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EPSRC Centre for Doctoral Training in Metamaterials

ANNUAL REPORT 2015

Metamaterials, fabricated microstructures having properties beyond those found in nature, are emerging as an important new class of materials with applications in many technology areas, from energy harvesting, through perfect imaging, to communication, and the much-hyped ‘cloaking’.

The EPSRC Centre for Doctoral Training in Metamaterials (XM2) is based on the Streatham Campus at the University of Exeter in the Departments of Physics and Engineering. We have a well-established and strong track record of relevant research, spanning a unique mix of interests, from microwave metasurfaces to carbon nanotubes, from the fundamental theory of electromagnetism, to new understanding in acoustics, from graphene plasmonics to spintronics, magnonics and magnetic composites, from terahertz photonics to biomimetics.

A list of projects to which we are currently recruiting is available at www.exeter.ac.uk/metamaterials Initial applications should be made by email to metamaterials@exeter.ac.uk, and should include:

• an outline of your research interests.

• a suggestion of your preferred areas of study, and/or your interest in a particular project.

• an indication of whether you have already been in contact with a potential supervisor.

• a discussion of why you would like to study for a PhD in Physics or Engineering, and why you would like to join a cohort-based doctoral training centre.

• an academic CV.

• relevant degree transcripts including a breakdown of module marks.

• the names and contact details of two academic referees.

Candidates will be short-listed by the Admissions Tutor against a set of agreed criteria detailed on our website. Short-listed candidates will be interviewed by a panel of two members of the management board. A second interview will be undertaken by the potential academic supervisors.

Note that applications will be processed as soon as they are received. Interviews will be held from November and offers will be made from December. You are therefore advised to apply as soon as possible.

APPLICATION DEADLINE:UK and EU: 1 JuneNon EU: 1 May (see note above)

ELIGIBILITY: Upper second or first class degree, or equivalent, in a relevant discipline.

VALUE:UK and EU: Tuition fee, and stipend (£14,057 in 2015/16)Non-EU: Tuition fee only, although a small number of international scholarships may be available.

DURATION OF AWARD: 4 years from September

HOW TO APPLY: www.exeter.ac.uk/metamaterials/apply

ADMISSIONS TUTOR:Professor Alastair Hibbins email: metamaterials@exeter.ac.uk phone: +44(0) 1392 726568

Key Information and Application Details

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Establishing, from scratch, one of the largest single-institution Centres for Doctoral Training in the country has been an interesting challenge! Our ambitious aims were to establish a new centre-of-excellence in the research of metamaterials, and to train doctoral scientists and engineers with relevant research skills, knowledge,

and professional attributes. We have built on our existing practices to develop a vibrant, supportive yet challenging, PhD training environment, and we look forward, in a few years’ time, to watching our graduates moving on to successful careers in industry and academia. They will leave Exeter with the ability to design and harness the functionality of metamaterials, maintaining and furthering the UK’s position in this rapidly developing field. They will have the additional benefit of deeper skills training and professionalism than students pursuing PhDs by the traditional route.

The process began in earnest in November 2013, 6 months before the funding officially started, with the urgent need to solicit, evaluate, prioritise and advertise PhD projects. We had a website to build, handbook to write, staff to recruit, and we had to ensure we sought input from our partners in industry. Worryingly, we had no desk space for our new students, so we worked with the University to develop a large, dedicated working space for 40 metamaterial researchers. The effort was worth it – the room was finished on time (albeit with the paint still wet during induction week), we had an administrator and technical director on board and, most importantly, 18 excellent new PhD researchers.

Since then, we have been working hard to deliver on our ambition. It has been particularly pleasing to see how our first intake of students have gelled into a ‘cohort’. Our first Induction Week was packed full of events, workshops and talks ranging from short presentations about metamaterial themes, to introductions about Creativity and Cognitive Behavioural Coaching. With our backing, the students have formed their own Student Advisory Group, and it’s been very encouraging to see them set up their own informal research meetings, organise social events, and even run the outward-facing Facebook page and Twitter account for the Centre.

Another encouraging aspect has been the way the students have taken on board the challenge of undertaking a substantive piece of research in their first six months, with the very significant aim of producing a research paper, despite having to balance this with attendance at formal training sessions. The first research papers from these projects are just now being prepared for publication, and have already been presented at conferences – a very impressive accomplishment, and one that the students should be extremely proud of.

Overall a very busy, challenging and exciting first year, no doubt next year will be even more exciting.

ROY SAMBLESDirector

It’s been a pleasure to watch the initial evolution of the EPSRC Centre for Doctoral Training in Metamaterials (XM2). The experience and innovative energy of the management/admin team, coupled with the outstanding cohort of students who comprise the first intake have resulted in the Centre becoming rapidly up to speed,

and the atmosphere amongst the team bodes well for the future. Through their informal and personal interactions with members of the Oversight Board, students on the programme have made

it clear how they appreciate the added-value components such as Cognitive Behaviour Coaching and the Creativity events, as well as the atmosphere engendered by the academics and the structures which have been put in place to enable their voices to be heard. The message that researchers should do things which matter, and do them to the highest standards, is very clear to hear.

JONATHAN KNIGHTChair of the Oversight Board

Foreword – the first 12 months

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PRINCIPLE CONTACTS

PROFESSOR ROY SAMBLES

(Director and External Relations)j.r.sambles@exeter.ac.ukex.ac.uk/jrs

PROFESSOR ALASTAIR HIBBINS (Admissions)a.p.hibbins@exeter.ac.ukex.ac.uk/aph

DR IAN HOOPER (Technical Director)i.r.hooper@exeter.ac.uk ex.ac.uk/irh

PROFESSOR DAVID WRIGHT (Progression and Monitoring)david.wright@exeter.ac.ukex.ac.uk/cdw

PROFESSOR BILL BARNES (Training)w.l.barnes@exeter.ac.ukex.ac.uk/wlb

ROSIE DIXON (Administrative Officer)r.dixon@exeter.ac.ukex.ac.uk/rdix

The Centre’s 1st cohort and Director during Induction Week

For general enquiries please contact +44 (0)1392 726568 or visit: www.exeter.ac.uk/metamaterials

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As the pace of technological development accelerates so does the need for novel materials functionality. XM2 is dedicated to ensuring that, when our graduates leave Exeter, they will move into industry and academia as highly trained professionals with an extensive breadth of knowledge in the field of metamaterials, helping to maintain and further develop the UK’s position in the field of metamaterial science.

The EPSRC Centre for Doctoral Training in Metamaterials (XM2) opened its doors to new PhD researchers is September 2014, and since then has been providing training to them in a challenging yet supportive cohort-based environment. Unlike traditional “lone scholar” PhDs, our researchers are working and learning together, while allowing individuals to flourish and discover their own potential (and limitations). The breadth of training has been chosen such that they will graduate with a wide knowledge of metamaterial physics, materials engineering, device production and characterisation. At the same time they are being formally trained in wider professional and personal skills such as innovation, engagement, industrial awareness, and time and programme management, enabling them to become highly skilled and talented researchers in their own right, as well as potential future leaders in industry and academia.

Each cohort of students will normally comprise around 15 students, approximately one-third of whom are from Europe and beyond. The academic team supervising them comprises over 30 academics spread across Physics and Engineering, ranging from world-level researchers through to early career academics, and whose focus ranges from fundamental theory to end-user applications. In order to maintain a cohort ethos and to encourage academics to be involved in multiple, interdisciplinary projects, our PhD researchers have two joint academic supervisors as well as a year coordinator and an independent mentor to provide advice about training, guidance concerning PhD progress, and importantly to give pastoral support.

One of our unique aspects is that our students undertake research from day one, whilst in parallel receiving the training that they need. The initial 6-month research projects are designed to enable the students to produce publishable results, a tough challenge but one that stimulates the students and drives them to be the best they can.

The XM2 Approach

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6

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6 MONTH RESEARCH PROJECT

COGNITIVE BEHAVIOURAL

COACHING

COMPLETE PhD

TECHNICAL TRAINING COURSES

(PROGRAMMING, COMPUTATIONAL

MODELLING ETC.)

