Deterministic coupling of quantum emitters in WSe$_2$ monolayers to plasmonic nanocavities

We discuss coupling of site-selectively induced quantum emitters in exfoliated monolayers of WSe$_2$ to plasmonic nanostructures. Squared and rectangular gold nanopillars, which are arranged in pitches of \SI{4}{\micro\meter} on the surface, have sizes of tens of nanometers, and act as seeds for the formation of quantum emitters in the atomically thin materials. We observe chraracteristic narrow-band emission signals from the monolayers, which correspond well with the positions of the metallic nanopillars with and without thin dielectric coating. Single photon emission from the emitters is confirmed by autocorrelation measurements, yielding $g^{2}(\tau=0)$ values as low as 0.17. Moreover, we observe a strong co-polarization of our single photon emitters with the frequency matched plasmonic resonances, indicating deterministic light-matter coupling. Our work represents a significant step towards the scalable implementation of coupled quantum emitter-resonator systems for highly integrated quantum photonic and plasmonics applications.


Introduction
Creating solid state quantum emitters and their integration in micro-and nanophotonic structures is one of the prime tasks in modern quantum engineering. Coupled solid state quantum emitter-cavity systems range among the most promising candidates for the realization of highly efficient single photon sources, 1-5 spin photon interfaces, 6,7 quantum sensing probes 8 as well as building blocks for quantum simulation 9 and surface code quantum computing. 10 While quantum emitters have been identified, studied and engineered in a variety of crystals including III-V 11 and II-VI quantum dots, [12][13][14] colour defects in diamonds, 15 impurities in SiC and organic polymers, 16,17 atomically thin materials [18][19][20][21][22] were recently established as a novel platform of quantum photonic devices. Quantum dots in III-V semiconductors and defect centers in diamonds certainly belong to the most mature implementations, 23 but the quality of site-controlled emitters leaves still needs to be improved, putting a serious thread regarding their scalable fabrication in ordered arrays. 24 Ordered InAs/GaAs quantum dot arrays have been realized by selective area growth methods and epitaxial growth on patterned substrates, 25 but in most cases the costly fabrication methods severly compromised their emission properties. Direct integration of positioned solid state quantum emitters with photonic resonators has been accomplished, [26][27][28] but only in very selected cases and genuine scalability has remained elusive.
The formation of quantum emitters in mono-and bilayers of transition metal dichalcogenides has now been observed in various implementations: initially, localized luminescence centers in exfoliated flakes were discovered close to their edges, and have been associated with strain wrinkles. [20][21][22] In epitaxially grown flakes, random positioning of such spots was observed, 18 indicating emission from defect bound excitons. Recently, the formation of quantum emitters on modulated metal substrates, 29,30 as well as nanopillars 31,32 was reported and associated with localized and engineered crystal strain fields, which outlines the unique possibility to deterministically induce quantum emitters in a straight forward manner by structuring the sample surface prior to the transfer.
While the ordered formation of quantum emitters thus far has been mainly observed on dielectric, nanostructured surfaces, spontaneous emission enhancement was reported on rough metallic surfaces and gold-coated nanopillars, giving rise to localized plasmonic modes. 30,33 Combining atomically thin materials which comprise either tightly localized excitons or strongly bound free excitons with nanoplasmonic cavities yields a promising pathway to study light-matter coupling on the nanoscale enabled by the enormous field enhancements provided by metallic nanostructures. 34,35 However, the deterministic coupling of well-ordered quantum emitters in atomically thin materials with resonant plasmonic modes has only now been achieved. 33,36 Sample Structure and Setup In this work, we demonstrate the feasibility to induce ordered arrays of quantum emitters by defined arrays of metallic nanopillars, fabricated on a SiO 2 substrate. Such structures directly represent a coupled quantum dot-nanocavity system, and act as polarization-controlled single photon sources. image of a prototype nanopillar array with a pitch of 2 µm is shown in Fig. 1a). On selected samples we additionally deposited a 3 nm thin layer of Al 2 O 3 via atomic layer deposition.
Next, we fabricated atomically thin layers of WSe 2 via mechanical exfoliation using adhesive tape, and transferred the layers on the pillar arrays via dry transfer 37 (Fig. 1b). We observe, that part of the pillars pierced the monolayer, while a substantial number of nanopillars (> 50 %) locally strained the layer, yielding the tent-like structure shown in Fig. 1c).
Spatially resolved optical spectroscopy was performed in a micro-photoluminescence setup with optional high spatial resolution (using fiber based confocal setting). The sample is excited by a frequency-doubled Nd:YAG laser at 532 nm, mounted in a liquid helium cooled flow cryostat. In the low-power regime, these emission lines exhibit a slightly sub-linear intensity increase with the pump power prior to their saturation level (Fig. 2b). This behaviour is mainly due to the gold pillars absorbing parts of the incident laser light, 38 but also the re-emitted light from the emitter, reducing their quantum efficiency.   Power-dependent study of a quantum emitter emission line before saturation starts above 10 µW. c) Spatial map of a WSe 2 flake covering the nano-pillar array, showing the integrated intensity from 700 − 800 nm. The enhanced PL coincides with the 4 µm pillar distance (black pattern). d) Spectral information extracted between the blue arrows in c), revealing a periodic increase in luminescence and the formation of additional localized emission centers.

Experimental Results and Discussion
The ordered formation of emitters on the nanopillar arrays is confirmed in a highly spatially resolved scanning microphotoluminescence study, applying the confocal configuration: Here, we carefully scan the sample's surface by utilizing a pair of motorized linear stages with a step width of 500 nm underneath the excitation and collection spot. The spectrally integrated map (700-800 nm) is shown in Fig. 2c). It clearly evidences a regular pattern of bright emission sites, perfectly coinciding with the positions of the metallic pillars (dashed black pattern). Spectral information is best illustrated in a selected linescan between the blue arrows in Fig 2c). Here, we clearly observe a two-fold effect by the nanopillars (Fig.   2d): A strong luminescence enhancement of the overall signal due to plasmons, 39 as well as the regular formation of the sharp peaks below the free exciton energy (<1.74 eV), which we associate with tight exciton localization due to strain. In order to provide evidence for the capability to emit single photons from the deterministically localized excitons, we performed second order correlation measurements by exciting the sample with a 532 nm CW laser (Fig. 3a). We selected a dominant emission feature from one square pillar (140 × 140 nm). The luminesence was spectrally filtered (bandwidth: ≈ 300 µeV, 300 grooves/mm grating) and passed to a fiber coupled Hanbury Brown and Twiss (HBT) setup. We observed a well-pronounced anti-bunching signal at zero delay time (τ = 0), allowing us to extract a g (2) (τ = 0) value of 0.17, which clearly puts our system in the regime of single photon emission.
Polarization resolved spectroscopy on different nanopillars revealed a strongly linear polarization of the luminescence from the emitters. In Fig. 3b two exemplary 90 nm × 30 nm pillars are shown which are aligned perpendicular to each other and covered by the monolayer. Comparing the polarization of several emitters from these two pillars shows a strong correspondence of the polarization and pillar orientation (Fig. 3c,d).

Summary
In conclusion, we demonstrated the formation of ordered arrays of quantum emitters in an atomically thin layer of WSe 2 , transferred on a metal nanopillar array. The gold nanopillars yield the formation of quantum emitters, and furthermore can act as plasmonic resonators granting active polarization control via deterministic light-matter coupling. Our work is a first step towards highly scalable cavity quantum electrodynamics with engineered quantum emitters in two dimensional materials.