Spectroscopy and accurate spatial positioning of quantum emitters hosted by two-dimensional semiconductors

Atomically-thin semiconductors offer intriguing technological advantages for quantum photonic applications. Advantages include a lack of dangling bonds, atomically-precise interfaces, the potential to design novel heterostructures with an absence of nuclear spins, and the ease of integration with photonic integrated chip platforms. These benefits offer a new opportunity to construct a scalable quantum architecture with a coherent lightmatter interface, an exciting prospect for future quantum technologies. This thesis takes the first steps in this direction. Atomically-thin flakes of transition metal dichalcogenides (WSe2 or MoSe2) are transferred to substrates with smooth and nanopatterned regions. Using cryogenic microphotoluminesce spectroscopy, a correlation between isolated quantum emitters and localised strain ‘pockets’ is observed. This observation is exploited to fabricate WSe2 arrays of highly pure single photon (g(2)(0) <0.5%) emitters at deterministic spatial positions (120±30 nm accuracy) with nearly 100% efficiency. The quantum emitters intrinsic optical properties are characterised via magnetic field and temperature dependent spectroscopy. The nanoscale strain engineering approach provides a universal scheme to create spatially and spectrally isolated quantum emitters in other two-dimensional materials. The thesis concludes with a discussion on the origin and dynamics of strain-tuned localized excitons in 2D semiconductors, presenting local disorder and exciton funnelling as important ingredients

Discrete quantum dot like emitters in monolayer MoSe2: Spatial mapping, Magneto-optics and Charge tuning

Transition metal dichalcogenide monolayers such as MoSe2,MoS2 and WSe2 are direct bandgap semiconductors with original optoelectronic and spin-valley properties. Here we report spectrally sharp, spatially localized emission in monolayer MoSe2. We find this quantum dot like emission in samples exfoliated onto gold substrates and also suspended flakes. Spatial mapping shows a correlation between the location of emitters and the existence of wrinkles (strained regions) in the flake. We tune the emission properties in magnetic and electric fields applied perpendicular to the monolayer plane. We extract an exciton g-factor of the discrete emitters close to -4, as for 2D excitons in this material. In a charge tunable sample we record discrete jumps on the meV scale as charges are added to the emitter when changing the applied voltage. The control of the emission properties of these quantum dot like emitters paves the way for further engineering of the light matter interaction in these atomically thin materials.Comment: 5 pages, 2 figure

Atomically-thin quantum dots integrated with lithium niobate photonic chips

The electro-optic, acousto-optic and nonlinear properties of lithium niobate make it a highly versatile material platform for integrated quantum photonic circuits. A prerequisite for quantum technology applications is the ability to efficiently integrate single photon sources, and to guide the generated photons through ad-hoc circuits. Here we report the integration of quantum dots in monolayer WSe2 into a Ti in-diffused lithium niobate directional coupler. We investigate the coupling of individual quantum dots to the waveguide mode, their spatial overlap, and the overall efficiency of the hybrid-integrated photonic circuit

Coulomb blockade in an atomically thin quantum dot coupled to a tunable Fermi reservoir

Gate-tunable quantum-mechanical tunnelling of particles between a quantum confined state and a nearby Fermi reservoir of delocalized states has underpinned many advances in spintronics and solid-state quantum optics. The prototypical example is a semiconductor quantum dot separated from a gated contact by a tunnel barrier. This enables Coulomb blockade, the phenomenon whereby electrons or holes can be loaded one-by-one into a quantum dot. Depending on the tunnel-coupling strength, this capability facilitates single spin quantum bits or coherent many-body interactions between the confined spin and the Fermi reservoir. Van der Waals (vdW) heterostructures, in which a wide range of unique atomic layers can easily be combined, offer novel prospects to engineer coherent quantum confined spins, tunnel barriers down to the atomic limit or a Fermi reservoir beyond the conventional flat density of states. However, gate-control of vdW nanostructures at the single particle level is needed to unlock their potential. Here we report Coulomb blockade in a vdW heterostructure consisting of a transition metal dichalcogenide quantum dot coupled to a graphene contact through an atomically thin hexagonal boron nitride (hBN) tunnel barrier. Thanks to a tunable Fermi reservoir, we can deterministically load either a single electron or a single hole into the quantum dot. We observe hybrid excitons, composed of localized quantum dot states and delocalized continuum states, arising from ultra-strong spin-conserving tunnel coupling through the atomically thin tunnel barrier. Probing the charged excitons in applied magnetic fields, we observe large gyromagnetic ratios (~8). Our results establish a foundation for engineering next-generation devices to investigate either novel regimes of Kondo physics or isolated quantum bits in a vdW heterostructure platform.Comment: Published in Nature Nanotechnology. 7 pages + 14 supplementary information pages. 14 figure

