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.