Livingston, United Kingdom
Livingston, United Kingdom

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Grant
Agency: GTR | Branch: EPSRC | Program: | Phase: Research Grant | Award Amount: 438.49K | Year: 2015

The isolation of single-atomic layer graphene has led to a surge of interest in other layered crystals with strong in-plane bonds and weak, van der Waals-like, interlayer coupling. A variety of two-dimensional (2D) crystals have been investigated, including large band gap insulators and semiconductors with smaller band gaps such as transition metal dichalcogenides. Interest in these systems is motivated partly by the need to combine them with graphene to create field effect transistors with high on-off switching ratios. More importantly, heterostructures made by stacking different 2D crystals on top of each other provide a platform for creating new artificial crystals with potential for discoveries and applications. The possibility of making van der Waals heterostructures has been demonstrated experimentally only for a few 2D crystals. However, some of the currently available 2D layers are unstable under ambient conditions, and those that are stable offer only limited functionalities, i.e. low carrier mobility, weak optical emission/absorption, band gaps that cannot be tuned, etc. In a recent series of pilot experiments, we have demonstrated that nanoflakes of the III-VI layer compound, InSe, with thickness between 5 and 20 nanometers, have a thickness-tuneable direct energy gap and a sufficiently high chemical stability to allow us to combine them with graphene and related layer compounds to make heterostructures with novel electrical and optical properties. The main goal of this project is to develop graphene-hybrid heterostructures based on this novel class of two-dimensional (2D) III-VI van der Waals crystals. This group of semiconductors will enrich the current library of 2D crystals by overcoming limitations of currently available 2D layers and by offering a versatile range of electronic and optical properties. From the growth and fabrication of new systems to the demonstration of prototype devices, including vertical tunnel transistors and optical-enhanced-microcavity LEDs, our project will provide a platform for scientific investigations and will contribute to the technology push required to create new routes to device miniaturization, fast-electronics, sensing and photonics. There is great potential for further growth of all these sectors as the fabrication of 2D systems improves and as new properties are discovered and implemented in functional devices.


Coles D.M.,University of Sheffield | Michetti P.,University of Würzburg | Clark C.,Helia Photonics | Adawi A.M.,University of Sheffield | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We explore the distribution of polaritons along the upper polariton branch of a strongly coupled organic-semiconductor microcavity as a function of temperature following nonresonant optical excitation. Measurements of polariton emission from a high-finesse cavity containing a thin film of a J-aggregated cyanine dye were performed as a function of external detection angle and temperature and compared with the results of detailed numerical simulations. We show that a full description of temperature-dependent upper-branch polariton emission can only be obtained by accounting for the interplay between two mechanisms that populate polariton states, namely, thermally assisted exciton scattering and direct radiative pumping of the photonic component of polariton states via the radiative decay of weakly coupled "reservoir" excitons. Our measurements provide a full description of the basic mechanisms at play in an organic microcavity, and may help guide the development of organic polariton-based devices. © 2011 American Physical Society.


Bauer R.,University of Strathclyde | Lubeigt W.,University of Strathclyde | Clark C.,Helia Photonics | McBrearty E.,Helia Photonics | Uttamchandani D.,University of Strathclyde
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

Multiple individually-controllable Q-switched laser outputs from a single diode-pumped Nd:YAG module are presented by using an electrostatic MEMS scanning micromirror array as cavity end-mirror. The gold coated, 700 μm diameter and 25 μm thick, single-crystal silicon micromirrors possess resonant tilt frequencies of ∼8 kHz with optical scan angles of up to 78°. Dual laser output resulting from the actuation of two neighboring mirrors was observed resulting in a combined average output power of 125 mW and pulse durations of 30 ns with resulting pulse energies of 7.9 μJ and 7.1 μJ. The output power was limited by thermal effects on the gold-coated mirror surface. Dielectric coatings with increased reflectivity and therefore lower thermal stresses are required to power-scale this technique. An initial SiO2/Nb 2O5 test coating was applied to a multi-mirror array with individual optical scan angles of 14° at a resonant tilt frequency of 10.4 kHz. The use of this dielectric coated array inside a 3-mirror Nd:YAG laser cavity led to a single mirror output with average Q-switched output power of 750 mW and pulse durations of 295 ns resulting in pulse energies of 36 μJ. © 2013 Copyright SPIE.


Coles D.M.,University of Sheffield | Coles D.M.,University of Oxford | Somaschi N.,University of Southampton | Michetti P.,TU Dresden | And 5 more authors.
Nature Materials | Year: 2014

Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light-matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes. © 2014 Macmillan Publishers Limited. All rights reserved.


Coles D.M.,University of Sheffield | Grant R.T.,University of Sheffield | Lidzey D.G.,University of Sheffield | Clark C.,Helia Photonics | Lagoudakis P.G.,University of Southampton
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We use angular-resolved photoluminescence excitation spectroscopy to probe energy relaxation towards the lowest lying states (k â̂¥=0) on the lower polariton branch (LPB) of two different strong-coupled J-aggregate organic semiconductor microcavities. We find that states along the upper polariton branch (UPB) are able to undergo efficient relaxation to populate states around kâ̂¥=0 on the LPB; a process that occurs through the rapid relaxation of UPB polaritons to the exciton reservoir. In contrast, LPB states having a photon fraction >0.5 are not apparently able to undergo further relaxation towards k â̂¥=0, constituting an effective relaxation bottleneck. © 2013 American Physical Society.