Smarter greener cities through a social-ecological-technological systems approach

Publisher Copyright: © 2022Smart city development is expanding rapidly globally and is often argued to improve urban sustainability. However, these smart developments are often technology-centred approaches that can miss critical interactions between social and ecological components of urban systems, limiting their real impact. We draw on the social-ecological-technological systems (SETS) literature and framing to expand and improve the impact of smart city agendas. A more holistic systems framing can ensure that ‘smart’ solutions better address sustainability broadly and extend to issues of equity, power, agency, nature-based solutions and ecological resilience. In this context, smart city infrastructure plays an important role in enabling new ways of measuring, experiencing and engaging with local and temporal dynamics of urban systems. We provide a series of examples of subsystems interactions, or ‘couplings’, to illustrate how a SETS approach can expand and enhance smart city infrastructure and development to meet normative societal goals.Peer reviewe

Out-of-plane orientation of luminescent excitons in atomically thin indium selenide flakes

Van der Waals materials offer a wide range of atomic layers with unique properties that can be easily combined to engineer novel electronic and photonic devices. A missing ingredient of the van der Waals platform is a two-dimensional crystal with naturally occurring out-of-plane luminescent dipole orientation. Here we measure the far-field photoluminescence intensity distribution of bulk InSe and two-dimensional InSe, WSe$_2$ and MoSe$_2$. We demonstrate, with the support of ab-initio calculations, that layered InSe flakes sustain luminescent excitons with an intrinsic out-of-plane orientation, in contrast with the in-plane orientation of dipoles we find in two-dimensional WSe$_2$ and MoSe$_2$ at room-temperature. These results, combined with the high tunability of the optical response and outstanding transport properties, position layered InSe as a promising semiconductor for novel optoelectronic devices, in particular for hybrid integrated photonic chips which exploit the out-of-plane dipole orientation.Comment: Published in Nature Communications. 12 pages, 5 figures. Link: https://www.nature.com/articles/s41467-019-11920-

Author Correction : Towards spontaneous parametric down conversion from monolayer MoS 2 (Scientific Reports, (2018), 8, 1, (3862), 10.1038/s41598-018-22270-4)

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper

The Doughnut for Urban Development:Manual, Appendix and Database

With the Doughnut for Urban Development we are using doughnut economics as a model for urban development and construction for the first time. Doughnut Economics has previously been used with great success globally and for urban strategies ranging from Amsterdam to Copenhagen.We have developed the Manual to provide the entire industry with a practical tool to evaluate the sustainability of their projects and what they can do to make them even more sustainable. The manual embraces both social and planetary sustainability and incorporates both local and global dimensions.The Doughnut for Urban Development is an open-source project and all the following resources can be downloaded for free:- The Manual- A scientific Appendix providing background for the Manual- A Database of impact areas used in the manual- A tool to assess a project's biodiversity impacts throughout its life cycl

Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor

An outstanding challenge in quantum photonics is scalability, which requires positioning of single quantum emitters in a deterministic fashion. Site positioning progress has been made in established platforms including defects in diamond and self-assembled quantum dots, albeit often with compromised coherence and optical quality. The emergence of single quantum emitters in layered transition metal dichalcogenide semiconductors offers new opportunities to construct a scalable quantum architecture. Here, using nanoscale strain engineering, we deterministically achieve a two-dimensional lattice of quantum emitters in an atomically thin semiconductor. We create point-like strain perturbations in mono- and bi-layer WSe2 which locally modify the band-gap, leading to efficient funneling of excitons towards isolated strain-tuned quantum emitters that exhibit high-purity single photon emission. These arrays of non-classical light emitters open new vistas for two-dimensional semiconductors in cavity quantum electrodynamics and integrated on-chip quantum photonics.Comment: 10 pages including supplemental information, 9 figure