PLASMONICS

SENSING AND SECURITY

FUNDAMENTALS OF ACOUSTICS

MAGNETIC METAMATERIALS AND DEVICES

ADVANCED MATERIAL CHARACTERISATION

TECHNIQUES

1ST CREATIVITY EVENT

LEARNING AND TEACHING IN HIGHER EDUCATION

STATISTICS

MASTERS LEVEL LECTURE COURSE (OF STUDENTS’ CHOICE)

LITERATURE REVIEW

PRESENTATION AND COMMUNICATION SKILLS

2ND CREATIVITY EVENT

LEADERSHIP COURSE

OUTREACH

PROJECT MANAGEMENT

COURSE

VITAE GRADSCHOOL

FULL PhD RESEARCH PROJECT

THESIS PREPARATION

Research

“Soft” skills

Scientific training

Metamaterials specific/Techincal training

THE PROGRAMME AT A GLANCE

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The four management board members have, between them, recruited and seen through to successful completion a total of over 110 PhD students, many of whom have subsequently attained high-level positions in industry or academia. Our previous success has been achieved by ensuring a high-quality, committed student intake coupled with high-quality supervision. However recruitment to a cohort-based doctoral training programme requires a somewhat different approach to that of a conventional “lone-scholar” studentship. Besides excellence in academic ability, candidates for our studentships will need to demonstrate commitment to working as part of a research team, alignment with the ethos of the programme, and demonstrate that they have thought about the direction of their career post-PhD. Applicants are also expected to have considered the available projects, and be able to describe metamaterials in simple terms. The eligibility requirements, application process and shortlisting criteria are detailed on the inside cover of this brochure.

Proposals for PhD projects are invited from academics across the Centre during September of the year prior to entry, and are then reviewed and prioritised (according to scientific

excellence and strategic relevance) by the members of the Management Board. Abstracts for all projects are detailed on our website www.exeter.ac.uk/metamaterials, and advertised on PhD recruitment websites and magazines. Through Google AdWords, online searches for metamaterials are directed to our website, and we have embraced social media through our student-run Twitter account (@XM2_CDT) and Facebook page (www.facebook.com/CDTMetamaterials).

During the 2013/4 recruitment round, 130 applications were received, from which we recruited 18 PhD students, 11 of which are from the UK, three are EU, and 4 international. Three of these 18 are funded from schemes outside of the Centre’s funding model (e.g. ICASE, research grant), but follow the same programme of study: they are termed “aligned” students. The second recruitment round has been even more successful, despite a more competitive market: We received 150 applications, and by June 2015, 17 offers had been accepted for the programme starting in September including one aligned-ICASE studentship, four EU and three international applicants.

Recruitment

It’s great to work alongside people from all over the world – I have learnt a lot more than just physics.

The double supervision allows for a greater flow of ideas and stimulating conversation.

LAUREN BARR, XM2 STUDENT

CHRIS KING, XM2 STUDENT

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THE UNIVERSITY OF EXETER

The University of Exeter is a Russell Group University, ranked in the top 10 of all the main UK University league tables – as well as being ranked in the top one per cent of institutions globally. The CWTS Leiden Ranking 2015 also places us 34th globally, a position based on the scientific impact of our research and on our involvement in scientific collaboration. The University is committed to a substantial expansion of its science base, with new buildings erected and under construction and extensive refurbishment of existing facilities. Within Physics and Engineering more than £15m has been spent on infrastructure since 2008. One of Exeter’s key Science Strategy themes across Physics and Engineering is Functional Materials, and the fields of electromagnetic materials, nanomaterials and graphene have seen extensive investment over the past five years with 20 new academic staff appointed during that period. The University is committed to continued investment in this important area for the next 5 years and beyond.

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TrainingWe have designed our programme so that our students start undertaking research from day one, yet we have also front-loaded much of the additional training programme to ensure that they can develop the skills they need at the earliest opportunity. One of the great strengths of doctoral cohort training is the additional benefits that we can provide our students over and above those available in a traditional PhD programme. In the first year our students undertake training in Creativity (as part of the EPSRC’s Creativity@Home programme), Cognitive Behavioural Coaching (CBC), and teaching, as well as taking courses targeted at the skills they will need to undertake their day-to-day research such as presentation skills, computational modelling and programming etc. As students progress into subsequent years their training will focus on Leadership and Project Management, and the opportunity to undertake scientific outreach. They will also attend the Vitae GRADschool where they will have the opportunity to broaden many of these non-technical skills further.

INITIAL 6-MONTH RESEARCH PROJECT

A key aspect of our programme is to ensure that researchers, many of whom will have Masters level degrees or have engaged previously with research, are not engulfed with formal lectures, but start their research work early. This means that the first year is exceptionally challenging as they balance the production of high quality research while also attending the additional training courses. Students are asked to produce research work of a quality that can be written up as a report in the form of a potentially publishable paper by the end of March. This is a major challenge, but our researchers have welcomed it and have produced some very impressive pieces of work. In the first cohort, two papers have been submitted directly as a result of these projects with more being finalised during 2015.

CREATIVITY

We want to make an early start on many of the elements of training that our students are likely to be unfamiliar with, in order to help build new habits and better working practices. We begin the Creativity training with an afternoon during the students’ induction week. This is run by Dennis Sherwood of Silver Bullet Machine, and the center-piece of the discussion is Arthur Koestler’s insight that creativity is not a ‘bolt from the blue’; rather, creativity is a process of forming different, and hopefully new, patterns from pre-existing elements. To those who have not come across this before, this is a surprise. Not only does it de-mystify creativity, but it further implies that, in a deep sense, nothing is new: all apparently new things are formed by bringing together pre-existing things. This is immediately evident in music – Beethoven didn’t invent any of the notes, but he did form some wonderful new patterns. Perhaps less obviously, it’s true in

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science too: so, for example, the X-ray diffraction pattern of a helix was known to be the characteristic X-shape (the “CCV” paper of 1952); Chargaff’s Laws had established that DNA is formed not of A-T-C-G tetramers but of A-T and G-C dimers (work published from 1949); and Rosalind Franklin’s PhD student, Raymond Gosling, took the critical X-ray picture in 1952, which was seen by Watson (1953). Watson, with Crick, put all these pieces together to describe DNA for the first time, but all of the pieces were there – anyone could have done it!

However, one creativity event is not enough – it needs to be worked on and become part of the habit of research. It also needs to become more personal, i.e. specific to the individual needs of the students and their projects. During the spring we run a number of other creativity events with Dennis as the facilitator. One is a student activity, where students sign-up to talk to Dennis about particular aspects of their own work. The aim in these sessions is to help students develop patterns of work that will enable them to solve their own problems and to come up with new ideas. We also run a creativity session with supervisors as working with students on creativity will not be effective if the student-supervisor relationship does not ‘keep up’. In the supervisors event the discussions typically go further, looking at how one can approach the student-supervisor relationship in ways that will encourage the generation of new ideas.

COGNITIVE BEHAVIOURAL COACHING

There is a growing evidence-base that postgraduate research progress can be enhanced through Cognitive Behavioural Coaching (CBC), and provision of this support has been designed into the XM2 programme. Staff in Exeter’s CEDAR (Clinical Education Development and Research) Centre offer CBC throughout the first year for each cohort aimed at strengthening academic adaptability and problem-solving commonly experienced obstacles to progression in both group and individual formats. The focus of these sessions can include topics such as goal-setting and self-monitoring, and enhancing skills to deal with issues such as procrastination and perfectionism or individual barriers to successful progression.

CBC is introduced to our researchers during a short session in their induction week. The goals for this session are simply to communicate the essentials; the nature of a coaching relationship and the use of CBC to enhance performance as well as dealing with setbacks. Before the Christmas break we run a follow-up workshop focusing on enhancing performance through strategic engagement using goal-setting and self-monitoring skills. We then continue to conduct CBC group and individual sessions, allowing the students to sign-up for the aspects they are interested in or find most helpful.

A BROADER SCIENTIFIC EDUCATION

Our students learn in their Creativity courses that all apparently new ideas are formed by bringing together pre-existing knowledge, so it is essential that we help them broaden their horizons both within the field of metamaterials, and without. To this end our students attend weekly seminars given by visiting academics on a wide-range of scientific topics, and we have developed a series of 2-day intensive workshops focused on the themes of Plasmonics, Magnonics, Acoustics, Fabrication and Characterisation of Functional Materials, and Sensing and Security. These workshops are designed to be interactive and stimulating and, in order for our students to gain a perspective of research in an industrial environment, some of these courses are being run by experts from industry.

