In the last years, the implementation of new and efficient calculator and computer is becoming a crucial topic in different sectors (scientific or economics). The accomplishment of this task requires a lot of computational resources that consist of a massive quantity of electronic circuits like silicon transistors. 2-dimensional transition metal dichalcogenides (TMDs) are the perfect candidate to achieve this task. In particular, they are optically efficient, multitasking, electronic integrable and economically synthesizable. Thanks to their flexibility TMDs can be also implemented in another research field, like the quantum networks. TMDs implementations were focused in the electronics field, but recently a new optical propriety, single-photon emission, was discovered and TMDs join also the sector of ¿single-photon sources¿. It has been shown that the origin of this new kind of solid-state quantum emitter is linked to the presence of strain fields, that favours exciton funnelling towards low-energy bandgap regions, where single localized excitons can be populated. Consequently, TMDs can be also developed in terms of quantum information, computation, and information sciences. In this project, we present a systematic study of the role of strain. Strain engineering is crucial in TMDs because we can control different proprieties like the energy quantum emitters¿ emission [1] or solar energy funnel achievement [2] for what concern, respectively, the single-photon emission and the electronic devices. In our devices strain can be varied and controlled dynamically via nano-machined piezoelectric actuators. A single layer of WSe2 will be deposited on a piezoelectric device on which strain fields can be controlled and modified dynamically and using confocal photoluminescence mapping and atomic force microscope measurements, we can investigate directly the strain control of our system.
[1] P. Tonndorf, et al, 2, 4, 347-352 (2015)
[2] J. Feng, Nature Photonics, 6, 866 (2012).
When strained TMD monolayers are cooled down to cryogenic temperatures, localized excitons appear in the optical spectrum. Several works [1] have demonstrated that these excitons behave as single-photon sources that can be potentially used for quantum communication, and therefore as building blocks of a quantum network. Single emitters in TMDs have several advantages indeed compared to other sources of non-classical light, such as QDs. They can be created using simple and cost-effective methods, their nucleation site can be controlled with nanometric precision, and they do not suffer from hurdles related to total internal reflection. Despite the physical origin of TMD quantum emitters are still under debate, some works [2], [3] have demonstrated that their appearance is tightly connected to spatial strain gradients (usually arising from wrinkles or buckles) which (most probably) favour exciton funnelling from high-energy towards low-energy bad gap regions where single localized excitons can be efficiently populated.
In this regard, this project will study the wrinkle geometries and follow, utilizing low-temperature -PL spectroscopy and AFM measurements, their evolution as the voltages applied to the nano-machined piezoelectric six legs are varied. Indeed, in previous works, the device could vary and control the strain gradient dynamically and reversibly. Therefore, we will investigate how strain gradients can lead to the appearance of localized excitons in SL TMDs. Therefore, we will investigate how strain gradients can lead to the appearance of localized excitons in SL TMDs and can be very helpful to investigate the theoretical origin of the single-photon emission. Our project is mainly focused on the QEs emission but it can be also useful for the electronic TMD engineering, for example, a specific strain
configuration can increase power efficiency conversion in the solar cell (this effect is also related to the recombination of the excitons that are depending on the strain variations).
. Strain to engineer is the key of emission properties of single emitters in TMDs and allows also the possibility to control the energy and the polarization of the emitted photons, as already demonstrated by our group in the QD community [4]. Previous results based on monolithic actuators (i.e., not nano-machined) show that the idea is feasible, and that strain induced by a voltage difference can be successfully delivered to a WSe2 SL [5]. This will allow us to create the core for the first-time ultra-compact device where we can induce QEs in a specific position with a specific tunable energy emission. No other device can accomplish this task because in other work we study the same property nano-machined piezoelectric nanopillars on whose top has been transferred a single layer of WSe2. Here we can tune the energy emission, but the strain variation is linked to the pillar profile variation that produces an out-plane strain variation and there isn't a direct relationship between the pillar profile variation and the strain applied on the attached monolayer.
Moreover, thanks to the expertise owned in our group of advanced quantum optics techniques such as photon correlation spectroscopy, quantum state tomography, and Hong-Ou-Mandel two-photon interference, a complete characterization of the quality of the emitted photons will be performed.
. On the other hand, instead, the emission and spin properties of localized excitons in TMDs are mostly unexplored, and the research that will be carried out in the present project might open new routes for quantum technologies. Finally, a natural step beyond will be the demonstration of site-controlled single-photon emitters in TMDs integrated on state-of-the-art photonic cavities.
[1] P. Tonndorf, et al, 2, 4, 347-352 (2015).
[2] L. N. Tripathi, et al, ACS Photonics, 5, 5, 1919'1926, (2018).
[3] J. Kern, et al, Advance Material, 28, 33, (2016).
[4] Trotta R. et al., Nat. Com. 7, 10375 (2016).
[5] J. Martín-Sánchez et al., Semiconductor Sciece. and Technology, 33, 013001 (2017).