PhD Graduate

PhD program:: XXXV

advisor: Rinaldo Trotta
supervisor: Antonio Polimeni

Thesis title: Engineering quantum emitters emission properties in 2D semiconductor materials

The first pivotal works reporting single photon emission in transition metal dichalcogenides (TMDs) have stimulated an explosion of research activities to investigate the possibility of using quantum emitters (QEs) in TMDs for ultra compact quantum photonics devices. Compared to other established solid state based quantum light sources, such as semiconductor quantum dots, QEs in TMDs have the clear advantage of being relatively simple and cheap to fabricate, and their spatial position across the substrate can be controlled with high precision. In addition, well-established processing techniques developed for conventional semiconductors can also be exported to TMDs to achieve, for example, coupling of QEs with nanophotonic cavities or tuning of QEs emission properties via external perturbations. However, QEs in TMDs still need to prove their real potential for quantum photonics. Even though single photon emissions in TMDs can be routinely observed, and no experiments reporting on the indistinguishability of those photons are available to date, an issue likely related to spectral diffusion. Even the generation of entangled photons, a possibility suggested by recent works but hampered by the presence of a sizeable exciton fine structure splitting, has still to be demonstrated. Therefore, it is quite clear that additional research activities aimed at understanding the origin and fundamental properties of QEs in TMDs and developing novel source-engineering methods are paramount. QEs in TMDs have been observed in WSe2, WS2, MoS2 and MoTe2 using various methods. Most of them use static strain gradients that switch on exciton funnelling towards strain-induced localized potential wells where single photon emission takes place. Whether strain alone is sufficient to create these potential wells or needs the aid of defects to enable the formation of localized intervalley bound states is still a question of theoretical debate. From the experimental side, on the other hand, strain gradients that allow for the formation of QEs are usually obtained upon transferring thin TMDs crystals (fabricated via mechanical exfoliation from bulk crystals) on textured substrates featuring nanopillars, metal nanostructures, nano-indentations and nanobubbles, to mention a few. Recent experiments using an AFM tip have also shown that it is possible to attain tight control over the strain profile, and the deterministic writing of QEs in TMDs has become a reality. However, in these schemes, the strain configuration is usually frozen. That leads to QEs whose emission properties are fixed by the local degree of bending of the monolayer, i.e., by the local strain configuration that has enabled their formation. Moreover, different QEs feature distinct emission properties (including energy, intensity, and polarization) due to slightly different local strain configurations at the QE location. That is not ideal for several quantum photonic applications requiring photonic states with the same energy. Moreover, previous attempts to attain dynamic control over the strain configuration have demonstrated the possibility of controlling exciton emission energy and polarization angle. However, considering the crucial role of strain in forming QE in TMDs, we demonstrate that strain dynamically controls the excitonic population, the brightness and the amount of light they generate.

Research products

  • 11573/1413195 - 2019 - Strain-Tunable Single Photon Sources in WSe2 Monolayers (01a Articolo in rivista)

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