LEADERSHIP & PROJECT MANAGEMENT

Whilst during their course, PhD students will be focused on undertaking their research project, when they graduate they will quickly find that they are expected to take on management roles; leading research teams and managing projects. In order to develop these often-ignored skills, in the 2nd and 3rd years of the programme our students will undertake courses in Leadership and Project Management.

The Leadership training takes place in the second year of the programme, with the first cohort receiving their training in the first half of 2016. The course, run by the Centre for Leadership Studies in Exeter’s Business School, will help students recognise different management styles and explore ideas in effective leadership. It will also involve visiting local companies to gain first-hand experience of different management approaches. The project management training is also offered by Exeter’s Business School and is undertaken in the 3rd year of the programme. It is a practical course where the students will develop their project management skills by working on a live project.

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THE NEED FOR A NEW TRAINING APPROACH

Nationally, around 35% of the UK’s doctoral graduates in physical sciences and engineering are employed in the higher education sector 3 years after graduating, and this has been a declining fraction since the mid 1990s. This decline has been accompanied by a marked transition in the role of PhD programmes. Emphasis has shifted from identifying and nurturing individual scholars, towards more closely matching the needs of industry. To embrace this change, we are training cohorts of doctoral scientists and engineers with relevant research skills, a broad knowledge base, and well-developed professional attributes, to become the UK’s future leaders in industry and academia in the field of material science.

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Facilities

The areas of electromagnetic materials, nanomaterials and graphene have been a strategic investment priority for the University for several years and, with over £10m of capital investment over the same period. This means that we have, in-house, all of the state-of-the-art materials, nanofabrication and characterisation facilities required by our students. From e-beam lithography, thin film deposition, plasma-etching systems, and associated photonic laboratories, through to a suite of 3D printers and fully equipped microwave and acoustics laboratories, we have the full range of facilities needed to fabricate and characterise samples on lengths scales from nanometres to metres.

All of these facilities are housed in purpose-built laboratories, including five dedicated clean rooms, three of which have been built in the last 5 years. Our students also have access to state-of-the-art computational facilities and software packages including the University of Exeter Zen Supercomputer and various Beowulf clusters.

In addition, during their studies our students will be expected to spend time in our academic and industrial partner institutions, and in central research facilities, in order to experience other working environments and allow access to an even greater range of specialist facilities.

All of our students are housed in newly re-furbished office spaces, with a dedicated training/meeting room, for at least the first two years of their training. This strengthens the cohort ethos of the Centre and encourages interactions and collaborations between the students.

My experience as an XM2 student has been great so far. Apart from our research, as a cohort we receive training in different areas helping us to broaden our knowledge in aspects such as creativity, teaching, etc.

ALBA PANIAGUA DIAZ, XM2 STUDENT

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STUDENT SUPPORT

A “buddy” scheme, established so that our researchers have additional pastoral support on arrival, provides a friendly face to advise new arrivals during their first few months in Exeter. The idea is simple: to pair each XM2 student with a current PhD student whom they feel comfortable approaching with questions and advice about the University, the departments, and Exeter itself, or even about life in the UK. Buddies are not people who are involved in supervising PhD students or running the Centre so that the student can ask things openly and, at the same time, expand their initial circle of contacts.

The Centre also has a full time administrator who handles the whole “life cycle” of the students’ programme and deals with finance, monitoring, and any non-academic issues that relate to procedures and paperwork. In addition there is a full-time technical director

who has responsibility for lab management and the day-to-day academic requirements of the students.

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Whilst the additional training available in a cohort-based PhD programme differentiates it from the traditional “lone scholar” PhD, the essence of the research aspect, i.e. the requirement to produce a piece of original research, remains unchanged. XM2 is interdisciplinary in nature, with projects taking place within our research groups in the Physics and Engineering departments along the following themes:

PLASMONICS

(i) light at the nanoscale(ii) nanoplasmonics for metamaterial applications (iii) graphene based devices

NATURAL AND DISORDERED PHOTONICS

(i) exploration of metastructures in wingscales and insect cuticles (ii) biomimetics of natural photonic structures(iii) use of nanocellulose to provide biodegradable optical

metamaterials

THZ PHOTONICS

(i) optically tuneable metamaterials (ii) subwavelength imaging

MICROWAVE METAMATERIALS

(i) metasurfaces and designer plasmonics (ii) broad-band metamaterials and thin metasurface absorbers(iii) compact antennas and resonators(iv) energy harvesting structures(v) mechanically activated meta-structures for dynamic control

MAGNETIC METAMATERIALS

(i) magnetodynamic modes in patch resonators(ii) electromagnetic properties of artificial magnetic materials and

magnetic composites(iii) tuneable antennas(iv) RFID(v) energy harvesting

ACOUSTIC METAMATERIALS

(i) making analogies with the microwave domain, including airborne and SONAR

(ii) metasurfaces, composite materials and light-weight absorbers (both in air, and under water)

SPATIAL TRANSFORMATIONS

(i) application of spatial transformation theory to electromagnetic and acoustic problems

(ii) theory of metasurfaces

GRAPHENE DEVICES

(i) flexible metastructured detectors and sources for infrared and THz

(ii) multifunctional ultra-lightweight energy harvesting coatings(iii) metastructures for underwater acoustics

NANOMATERIALS AND NANOCOMPOSITES

(i) nanometamaterials, nanorods, nanowires, nano-tubes of carbon or carbon based structures

(ii) carbon nanotubes and graphene for energy conversion and storage

(iii) nanocomposites for control of electro-magnetic radiation

MAGNONICS AND SPINTRONICS

(i) programmable magnonic metamaterials(ii) spintronics(iii) spin wave based data and electromagnetic signal processing

Research

In the following pages, we provide summaries of the exciting work undertaken by our 2014 cohort of PhD researchers during their first 6 months of study.

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Microphononic crystals BENJAMIN J. ASH, PETER VUKUSIC, GEOFFREY R. NASH

Phononic Crystals (PnCs), analogous to photonic crystals, can inhibit the propagation of acoustic waves for specific frequency bands by inducing bandgaps1,2,3. We are investigating a new approach to the realisation of PnCs for surface acoustic waves (SAWs), where we use unit cells based on annular holes in order to support low frequency resonant modes. This approach offers a potential route to the much easier fabrication of PnCs

in the very hard materials used for SAW devices.

SAW devices are used in a wide range of applications including sensing and in RF signal processing. The aim of this work is to realise PnCs working in the MHz range, leading to increased and new functionality of SAW components, where the PnCs are used as waveguides or resonant cavities.

For bulk acoustic studies, periodic elastic composites of circular inclusions within a host matrix leads to phononic bandgaps. However when this approach was applied to SAW studies in the form of an array of cylindrical void holes in a host surface, which are technologically challenging to fabricate, PnC characterisation was not realised because coupling of surface and bulk modes was found at frequencies within and above the predicted bandgap, meaning propagation at higher frequencies than the bandgap couldn’t be verified4. We have used finite element modelling to show that an array of annular holes can be used to reduce the energy, and therefore frequency, of propagating modes by introducing out of plane resonances in the inner annular structure, evident in figure 1, meaning SAW PnC characterisation will be possible. We are now patterning such phononic crystals into commercial SAW devices consisting of a piezoelectric substrate and two inter-digital transducers used to excite and detect SAWs, figure 2.

1. M. S. Kushwaha et al., Phys. Rev. Lett. 71, 2021 (1993)2. F. R. Montero de Espinosa et al., Phys. Rev. Lett. 80, 1208 (1998)3. Y. Achaoui et al., J. Appl. Phys. 114, 104503 (2013)4. S. Benchabane et al., Phys. Rev. E 73, 065601 (2006)

Figure 1 Unit cell cross-section displacement plots for resonant low energy propagating SAW modes for a square array of annular holes. Lattice parameter = 10μm, inner radius = 3.6μm and hole depth = 5.6μm. a) is the first harmonic resonance at 92MHz b) is the second harmonic at 131MHz.

