Microcavity polaritons in atomically thin semiconductors and heterostructures: many-body and nonlinear phenomena

Atomically thin materials offer a new paradigm for control of electronic excitations in the extreme two-dimensional (2D) limit in condensed matter. Recently this concept has been developed further when artificial potentials for electrons were created in heterostructures consisting of stacked 2D layers held together by van der Waals forces, and light was used to access and manipulate electronic spin and valley degrees of freedom in atomically-thin semiconducting transition metal dichalcogenides (TMDCs). A significant world-wide effort in the last 5 years has resulted in intense studies of optical properties of TMDC atomic layers in the linear regime. Here, we propose to use this new class of (2D) semiconducting crystals to demonstrate unexplored approaches to exploiting nonlinear optical phenomena on the nano-scale in regimes unattainable by other ultra-fast photonic materials. To achieve this, we will exploit robust excitonic complexes observable up to room T, which will be generated and controlled in artificially created vertical stacks of 2D atomic layers. Giant nonlinearities enabling ultra-fast control of light with light of low intensity will be realised and explored in such van der Waals heterostructures placed in optical microcavities, operating in the strong light-matter coupling regime that we demonstrated recently. In this regime part-light-part-matter polaritons are formed, with the exciton part responsible for the strong nonlinearity and the photon part providing efficient coupling to light. This work will open a new route to development of highly nonlinear nano-photonic devices such as miniature ultra-fast modulators and switches, with high potential to impact on a new generation of signal processing and quantum technology hardware.

Our Impact strategy will have four main components:

1. Knowledge dissemination within the scientific community outside the consortium will be achieved via publications in leading journals including Nature family and Science; talks at international conferences and workshops; participation in large-scale initiatives including (i) the European Graphene Flagship, (ii) Sir Henry Royce Institute for Advanced Materials (SHRI), (iii) National Graphene Institute (NGI), (iv) Horizon-2020 Spin-NANO Marie Sklodowska-Curie ITN, (v) ERC Synergy grant Hetero2D, (vi) EPSRC Engineering Grand Challenges grant 'Engineering van der Waals heterostructures: from atomic level layer-by-layer assembly to printable innovative devices' (EP/N010345/1).

2. Knowledge and technology transfer and direct engagement with industry. Our project concerns fundamental research into 2D materials with a strong potential to impact development of nonlinear optical devices for novel photonic integrated circuits, miniature and energy efficient active photonic elements, which are conceptually essential for future development in telecommunications and may also be used in a long-run to boost speed in supercomputers. At this early stage in the exploration of fundamental properties of 2D materials, our industrial engagement strategy will consist of establishing and maintaining long-lasting contacts with relevant industry. This will be achieved by providing the flow of information between our project and industry, as well as directly engaging with several industrial partners: Helia Photonics, Attocube Systems and HQGraphene. It is important to emphasize that this consortium has already established extremely reliable routes for engagement with industry: we will connect with industry via the national and international hubs and large-scale projects (i)-(vi) above, all addressing 2D materials. These multi-lateral initiatives and projects are intrinsically designed to encourage interaction between academics and industry.

3. Training of highly qualified personnel. We aim to train 4 postdoctoral researchers and 3 PhD students to high levels in physics, optics, theoretical analysis and device fabrication during the course of the grant. All these topical areas are highly relevant industrially and academically. The training in professional skills both in Sheffield and Manchester will be supplemented by attendance at university-run training in transferable skills including written and presentational skills, CV preparation etc. University-wide career coaching will be available both in Sheffield (the Think Ahead programme) and Manchester (the Researcher Development programme).
The combination of participation in research at a high level, together with the oral, written and organisational skills, as well as direct involvement with the industrial partners, will prepare outgoing members very well for future careers in industry and academia, which is of considerable benefit to the UK economy. Researchers supervised by the investigators have taken up positions in industry and academia (~50/50 split), as well as started several spin-off companies.

4. Public engagement. We will participate in various outreach activities targeting the general public, the school pupils and undergraduates in universities. Recently, we have produced a series of animations on polariton physics and quantum optics, and more recently on 2D materials, which have obtained more than 55,000 online hits. These videos were used at the Royal Society Summer Science Exhibition and the Festival of The Mind (Sheffield, 2016). Within this proposal we will create an animation describing the importance of nonlinear optics in modern technology and potential of 2D materials for such photonic applications. At Manchester, we collaborate closely with Museum of Science and Industry, where suitable exposits will be placed related to the outcomes of the proposed research.