Polariton physics in novel 2D materials

Lead Research Organisation: University of Sheffield
Department Name: Physics and Astronomy


In this project the student will explore the strong light-matter interactions in atomically thin two-dimensional semiconducting transition-metal dichalcogenides (TMDs). The principal objectives are (1) Fabrication of TMD-microcavity devices comprising gated heterostructures enabling electric field control of exciton complexes; (2) Observation of robust gate-controllable TMD polaritons based on trions and indirect excitons. If the two goals are reached on a reasonable time-scale, the student will then study non-linear phenomena in the polariton regime.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509735/1 01/10/2016 30/09/2021
1950950 Studentship EP/N509735/1 01/09/2017 28/02/2021 Daniel Gillard
Description At the start of this award, the observation of exciton-polaritons in two dimensional materials was only just being demonstrated and discussed. The only viable fabrication method to show such novel physics was by employing the micro-mechanical (or scotch-tape) exfoliation technique. Since then, the development of alternative fabrication methods, such as chemical vapor deposition, has advanced remarkably. I have shown, for the first time, observational evidence of polariton production (or strong light-matter coupling) in two dimensional semiconductors that are not fabricated through micro-mechanical exfoliation. The observation of strong coupling in these monolayer semiconductors is strongly linked with high optical quality with few defects, grain boundaries and inpurities within the crystaline lattices (usually only seen in samples produced by micro-mechanical exfoliation), therefore demonstrating the significant progress made by CVD fabrication methods. CVD is useful as a growth method because of the inherent scalability of growing monolayers and heterostructures across an entire substrate, instead of building a single heterostructure by hand. Only by creating a method capable of producing many devices at once will the 2D materials field be viabale as a commercial industry.

Other projects include research into two dimensional (chromium tribromide) and thin film (Europium Sulfide) ferromagnetic materials, demonstrating coupled spin valley physics and valley dependent charge transfer due to proximity effects of the ferromagnetic materials. This is significant, as valleytronics (using the valley psuedospin as an information 'unit') is one of the major potential applications of these two dimensional semiconductors. Providing a method to control the valley physics within these materials is vital for acheiveing this goal.

Finally, the last major project has seen investigations into dipolar-polaritons take place. Exciton-polaritons in two dimensional materials usually consist of an intralayer exciton (both charge carriers, electron and hole, exist in the same semiconductor layer providing a large oscillator strength to 'encourage' interactions with photon) strongly coupled to a cavity photon. The properties of the exciton are shared with the resultant polariton. One of the issues with this is that the intralayer excitons, due to being so confined, have a low dipole moment meaning that the coloumb interations with other excitons are small and novel non-linear physics is not usually seen.
To acheive the required dipole moment, an interlayer exciton, with electron and hole in seperate layers, is used and exciton-exciton interactions can be observed. However, the oscillator strength of interlayer excitons are low due to the low confinement and interactions with cavity photons are not seen. By encorperating band structure engineering, it is possible to hybridise the interlayer and intralayer excitons in MoSe2 / WS2 heterostructures producing a many body quasiparticle that can couple efficiently to light and have a coulomb exchange with other excitons. By coupling this to a cavity photon, it is possible to produce a highly interacting dipolar-polariton (or dipolariton), named as such due to the increased dipole moment. Mutual self-interactions within a dipolariton population will lead to novel non-linear physics such as bose-einstein condensation within a semiconductor monolayer. This project provides investigations into fundamental physics.
Exploitation Route Within this award, I have optically tested the progress of large scale fabrication processes, data which will be used by potential future start-up companies that want to bring optoelectronic devices with 2d materials into the consumer market. They will need to employ these large scale fabrication methods, and I have shown that CVD growth is a viable alternative to micro-mechanical exfoliation by overcoming the inherent scalability issues associated with exfoliation methods.

The incorporation of semiconductors and ferromagnetic materials have previously provided advances in both advanced spintronic research and real-world applications in information technologies such as information storage devices. By recreating the the foundation of such devices with atomically thin semiconductors and ferromagnetic materials, it is possible to shrink existing storage techonologies and also help develop new devices.
Sectors Digital/Communication/Information Technologies (including Software),Education,Electronics,Energy