Figure 2 Schematic of SAW filter consisting of a pair of aluminium inter-digital transducers (white) and lithium niobate substrate (blue) with zoomed inset of an annular hole array phononic crystal.

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Electromagnetic chirality and the origin of optical rotation in the near-field of handed metasurfaces

LAUREN BARR, BEN TREMAIN, ANA DIAZ-RUBIO1, EUAN HENDRY, ALASTAIR P. HIBBINS1WAVE PHENOMENA GROUP, DEPARTMENT OF ELECTRONIC ENGINEERING, UNIVERSITAT POLITÈCNICA DE VALÈNCIA, SPAIN.

Chirality, a property of a system that lacks any plane of inversion symmetry1, has recently sparked great interest in the area of metamaterials2,3. The structure we have investigated produces broadband optical rotation on transmission through a thin metasurface. In

order to fully explain the origin of this effect, we have studied the chirality of the electromagnetic fields at the surfaces of such structures.

An electromagnetic field has chirality when components of the electric and magnetic fields are parallel and out of phase with each other4. It has been found that a metamaterial comprised of geometrically chiral unit cells can provide giant, pure optical rotation3 across a wide range of frequencies, although the origin of this phenomena remains unexplained. We aim to shed light on this problem through the experimental and computational study of a chiral metasurface at microwave frequencies. Our chosen structure is a bilayer consisting of a square array of metallic crosses above an array of identical, aligned but twisted complementary crosses patterned in a metallic sheet, separated by a thin layer of dielectric (inset, figure 1).

It was found that this metasurface produces optical rotation in transmitted waves with no ellipticity induced in the beam at frequencies where the transmission is maximum, as illustrated in figure 1. This is due to competing even and odd resonances which act to rotate the plane of polarisation in opposite directions. The currents displayed in figure 2 show that a current loop is set up between the top and bottom surfaces, as the currents in the top cross are π out of phase with those directly beneath, producing a magnetic moment through the centre of the loop. The perturbations to the currents in the bottom layer caused by the twisted complementary-cross induce parallel components in the electric and magnetic fields, and hence the chirality needed for optical rotation. In future studies we will investigate the interactions of chiral near-fields with 3D chiral scattering objects, such as helices.

1. L. D. Barron, Molecular Light Scattering and Optical Activity, Cambridge University Press, Cambridge, 2nd Ed (1982)

2. J. B. Pendry, Science 306, 1353 (2004)3. A. Rogacheva et al, Phys. Rev. Lett. 97, 177401 (2006) 4. Y. Tang and A. Cohen, Phys. Rev. Lett. 104, 163901 (2010)

Figure 1 a) Transmission and b) optical activity of a wave transmitted through a square array of unit cells as illustrated in inset a). In the frequency range 16 - 35 GHz the ellipticity is close to zero but the optical rotation remains finite (pure optical rotation), due to close resonant modes rotating the plane of polarisation in opposite directions.

Figure 2 Currents, represented by arrows, and electric field intensity, represented by the colour of the arrows, in the bottom layer of a unit cell of the metasurface at a) 7.8 GHz, corresponding to a peak in transmission and b) 14 GHz, corresponding to a peak in optical rotation and ellipticity respectively. In the upper cross, a simple dipole along the x-arms is seen at both frequencies. Half-loops of current seen in the lower surfaces signify a magnetic resonance in the complementary cross at 7.8 GHz.

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Time resolved imaging of coupled nano-contact spin transfer vortex oscillators

ERICK BURGOS PARRA, PAUL S. KEATLEY, SOHRAB R. SANI1, JOHAN ÅKERMAN1 AND ROBERT J. HICKEN1 MATERIALS PHYSICS, SCHOOL OF ICT, ROYAL INSTITUTE OF TECHNOLOGY, ELECTRUM 229, 164 40 KISTA, SWEDEN

The spin transfer torque effect is a relatively new spin effect predicted1 a decade after the birth of spintronics2 and observed experimentally in just the last decade3. In the

present study the change in magnetisation induced by spin transfer torque within a spin transfer vortex oscillator was imaged by time resolved Kerr microscopy (TRSKM).

The angular momentum transferred by a spin polarised current can exert a torque on the magnetisation of a ferromagnetic material. This effect is called spin transfer torque (STT) and it is the fundamental interaction that drives spin torque oscillator (STO) devices. STOs are a new kind of nano-device in which the emission of microwaves is stimulated by a dc electrical current. Although STOs may emit across a broad range of frequencies, the low power output and poor phase stability are significant problems for practical applications.

If the applied current density is sufficiently large, the local magnetisation can be modified without application of an external magnetic field. In order to produce high current densities current is often passed through a nano-contact. The formation of a non-uniform vortex state in the vicinity of a nanocontact4 is induced by the field produced by the DC current, while gyration of the vortex results from the associated STT. This can lead to stable sub-gigahertz microwave emission that avoids some of the problems usually associated with STOs5. This type of device is known as a Spin Transfer Vortex Oscillator (STVO).

A pair of STVO nano-contacts has been studied to understand how coupling the dynamics of a pair of vortices might change the amplitude and stability of microwave emission. The dependence of the gyration frequency upon the applied current and the distance between the nano-contacts has been explored. Finally, Time Resolved Scanning Kerr Microscopy (TRSKM) was used to image the dynamic magnetisation around the nano-contact to better understand the complex behaviour of the STVO pair.

1. J. C. Slonczewski, J. Magn. Magn. Mater., 159:L1, (1996)2. M.N Baibich et al., Phys. Rev Lett., 61 (21):2472-2475, (1988)3. W. Rippard et al., Phys. Rev .Lett., 92, 027201 (2004)4. M. R. Pufall et al., Phys. Rev. B, 75, 1 (2007)5. M. W. Keller et al., Appl. Phys. Lett. 94, 10 (2009)

Figure 1 Schematic of a pair of nano-contact STVOs. The device is comprised of a pair of nano-contacts fabricated in a layer of SiO2 (30 nm) on top of a 16 μm2 x 8 μm2 spin valve mesa. The current is delivered to the system by a Cu (1200 nm)/Au (20 nm) coplanar waveguide (CPW) structure with a top signal contact in the center of the mesa. The distance S between the nano-contacts is 200 nm.

Figure 2 (a) The three components of the phase modulated magnetisation are shown. The grayscale spans the voltage range -0.5 V to 0.5 V. The dashed yellow mask in each image denotes the position of the signal contact of the CPW and was taken to lie at the bottom of the borders of the AFM contact image in (b). (b) AFM image of the CPW. The zoomed image is the reflectance image acquired during the TRSKM measurements. The labels A and B mark the centers of the regions of strong magnetic contrast.

17

The design of practicable phase-change absorbers and modulators for near-infrared wavelengths

SANTIAGO GARCIA-CUEVAS CARRILLO, C. DAVID WRIGHT AND GEOFFREY R. NASH

Phase-change materials, such as the alloy Ge2Sb2Te5 (GST), have very different optical properties in their amorphous and crystalline phases, and can be switched rapidly and repeatedly between these phases by electrical and/or optical excitation. Combining these materials with metamaterial structures (plasmonic resonators) means that they also have much (as yet little explored) potential for applications as tunable photonic devices.

The aim of this work therefore is to design chalcogenide-based electromagnetic absorber and modulator devices, tuned for use in the near infra-red (NIR), specifically at 1550 nm as used in optical fibre communications. Our design (see figure 1) uses aluminium metal layers for the plasmonic resonator structure, rather than the more commonly used silver or gold layers, since aluminium is better suited to traditional microelectronic manufacturing processes. We also include a suitable means, via an ITO layer, for protecting the GST layer from environmental oxidation while still enabling electrical access to switch the phase-change layer. In terms of manufacturability we also take into account, for the first time and via a detailed optimisation and tolerance analysis, the effects of manufacturing tolerances on key performance parameters (see figure 2). This study offers some important insights into the performance that can be achieved with these types of phase-change metamaterial devices at NIR wavelengths, and shows how sensitive are the optimal designs to the imperfections produced during the manufacturing process.

