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The demonstration of stable quantum emitters in semiconductor transition metal dichalcogenides (TMDs) [1] and insulating hexagonal boron nitride (hBN) [2] has begun to impact the field of integrated quantum optics due to the promising properties of two-dimensional materials, such room-temperature operation, stretchability, heterogeneous device assembly and straightforward integration with photonic circuits. Nevertheless, major questions remain. Single photon emitters (SPEs) in hBN are associated with atom-like defects that confine electronic levels deep within the wide band gap. As recently reported, hBN can host several different families of emitters with emission energy that spans over a large spectral band [3]. On the other hand, quantum emission in TMDs is attributed to individual excitons bound to defects or impurities of the crystal structure. Recently, it has been shown that strain pockets can create potential traps able to confine single excitons producing SPEs with high spatial control [4,5]. Nevertheless, the relation between strain and emission energy is still not fully clear. The lack of a clear understanding of the nature of these defects [6] brings along the challenge of controlling the emission energy and of the deterministic creation of such emitters. These crucial aspects present central problems for developing identical single photon sources and for integration with photonic platforms.
We developed a material processing based on ion irradiation and high temperature annealing of exfoliated hBN that sharply improves the single-photon purity with g(2)(0) = 0.08, and brightness with emission rate exceeding 10^7 counts/sec at room temperature. We also showed that the emitters persist material transfer process allowing to integrate them onto different platforms. To investigate the wide span of the emission energy we applied external compressive and tensile strain. Emitters in hBN show different tuning coefficients up to 6 meV per strain unit [7]. Furthermore, we performed photoluminescence excitation experiments at cryogenic temperature on different families of emitters. Our experimental results allow to identify the excited states of these atom-like systems and shine light on the characteristic level structures of the different families of emitters in hBN and can potentially help to reduce the spectral wandering with more efficient resonant excitation.
Recently, new methods to develop quantum emitters in monolayer TMDs have been reported [4,5]. They rely on the strain induced by nanostructures in the substrate. This technique allows to create emitters with high spatial resolution. Unfortunately, it is still not possible to engineer the single photon energy with strain since the relation between strain and the emission energy of the localized excitons in TMDs is still not fully understood. To answer this question, we transferred CVD grown WSe2 on a pre-patterned Si3N4 substrate with pillars of 200 nm in diameter. We combined hyperspectral measurements, which allow to locate a single energy emission peak with a spatial resolution of around 10 nm, with strain measurements. Strain is measured optically by resolving in polarization the intensity of the second harmonic signal. This method allows to measure the photoelastic tensor of TMD monolayers and to retrieve the magnitude of biaxial strain with a spatial resolution of ~ 200 nm [8]. The combination of these techniques opens a new way to investigate the correspondence between the strain magnitude created by a nanopatterned substrate and the emission energy of the single photon emitted due to exciton localization in the nanostructures.
[1] Perebeinos, V. Nat. Nano. 10, 485 (2015)
[2] Tran, T. T., et al. Nat. Nano. 11, 37 (2016)
[3] Tran, T. T. et al. ACS Nano 10, 7331 (2016)
[4] Branny, A. et al. Nature Comm. 8, 15053 (2017)
[5] Palacios-Berraquero, C. et al. Nature Comm. 8, 15093 (2017)
[6] Tawfik, S.A. et al. Nanoscale 9, 13575 (2017)
[7] Grosso, G., Moon, H, et al., Nature Comm. 8, 705 (2017)
[8] L. Mennel, et al. Nature Comm. 9, 516 (2018)
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