emitters in atomic monolayers. The absence of the distinct free exciton resonances could be a result of exciton diffusion towards such distinct localization sites, which form an attractive potential for excitons. We believe, that the bended monolayer effectively leads to an exciton funneling effect, which particularly exposes some crystal defects and nanowrinkles as emitters of narrow band spectral features.
In order to analyze the optical quality of our emitters, we study the linewidth of each emerging peak in our monolayer. To reduce significant power broadening and laser induced heating, the power was reduced to 708 W/cm 2 and the spectrum was integrated for 10 sec. The acquired linewidth statistics is shown in Fig. 2a. Although we identify emission features scattered over a wide range of linewidths (few 100 µeV to few meV), we observe various emitters close to the resolution limit of our optical spectrometer (75 µeV , see inset of Fig. 2a). We plot the emission intensity (Fig. 2b) and linewidth (Fig. 2c) of a selected emitter as a function of the excitation power of the driving laser. The excitation power dependent photoluminescence intensity (Fig. 2b) can be approximated by an equation for a two level system, 17 I = I sat (1/(1 + P n /P exc )) where, I sat is the saturation intensity and P exc is excitation power. P n is a fit parameter normalizing the excitation power. In order to assess the polarization properties of the emitters, we measured a series of polarization dependent PL spectra using a half wave plate and a linear polarizer. The resulting polar plot of two of such emitters, called P 1 (786.2 nm) and P 2 (791.5 nm) as a function of the detected polarization angle is shown in Fig. 2f. We can see that the emitters are highly linearly polarized (with a degree of polarization (DOP) of 92 % for P 1 and 94% for P 2 ), and differ in orientation by an angle of ≈ 80 • . One plausible origin of the orientation and high directionality could be an effect of light matter coupling of the emitters and silver nanoparticles. Different orientation of adjacent emitters further suggest that emitters P 1 and P 2 might be located at strained edges around the same silver nanoparticle. However, we also note that high degrees of linear polarization have been reported on for emitters in WSe 2 on dielectric substrates, which has been associated with anistotropic strain in the crystal. 18 To test our system with regard to the statistics of emitted photons, we performed second order correlation measurements by exciting the sample with a 532 nm CW laser (Fig. 3a).
We selected a dominant emission feature (shown in Fig. 3b), which was spectrally filtered (bandwidth: ≈ 300 µeV, 300 grooves/mm grating) and passed it to a fiber coupled Hanbury Brown and Twiss (HBT) interferometer. We observed a sharp and narrow anti-bunching feature at zero delay times (τ = 0). In addition, the correlation exhibits a slight bunching behavior at finite delay times. The experimental data, as well as a fit to the data are depicted in Fig. 3a. There, we explicitly accounted for the finite time resolution of our single photon detectors (350 ps) by introducing a Gaussian response function g d . 16 The overall fitting function thus writes have been reported to be on the order of several nanoseconds (see table 1). Reference Decay time (ns) Substrates Ref. 17 4.1 bare silicon 1 Ref. 14 3.6 2 SiO 2 on silicon Ref. 12 (1.5-2.5) SiO 2 on silicon Ref. 20 2.8 Polymer pillars array on silicon Ref. 54 1.8 SiO 2 on silicon Ref. 55 3
The TMDC-metal hybrid is quite stable against atmospheric (air) and temperature conditions, allowing us to measure the localized emission spectra even after repeated thermocycles (cryostat-temperature, He flow at 5 K to 297 K) and laser intensities, without damaging the strained monolayer for nine months.
The extracted emitter lifetime of only 300 ps suggests that the spontaneous recombination rate in our quantum emitter is subject to some enhancement by coupling to plasmonic modes in the silver layer. The observed bunching at much longer characteristic time scales is indicative that longer lived metastable states also play a role in our system. We believe, that the contribution of a localized dark exciton, which was recently observed in W Se 2 based quantum emitters, could yield such a behavior. 57 In addition, we furthermore believe that the presence of such long lived characteristic timescales in our photon statistics excludes very strong non-radiative recombination as a sole source of significant lifetime shortening in our experiment. The extracted second order correlation function at zero delay, g 2 (0) is 0.25, adds evidence that our system acts as a single photon source.
In order to confirm the fast recombination of our quantum emitters, we additionally performed time correlated single photons measurements of 20 emitters at different positions of the monolayer. Since the time resolution of the single photon detector is on the same order of magnitude as the lifetime of the emitters, the measured decay M(τ ) is determined both by the instrumental response function IRF(τ ) and the time-resolved emission of the sample E(τ ). Therefore each measured decay was fitted by iterative re-convolution of the IRF with a bi-exponential decay model where Θ describes the Heaviside step function. The instrument response was experimentally extracted by measuring the reflected laser signal and for the re-convolution it was fitted with an exponentially modified Gaussian function. Indeed, we observed two different classes of quantum emitters in our study , which are exemplary depicted in Fig 3c) and 3d). For the class of emitters corresponding with the time-trace in Fig 3c), we observe a very fast emission decay between 100-500 ps, being significantly faster than reported values in the same material system on dielectric surfaces (see table 1). The statistics of the recombination time from these emitters is plotted in the inset of Fig 3e). The second class of emitters in our study yields decay times on the order of various nanoseconds (Fig 3d). We believe, that these emitters emerge from wrinkles in our strained monolayers which do not couple to the silver surface.
In order to add further confidence, that the origin of the enhanced radiative decay rates is a result of coupling of our quantum emitters to plasmonic resonances evolving in the silver surface in the close proximity, 32  A sketch of the assumed scenario is shown in Fig. 4a. A horizontal dipole emitter is placed on top of the nanocone structure with a wavelength of 800 nm and we calculated the output  power in the nanostructure, as we move the position horizontally with a spatial resolution of 3 nm. The reference output power of the dipole is calculated by putting the dipole in the background of air. The grid resolution is 1 nm. The decay rate enhancement map for a dipole emitter around a silver nanocone is shown in Fig. 4b. We observe, that nearly 100 times decay rate enhancement is expected for a dipole, which is located 3 nm away from the tip of the cone, while a negligibly small value is observed for a non-metallic reference structure purely comprising a sapphire cone (not shown). The vector field distribution and the field enhancement E/E 0 for our metallic, Al 2 O 3 coated device is shown in Fig. 4c  24 nm). In fact, these sizes match the approximate structure dimensions of the smallest class of our metallic particles (e.g. depicted in Fig 1d). We note, that considering the dielectric environment due to WSe 2 on the nanostructure which is known to induce a further redshift on the spectrum of several 10 nm, 43,44 the smaller silver nanoparticles (diameter: 12 nm) can be expected to approach the resonance frequency range (720-800 nm) of our quantum emitters. Furthermore, it is indeed known that the spontaneous emission enhancement of a plasmonic device can be the strongest on the low energy side of the actual plasmonic resonance. [45][46][47] In conclusion, we demonstrated the formation of quantum dot-like structures in a monolayer of WSe 2 exfoliated onto a rough metallic surface. The metallic surface serves a dual purpose of not only creating quantum emitters in the monolayer via nanoscopic strain engineering, but also by enhancing the spontaneous emission rate. The latter is experimentally confirmed by time resolved studies, yielding decay times down to 100 ps. Our numerical calculation suggests decay rate enhancements due to a nanocone antenna. We verify that the coupling efficiency is optimum near the edges of the cone, rather than the center of the tip. Our work on the hybrid device will motivate the further research on integration of TMDC-based single photon sources and plasmonic waveguide/nano-circuits for quantum photonics/plasmonics application on a fully metallic substrate.