1. C. Rios et al., Adv. Mater. 26(9), 1372-7 (2014)2. P. Hosseini et al., Nature 511(7508), 206-211 (2014)3. C.D. Wright et al., Adv. Mater. 23(30), 3408-13 (2011)4. C.D. Wright et al., Adv. Mater. 23(18), 2248-54 (2013)

Figure 2 Specimen results of the tolerance analysis showing scatter of optical modulation amplitude (OMAR) (an optimised figure of merit) as a function of (left) variations in the thickness of the ITO layer and (right) variation in the width of the Al stripes.

Figure 1 (top) Schematic view of the proposed structure, here acting as an optical modulator; (middle) cross-section of the device, showing the material layers and key dimensions; (bottom) simulated reflectance of the structure for the GST layer in both amorphous and crystalline phases.

18

Graphene oxide/zeolitic imidazolate framework nanocomposites and their CO2 uptake capacity

LAICONG DENG, YANQIU ZHU, YONGDE XIA

The aim of this project was to synthesise and characterise nanocomposites of graphene oxide (GO) with porous zeolitic imidazolate framework-67 (ZIF-67) and their derived graphene-based composites, and to evaluate their adsorption capacities and electrochemical behaviours.

Recently, a new sub-family of metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) has attracted great attention. ZIFs are formed by a self-assembly approach and show diverse structures that are similar to a traditional aluminosilicate zeolite, where typically Co2+ ions play the role of silicon while the imidazolate anions form bridges that mimic the role of oxygen in zeolite frameworks, with the M-Im-M angle around 145˚. Consequently, ZIFs are novel porous materials with ultrahigh surface area, and exhibit unique crystal structures, abundant functionalities as well as exceptional thermal and chemical stability1. On the other hand, graphene based materials have attracted much interest in the past few years2. Graphene oxide (GO), containing functional groups including both hydroxyl and epoxy groups exist in the basal plane, and carboxy, carbonyl and lactone on the edge of sheet3,4, has been widely considered as an excellent precursor for graphene.

Thermogravimetric analysis (TGA) and corresponding mass spectroscopy (MS) signals (figure 1) of the in-situ synthesized GO/ZIF-67 series composites provided clear evidence of the strong interaction between GO and ZIF-67 in the in-situ synthesized GO/ZIF-67 composites. This is evidenced by the controlled in-situ synthesized GO/ZIF-67 composites exhibiting similar weight loss, decomposition temperature and MS signals with pure ZIF-67, without any feature of pristine GO. Nitrogen sorption isotherms are shown in figure 2, suggesting that the in-situ synthesized composites are not simply a physical mixture of GO and ZIF-67. In addition, the GO contained composites obtained via the in-situ synthesized method effectively modify the textural properties, markedly influencing the CO2 uptake capacities of such composites.

1. B. Chen et al., J. Mater. Chem. A 2, 16811 (2014)2. S. Stankovich,et al., Nature 442, 282 (2006)3. D. Chen et al., Chem. Rev. 112, 6027 (2012)

4. G. Eda and M. Chhowalla, Adv. Mater. 22, 2392 (2010)

Figure 1 (a) TGA curve of GO/ZIF67 series samples and corresponding MS signals of (b) CO2, (c) NO2 and (d) H2O. (20 nm) coplanar waveguide (CPW) structure with a top signal contact in the center of the mesa. The distance S between the nano-contacts is 200 nm.

Figure 2 Nitrogen sorption isotherms for GO/ZIF67 series composites.

19

Permittivity and permeability of carbonyl iron powder – ptfe composites

CAMERON. P. GALLAGHER, J. ROY SAMBLES, ALASTAIR P. HIBBINS, HEATHER LEWTAS1

1 BAE SYSTEMS, BURCOTE ROAD, TOWCESTER, NN12 6TF

Carbonyl Iron Powder (CIP) has become of interest in recent years for having an ‘onion ring’ structure, giving the particles a relative permeability greater than unity in the microwave regime (1 -10 GHz)1-3. A cold-press technique for creating composites has been adopted

alongside use of a strip-line for transmission/reflection measurements in order to investigate the permittivity and permeability of composites consisting of varied percentage volume mixes of CIP with PTFE.

Carbonyl Iron Powder was first created by BASF in the 1920s for use in powder metallurgy before its microwave properties began to be exploited. Interest in microwave applications has become more prevalent due to absorption modes being discovered at GHz frequencies that were previously inaccessible4. Carbonyl Iron Powder comprises of alternating spherical shells of iron and iron carbide, giving insulated, spherical regions of magnetic material5. This separation of magnetic regions within particles significantly reduces the conduction length, minimising the effect of eddy currents and increasing the frequency range of the magnetic response.

By mixing these particles with a polymer matrix at different percentage volumes and recording the change in permittivity and permeability, the relationship between particle size and particle loading is explored, and the understanding gained will benefit the aim of producing materials with tailored values for permittivity and permeability able to be used in microwave devices. Figure 2 illustrates the relative permittivity and permeability deduced from strip-line measurements for composites formed from particle loadings across the range 10%vol. – 50%vol. A ferromagnetic absorption mode at 2 GHz is seen due to the rotation of magnetisation having an upper frequency limit that is dependent upon the static value for permeability6. Future work will involve optimising the strip-line geometry such that it will be useful for characterising samples at frequencies beyond 10 GHz, as well as investigating the effects of particle size on the properties of these composites, where number and thickness of layers will be taken into consideration, with the final aim of increasing the frequency range for composites possessing a relative permeability

greater than 1.

1. M. A. Abshinova et. al., Composites: Part A 38, 2471 (2007) 2. R. Han et. al., Journal of Alloys and Compounds 509,

2734 (2011)3. Q. Yuchangn et. al., Physica B 406, 777 (2011) 4. A. N. Lagarkov et. al., Journal of Magnetism and

Magnetic Materials 324, 3402 (2012) 5. V. G. Syrkin, Poroshkovaya Metallurgiya No. 3, 21å (1965)6. J. L. Snoek, Physica 14, 207 (1948).

Figure 1 a) SEM image showing spherical shape of carbonyl iron particles b) TEM image of ‘onion ring’ structure for typical carbonyl iron particles4

Figure 2 Graphs showing complex relative permittivity and permeability for composites formed from particles loadings of 10%vol. 30%vol. and 50%vol.

20

Observation of the ultrasonic cut-off frequency of a water filled pressure-release cylindrical waveguide

THOMAS GRAHAM, SIMON HORSLEY, ALASTAIR P. HIBBINS, J. ROY SAMBLES

In this project an underwater ultrasonic cut-off frequency has been directly observed for the first time for a variety of lengths of cylindrical waveguides.

Cylindrical waveguides having pressure-release boundaries1,2 only allow acoustic waves to propagate through them above a certain cut-off frequency, determined by the radius. The first experimental work on such waveguides was published in 1948 by Jacobi3. Jacobi investigated both theoretically and experimentally the dispersion of sound travelling through water-filled cylindrical waveguides with both rigid and pressure-release boundary conditions. Only recently has there been renewed interest in such structures, with the help of advancements in computing power4-6. Yet, until now, the direct observation of the cut-off frequency of a cylindrical acoustic waveguide had not been observed.

Polyurethane, closed cell, parallel faced foam samples of different thicknesses were fabricated and a 4 mm radius hole drilled normally through them. Hydrophones were used as both sources of pulsed ultrasound and as detectors to determine the frequency dependent transmission under water through the holes. Computational and analytic results predict that the lowest order cut-off frequency of such structures should be 142 kHz. This was experimentally observed for thicknesses of 10, 20, 40 and 60 mm of polyurethane foam. These results show that polyurethane foam can be used as a pressure-release material underwater and that the analytic and computational model of the system accord with the data on the lowest order cut-off frequency.

1. Lord Rayleigh, “The theory of sound”, Vol. 2, 1896. Dover, New York (1945)

2. T. D. Rossing and N. H. Fletcher, “Principles of Vibration and Sound”, New York, Springer-Verlag (1995)

3. W.J. Jacobi, J. Acoust. Soc. Am. 21, (1948)4. K. Bai et al., J. Acoust. Soc. Am. 128, 2610 (2010)5. J. Jiang et al., J. Acoust. Soc. Am. 130, 695 (2011)

6. K. Balk et al., J. Acoust. Soc. Am. 133, 1225 (2013)

Figure 1 A schematic of the underwater experiment. The hydrophones are positioned 10 cm away from each other, with each sample being central and of area 20 by 10 cm.