Dufferwiel S.,University of Sheffield | Schwarz S.,University of Sheffield | Withers F.,University of Manchester | Trichet A.A.P.,University of Oxford | And 13 more authors.
Nature Communications | Year: 2015

Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe 2 /hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe 2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe 2 monolayer, enhanced to 29 meV in MoSe 2 /hBN/MoSe 2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized. © 2015 Macmillan Publishers Limited. All rights reserved.


Coles D.M.,University of Sheffield | Michetti P.,University of Würzburg | Clark C.,Helia Photonics | Tsoi W.C.,Imperial College London | And 3 more authors.
Advanced Functional Materials | Year: 2011

If a semiconductor with an electronic transition that approximates a two-level system is placed within an optical cavity, strong coupling can occur between the confined photons and the semiconductor excitons. This coupling can result in the formation of cavity polariton states that are a coherent superposition of light and matter. If the material in the cavity is an organic semiconductor, it has been predicted that interactions between Frenkel excitons, polaritons, and molecular vibrational modes will have a profound role in defining the overall relaxation dynamics of the system. Here, using temperature-dependent spectroscopy on a microcavity containing a J- aggregated cyanine dye, it is shown that a spectrum of localized vibrational modes (identified by Raman scattering) enhances the population of certain polaritonic modes by acting as an energy-loss channel to the excitons as they undergo scattering. Our work demonstrates that simultaneous control of the optical properties of a cavity and the vibrational structure of a molecular dye could promote the efficient population of k = 0 polariton states, from which lasing and other cooperative phenomena may occur. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Paterson A.,University of Strathclyde | Bauer R.,University of Strathclyde | Li R.,University of Strathclyde | Clark C.,Helia Photonics | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2016

Active control of the spectral and temporal output characteristics of solid-state lasers through use of MEMS scanning micromirrors is presented. A side-pumped Nd:YAG laser with two intracavity scanning micromirrors, enabling Q-switching operation with controllable pulse duration and pulse-on-demand capabilities, is investigated. Changing the actuation signal of one micromirror allows a variation of the pulse duration between 370 ns and 1.06 μs at a pulse repetition frequency of 21.37 kHz and average output power of 50 mW. Pulse-on-demand lasing is enabled through actuation of the second micromirror. To our knowledge this is the first demonstration of the use of multiple intracavity MEMS devices as active tuning elements in a single solid-state laser cavity. Furthermore, we present the first demonstration of control over the output wavelength of a solid-state laser using a micromirror and a prism in an intracavity Littman configuration. A static tilt actuation of the micromirror resulted in tuning the output wavelength of an Yb:KGW laser from 1024 nm to 1031.5 nm, with FWHM bandwidths between 0.2 nm and 0.4 nm. These proof-of-principle demonstrations provide the first steps towards a miniaturized, flexible solid-state laser system with potential defense and industrial applications. © 2016 SPIE.


Grant
Agency: GTR | Branch: STFC | Program: | Phase: Research Grant | Award Amount: 4.73K | Year: 2016

Project FABs main aim to achieve an increase in the enjoyment of science lessons for primary school students. By doing this, we aim to increase the number of children wanting to pursue a scientific career. We also aim to increase the confidence of senior high school students in communicating scientific ideas, and teaching them to younger pupils. We hope that this will not only boost their chances of employment or acceptance to higher/further education, but will fuel their individual passion for scientific subjects. Another of our aims is to increase the confidence of primary school teachers in teaching science in a fun and engaging manner. One of the main aims of our project is to inform young people about science. We plan to teach senior high school students the intricacies of the project and recruit them to help teach primary school children. This will give pupils a basic understanding of atomic structure, gravity, the relationship between temperature and pressure, greenhouse gases, and particle physics. Furthermore, we will help older students to expand this understanding by learning about cosmic rays, gravitational waves, and climate change. The high school students will help to lead lessons about high altitude ballooning, which will provide them with the opportunity to become more confident in their scientific knowledge and get first-hand experience teaching. During these lessons, we will carry out exciting experiments which will engage the children and make some of the more difficult aspects of our project easier for them to understand. We also aim to run an after school club for the high school students which will give them the opportunity to learn about microcontroller programming. We hope that giving pupils this basic knowledge of physics will inspire them to become more involved in science and pursue a career in science. We aim to carry out four major experiments, consisting of three high altitude balloons, each measuring a different property of the atmosphere with the equipment attached. With one balloon we aim to measure the change in magnitude of radiation, caused by high energy cosmic ray collisions in the upper atmosphere, as a function of altitude using a scintillating plastic. With a second balloon we aim to measure the change of gravitational force as a function of altitude, and demonstrate that objects do in fact fall at the same rate. We also hoe measure atmospheric temperature and pressure as a function of altitude. Our aim for the third and final balloon is to measure the concentration of Carbon Dioxide, Ozone, and Methane. Using the concentrations of Ozone, we aim to show that the absorption of UV radiation raises atmospheric temperature. Also, by comparing levels of Methane and Carbon Dioxide to pre-industrial revolution levels we aim to show that levels of these gases do contribute to global climate change. Our hope is that we can use these experiments to illustrate the things the pupils have learned in the classroom, as well as using images captured on-board to really spark their imaginations and love of science.


PubMed | University of Sheffield, Helia Photonics, University of Manchester, CNRS Pascal Institute and University of Oxford
Type: | Journal: Nature communications | Year: 2015

Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20meV for a single MoSe2 monolayer, enhanced to 29meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.

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