Figure 2 The relative transmission intensity from experimental (red) and computer model (dashed blue) for pressure-release cylindrical waveguides filled with water, with radius 4 mm and length 20 mm. The dashed vertical line (green) is at the analytic predicted value for the cut-off frequency, 142 kHz.

21

Reflectionless materials and the propagation of waves in the complex plane

CHRISTOPHER G. KING, SIMON A. HORSLEY, TOM G. PHILBIN

The propagation of electromagnetic waves through inhomogeneous media has been approached in many different ways, often with the aim of finding materials which do not reflect radiation from any angle of incidence. We have used the WKB approximation1 to investigate the reflection and transmission through some complex permittivity profiles.

Such materials may be used for applications requiring maximal absorption or transmission of light, such as in anti-reflective coatings2 (for example in solar cells), or to make an object appear invisible3. We have calculated the transmission through, and reflection from, some spatially varying permittivity profiles in one dimension. We aim to use the WKB approximation to the wave equation governing this propagation to understand why some profiles reflect radiation and others do not. The WKB approximation is the first term of an infinite series solution to the wave equation valid when the permittivity changes significantly on a scale smaller than the wavelength.

To use this approach, we allow the spatial coordinate to be complex and, by investigating how the WKB approximation changes in the complex position plane, we can deduce the behaviour of the travelling wave solutions in real space. The properties of permittivity profiles in the complex position plane has been shown to help determine whether or not a material is reflectionless4.

The profiles we have considered have a non-zero imaginary part and have poles in the complex plane, such as the case of a single pole in figure 1. The numerical solution for waves propagating through this profile is shown in figure 2. The phenomenon of reflection occurs due to the breakdown of the WKB approximation at the zeros of the permittivity. However, by varying the parameters in the permittivity, we can obtain profiles which don’t reflect radiation from either the left or the right.

1. J. Heading, An Introduction to Phase Integral Methods Dover Publications, (New York, 2013)

2. H. A. Macleod, Thin Film Optical Filters, Institute of Physics Publishing, (London, 2001)

3. J. B. Pendry et. al., Science, 312, 1125907, (2006)4. S. A. R. Horsley et. al., arxiv:1503.00152 (2014)

Figure 1 : (i) The real and imaginary parts of the permittivity profile (z)=1-1/(z+i). In this case Im[(x)]>0, so waves propagating through the medium will undergo loss. (ii) The permittivity in complex position space, where brightness represents amplitude and colour represents phase. The yellow colour along the real axis indicates the negative imaginary part of the permittivity in this region.

Figure 2 The numerical solution of the wave equation for the profile . The absolute value of a right and a left travelling wave are plotted. The lack of oscillations for a right travelling wave indicates that there is no interference with a reflected wave, and thus there is no reflection for waves incident from the left.

22

Modelling the behaviour of ferromagnetic composites at microwave frequencies

CONOR MCKEEVER, MUSTAFA AZIZ, FEODOR OGRIN, ALASTAIR P. HIBBINS, J. ROY SAMBLES, HEATHER LEWTAS1

1 BAE SYSTEMS, BURCOTE ROAD, TOWCESTER, NN12 6TF

Recently the electromagnetic properties of ferromagnetic composites with different types of carbonyl iron particles have been investigated as microwave absorbing materials for frequencies up to 20 GHz1,2.

This project is concerned with theoretically understanding the magnetic and electromagnetic behaviours of the complex, multi-layered carbonyl particles in isolation and embedded in a dielectric matrix.

The permeability of composites containing primary and processed carbonyl iron powders shows significant differences in the high frequency range despite possessing comparable chemical composition. Processed carbonyl iron powders exhibit two regions of magnetic dispersion and higher absolute values of permeability and permittivity in the radio-frequency band. These large differences have been attributed to an “onion-like” multi-layered microstructure of the particles. This multi-layered microstructure is comprised of electro-conductive iron layers and dielectric layers, which form the alternating concentric shells of the “onion-like” morphology. The approach we adopted here to model this complex system includes simulating the static and dynamic magnetic behaviours of single and arrays of particles at the micro-scale using numerical micromagnetics .

The RF behaviour of a two-dimensional analogue was studied to provide foundation for further investigation into these onion-like multi-layered metamaterials. The dynamic behaviour of a permalloy two-ring structure of outer ring diameters 1800 nm and 1000 nm, ring width 200 nm and thickness 20 nm is investigated by the application of a 100 ps Gaussian field pulse (see figure 1, figure 2). Our results reveal a linear oscillation of the onion state domain wall moments at frequencies above 1 GHz following pulse excitation. Future work will extend the micromagnetic simulations to three-dimensions to model the magnetic behaviour of complete spherical particles, and incorporate the use of Maxwell’s equations solvers to simulate and study the interaction between the embedded ferromagnetic particles and dielectric matrix with the incident electromagnetic fields2.

1. A. N. Lagarkov et al., JMMM 324, 3402 (2012)2. Y. Feng et al., JMMM 324, 3034 (2012)3. OOMMF, http://math.nist.gov/oommf/

4. M. M. Aziz, PIER B 15, 1-29 (2009)

Figure 1 Two thin-rings of outer diameters 1800 nm and 1000 nm, thickness 20 nm and width 200 nm with onion domain structure; arrows correspond to the direction of magnetization in the xy-plane; red corresponds to magnetization in the x-direction.

Figure 2 Fourier transform of average magnetization of double ring system following Gaussian pulse excitation of strength 2 mT and width 100 ps. Peaks at 880 MHz and 2.1 GHz correspond to oscillation of the domain wall moments in the inner and outer rings, respectively.

23

Novel opto-electronic devices based on graphene plasmonics

JAKE MEHEW, MONICA CRACIUN, SAVERIO RUSSO

Functionalised graphene offers a unique and tuneable platform for many potential novel applications1, particularly within optoelectronics2. Plasmonic arrays incorporated onto the surface of graphene could lead to the realisation of ultrasensitive photodetectors.

Graphene possesses a host of unique mechanical, electric and thermal properties resulting in a massive amount of interest within the academic community as well as industry. These properties allow graphene to fulfil the requirements of a multitude of applications and within these the focus currently lies on optoelectronic devices; for example graphene could replace ITO, the current industry standard transparent conductor, with one realisation achieved through ferric chloride intercalation of graphene, dubbed GraphExeter1-3. This functionalisation is one of many avenues available to researchers to fine-tune the properties of graphene to suit specific requirements.

Other functionalisations are possible including the adsorption of hydrogen and fluorine onto graphene’s surface4. This process has been shown to open a gap in the electronic band structure through the disruption of the -orbital and subsequent localisation of electrons with figure 1 demonstrating a manifestation of this energy gap. Incorporating these materials into photodetectors, figure 2, prevents ultrafast recombination of excited charge carriers thus overcoming a common problem in pristine graphene devices.

The beauty of this functionalisation is that it is tuneable down to the nanoscale and this has been demonstrated by means of electron beam patterning of highly conductive channels in an otherwise insulating material5. Our current research will focus on exploiting collective oscillations of charges, i.e. surface plasmons, in arrays of these channels to enhance the light-matter interaction. These plasmonic graphene arrays will be employed as novel ultra-sensitive photodetectors.

1. I. Khrapach et al., Adv. Mater. 24, 2844 (2012)2. F. Withers et al., ACS Nano 7, 5052 (2013)3. D. Wehenkel et al., Scientific Reports 5, 7609 (2015)4. F. Withers et al., Nano. Res. Lett. 6, 526 (2011)5. F. Withers et al., Nano Letter 11, 3912 (2011)

Figure 1 a) Current-voltage characteristics of multilayer fluorinated graphene device. The non-linear nature is indicative of the presence of a bandgap and this is observed in traditional wide gap semiconductors. b) Photocurrent map of a fluorinated graphene device. The greatest photocurrent is collected at the interface between the gold electrodes (overlaid in black) and fluorinated graphene.

Figure 2 Cross-sectional view of the photodetector with the electrical connections used to create the photocurrent map. A silicon substrate with a thermally oxidised surface layer is used to allow gate modulation (Vbg) of the charge carrier density in the deposited fluorinated graphene. The photocurrent (Ids) is collected from one of the gold electrodes whilst the laser beam (λ=514 nm) is rastered across the device. Inset: side view of the crystal structure of fully fluorinated graphene.

24

Few-layer MoTe2 photodetectorsTOBY J. OCTON, MONICA F. CRACIUN, C. DAVID WRIGHT

The field of two dimensional electronics has expanded greatly since the awarding of the Nobel Prize in 2010 for the discovery of graphene1. A similar material, Molybdenum Ditelluride (MoTe2), was examined for use as a transistor and photodetector in the visible regime.

Transition Metal Dichalcogenides (TMDCs) are layered semiconductors with band gaps that expand into the visible and near infra-red regime (0.7 eV - 2 eV). TMDCs can be fabricated by the same methods as graphene and their properties can be tuned by their layer thickness2. MoTe2 has a band gap between 1 eV – 1.2 eV3, which increases as layer thickness decreases. MoTe2 is more suitable than other TMDCs for photodetection in the visible regime due to the band gap corresponding to the near infra-red. Light impinging on a semiconductor of energy that exceeds the band gap creates excitons which increase the current across the device. In figure 1 a fabricated four layer thick MoTe2 device and schematic is shown. Devices are fabricated in a back gated configuration, and photodetection properties are examined via incident laser illumination.

In figure 2 the electronic properties show MoTe2 to have a current ON/OFF ratio of 105, and a measurable photocurrent at low illumination intensity. With current research on the deterministic thinning of samples via laser illumination ongoing4, it is hoped that TMDCs could be used as the next generation of semiconductors, with all the benefits of a two dimensional material. The addition of gold nanoparticles on top of the device has been seen to enhance the photoresponse of TMDCs5 but also change their structure to a more conductive phase and we hope to examine a similar response in MoTe2.

1. K. S. Novoselov et al., Science 306, 666 (2004)2. K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)3. A. Kumar et al., Eur. Phys. J. B 85, 186 (2012)4. A. Castellanos-gomez et al. Nano Lett. 12, 3187 (2012)5. J. Lin et al., Appl. Phys. Lett. 102, 2013 (2013)6. Y. Kang et al., Adv. Mater. 6467–6471 (2014)

Figure 1 (a) Optical microscopy image of two four layer MoTe2 flake devices. The purple colour shows a 3 nm thick four layer flake, yellow shows the location of electrical contacts made of gold and the orange shows the SiO2 substrate. (b) Schematic diagram of fabricated devices and electrical connections for measurements.

Figure 2 Back gated electronic characteristics of few-layer MoTe2. MoTe2 is seen to be strongly hole doped. b) Saturation of the photocurrent for a few-layer MoTe2 device under 514 nm laser light. Highest photoresponse seen for lowest power incident radiation.

25

Increasing the penetration depth of light in scattering media by wavefront shaping

ALBA M. PANIAGUA DIAZ, WILLIAM L. BARNES, JACOPO BERTOLOTTI

Random scattering of light in biological media poses a problem for imaging, since it reduces the penetration depth of light in the medium. As a result, many imaging techniques are limited to work near the surface of the material. We use wavefront shaping techniques to increase the penetration depth of light into such media.

When light impinges on biological tissue, it gets scattered in all directions. Since elastic scattering preserves coherence, interference determines the distribution of light in the medium. If the material scatters strongly enough, the penetration depth of light is limited to a region close to the illuminated surface (figure 1(a)), which is a limitation for different imaging techniques. Given that scattering events are deterministic, we can control interference by shaping the wavefront of the incident beam, which can be used to focus light1 or increase the transmission of light2 through scattering materials (figure 1(b)).

Wavefront shaping can be performed by modifying the amplitude and/or phase of the incoming electric field, which gives us control of the interference inside the materials. In this project we are using a Digital Micromirror Device (DMD) to perform the wavefront shaping. DMDs consist of a number of micromirrors that can be addressed independently. In this way, these devices spatially divide the incoming wavefront into different segments (groups of mirrors), where an algorithm assigns to them the appropriate values of amplitude in each case. Since phase modulation is more effective than amplitude modulation1,3, we are using the Lee holography method4 to encode the phase using the DMD. The reflected light from the sample is then collected by a microscope objective, where an algorithm minimises the reflected intensity (figure 1(c)). Since energy has to be conserved, this results in an increase in the transmission of light through the sample (figure 1(d)).

1. I.M. Vellekoop et al., Opt. Lett. 32, 2309 (2007)2. H. Yu et al. Optics Express, 22, 7514 (2014)3. D. Akbulut et al. Optics Express, 19, 4017 (2011)4. W. Lee. Applied Optics, 18, 3661 (1979)

Figure 1 Schematic of the propagation of light when the wavefront is not shaped (a) and when it is shaped (b). Graph (c) shows the reflected intensity after running the algorithm that minimises the reflected intensity from the sample. In (d) is plotted the transmitted intensity after running the same algorithm, showing an increase in transmission.

26

Anderson localisation in disordered chains of patch antennas

SATHYA S. SEETHARAMAN, IAN R. HOOPER, WILLIAM L. BARNES

Anderson localisation of electromagnetic waves in disordered photonic lattices has been well established in recent years1,2. We have extended these studies to directly observe the localised eigenmodes of disordered chains of coupled radiating patch antennas at radio frequencies.

Anderson localisation was first introduced in the context of electron localisation within semiconductors when there is a sufficient degree of randomness associated with any defects3, but is a more general phenomenon associated with wave interference that has proven to be relevant in many other contexts4. A simple manifestation of Anderson localisation can be observed in toy-models of coupled oscillators, and is evident in figure 1 where we show the eigenmodes of a chain of 50 coupled dipoles with randomised nearest-neighbour coupling strengths. Unlike the eigenmodes of the equivalent regular lattice, which are well known and have dipole moment distributions that are extended throughout the chain, the higher and lower frequency eigenmodes of the randomised chain show a high degree of localisation.

Patch antennas were first popularised for commercial applications in the 1970s5, and for our research we can consider them to be simple dipole oscillators. In a chain of such antennas near-field interactions couple nearest-neighbour patches together and, by randomising the distance between the patches, we can mimic the interactions of the toy-model. By driving a single patch at the frequency of the eigenmode of interest and spatially probing the local electric field above the chain of patches we can directly observe the spatial profile of the mode. In figure 2 we show the measured electric field profile of the highest frequency eigenmode of a disordered chain of 50 patch antennas showing the localisation of the electric fields to a central portion of the chain. Future work will explore more complex structures relevant to both meta- and nano-materials.

1. Y. Lahini et al., Phys. Rev. Lett. 100, 013906 (2008)2. J. Carbonell et al., Phys. Rev. Lett. 113, 233901 (2014)3. P. W. Anderson, Phys. Rev., 109, 1492 (958) 4. A. Lagendijk, Physics Today 62, 24 (2009)5. B. A. Balanis, Antenna Theory Analysis and Design, 3rd Edition,

(Wiley, 2005)

Figure 1 The dipole moment distributions of the eigenmodes of a disordered chain of 50 coupled dipoles with randomised nearest-neighbour coupling strengths. Localisation of the dipole moments is clearly evident in the higher and lower frequency eigenmodes.

Figure 2 a) A schematic of the experimental apparatus. b) Modeled electric field distribution for a disordered chain of patch antennas for the highest frequency eigenmode (obtained using COMSOL Multiphysics). c) The measured electric field distribution corresponding to the b).

27

Metamaterial concepts for the control of hydrodynamic flow

SAMUEL SHELLEY, ALASTAIR P. HIBBINS, SIMON HORSLEY, J. ROY SAMBLES, JOHN D. SMITH1

1 DSTL PORTON DOWN, SALISBURY, WILTSHIRE SP4 0JQ

Reducing the drag around objects is of great importance for a multitude of reasons. Recent work has dealt with trying to make surfaces “slippy” so that a fluid flows faster along the surface; one possible method for doing this is by structuring the surface. Here a previously untested theory for quantifying the

slip of a surface is compared with results from numerical simulations.

It was a long held belief that when a fluid flows over a flat solid surface there would be zero relative velocity between the fluid and the surface along the boundary. However this is not always the case. Interfacial slip, where the fluid has a non-zero relative velocity along the surface, is of key importance for reducing the drag around objects. An avenue of interest for potentially increasing surface slip is that of structuring surfaces. Reasonably complicated surface structures, such as an idealised model of shark scales1 or sinusoidal riblets2, have been researched experimentally in the turbulent regime showing drag reduction. However a theoretical understanding of the observed flow and the surface slip is generally lacking due to the complexities of the surface structures and the full Navier Stokes equations needed to describe the turbulent regime.

Bazant and Vinogradova3 proposed a general tensorial relation between the slip velocity and the shear stress via the surface mobility tensor (M), a 2x2 matrix. Kamrin et al.4 used a domain perturbation expansion to find the flow above an arbitrary periodic surface in the Stokes regime, from which they derived the mobility tensor. Despite deriving the mobility tensor, the aforementioned work does not provide any numerical or experimental evidence to show the region of validity of their solution. The present work provides a comparison between their theoretical predictions and a full numerical model, confirming that the theory is valid but only in the limit of very shallow structuring.

1. D. W. Bechert et al., Experiments in Fluids 28 403 (2000)2. M. Sasamori et al., Experiments in Fluids 55 1828 (2014)3. M. Z. Bazant et al., Physical Review Letters 102 026001 (2009)4. K. Kamrin et al. Journal of Fluid Mechanics 658 409 (2010)

Figure 1 Comparison between theoretical (blue) and modelled (red) values of the diagonal terms of the mobility tensor for flow over a sinusoidal surface. Solid lines correspond to flow transverse to the structuring (M11) whilst dashed lines are for flow parallel (M22) to the structuring.

Figure 2 Streamlines for flow transverse to the structuring at different aspect ratios (Top A/L = 0.1, bottom A/L = 0.39). For small aspect ratios the flow follows the shape of the boundary whereas at large aspect ratios vortices form above the surface. The earlier breakdown of the theory for transverse flow compared to parallel flow is attributed to the formation of these vortices. Colour shows the value of the x component of the velocity.

28

Excitation of picosecond magnetisation dynamics by spin transfer torque

TIM SPICER, PAUL KEATLEY, TOM LOUGHRAN, ROB HICKEN, PHILIPP. DÜRRENFELD1, AFSHIN HOUSHANG1, MOJTABA RANJBAR1, AHMAD A. AWAD1, RANDY K. DUMAS1, JOHAN ÅKERMAN1AND ROB HICKEN1 PHYSICS DEPARTMENT, UNIVERSITY OF GOTHENBURG, FYSIKGRÄND 3, 412 96 GOTHENBURG, SWEDEN

The injection of spin-polarised electrons can be used to induce picosecond timescale motion of the magnetisation of nanoscale ferromagnets that are the building blocks for dynamic

microwave and magnetic metamaterials. Picosecond processes are of vital importance for the development of high-speed magnetic data storage technology and microwave frequency wireless telecommunications.

The aim of spin electronics (or spintronics) is to develop devices that work by the transport of electron spin rather than charge, and to describe the new physics that arises1. In such devices the flow of charge and angular momentum are decoupled. Spin currents can be used to excite magnetisation dynamics through a mechanism known as spin transfer torque2.

Recently Spin Torque Nano-Oscillators (STNOs) have been demonstrated to exhibit microwave emission and show great potential as low-power current-controlled oscillators3,4. The Spin Hall Effect (SHE)5,6 has been shown to generate spin currents capable of driving magnetic oscillations, and as such STNOs driven by the SHE have become known as Spin Hall Nano-Oscillators (SHNOs)7. An example of such a device is shown in Figure 1. It is possible to detect the magnetic oscillations as changes in electrical resistance, as a result of the anisotropic magneto resistance.

Figure 2(a) shows that a mode with frequency of about 6 GHz forms once a critical current value has been reached. As the electrical current (and hence the size of the spin current) is increased the mode appears to redshift. Similarly if the applied current value is fixed and the field swept in the presence of an injected RF (12 GHz) current we obtain the results in figure 2(b). In this graph the oscillation frequency can be seen to plateau at 6 GHz, indicating that the oscillations are ‘locked’ (or synchronised) to the injected RF.

1. S. Maekawa, in Concepts in Spin Electronics (Oxford Scholarship Online, 2006), vol. 9

2. A. Brataas et al., Nature Materials 11, 372 (2012)3. W. H. Rippard, et al., Phys. Rev. Lett. 92, 027201 (2004)4. M. D. Stiles and J. Miltat, Topics in Applied Physics 101,

225 (2006)5. J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999)6. Y. K. Kato, et al., Science 306, 1910 (2004)

7. V. E. Demidov et al., Nature Materials 11, 1028 (2012)

Figure 1 Scanning Electron Microscope image of a Spin Hall Nano-Oscillator. The inset shows the device structure and dimensions. Large copper pads deliver current to the gold nano-contacts.

Figure 2 Microwave emission with a static magnetic field applied at 60˚ to the direction of the current and in the plane of the disk. Colourbars show Power Spectral Density in W ⁄ Hz. (a) Current sweep for a field of 630 Oe. (b) Field sweep with current maintained at 18mA and injected RF power of 11 dBm with frequency of 12 GHz.

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Correlation properties of the electromagnetic field ILYA STARSHYNOV, JACOPO BERTOLOTTI, JANET ANDERS

Quantum correlations play a crucial role in quantum computation, communication and cryptography, however the most extreme case of these correlations – quantum entanglement – is hard to create and handle. Quantum discord1 takes into account correlations that are due to quantum effects but do not necessarily involve quantum entanglement. We predict the presence of quantum discord between the modes of light

scattered by disordered material in case when input light is in a mixed quantum state.

Traditionally most practical applications of quantum effects focus on the systems which are in pure quantum states. In this case the only possible type of correlation between the subsystems is entanglement. However in realistic experimental conditions it is hard to keep the state pure. This stimulates the study of application of mixed states in quantum tasks2. Modes of light are ideal candidates for these studies since quantum operations on them can be performed with linear optical instruments – mirrors, phase shifters, beam splitters and multiple scattering media can be considered as a set of such instruments.

We consider a single-mode light beam incident on a scattering material. Scattering is fully described by the scattering matrix, which relates all the input modes with all the output modes (figure 1). We calculate the covariance matrix3 for the arbitrary pair of output modes, assuming that only one input mode is occupied and all others are in the vacuum state. We consider different quantum states of the input mode and characterise quantum correlations for three types of them: coherent, thermal and squeezed. Figure 2 shows possible quantum correlations depending on the parameters of the correlation coefficients between the quadratures of the output modes. For a coherent input state no correlations are possible and the output modes are in a product coherent state. If the input mode is in the squeezed state any two output modes are entangled (gray regions on figure 2). In case of thermal state input, the output state shows non-zero quantum discord, which means that quantum correlation are present between these modes.

1. H. Ollivier and W. H. Zurek, Phys. Rev. Lett. 88, 017901 (2001)

2. A. Datta and G. Vidal, Phys. Rev. A, 75, 042310 (2007)3. J. Laurat, at al., Journal of Opt. B: 7, S577 (2005).

Figure 1 The process of multiple scattering. The vector of output modes is defined by multiplication of the vector of input modes by the scattering matrix.

Figure 2 The map of possible correlations for two-mode Gaussian states with different correlation coefficients between the quadratures cx and cp. The regions within the thick solid curves outline all physically realisable quantum states. When the values of the cx and cp lie outside the regions outlined by dotted lines (gray areas), the joint state is entangled.

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Corrosion, dose-dependent in vivo, in vitro cytotoxicity and drug therapeutic effects to water soluble self-passivated graphene nanocrystals

TANVEER. A. TABISH, LIANGXU LIN, M. ALI1, CHRIS SCOTTEN2, YONGDE XIA, SHAOWEI ZHANG1 GOVERNMENT COLLEGE UNIVERSITY, FAISALABAD, 38000, PAKISTAN. 2 INSTITUTE OF BIOMEDICAL AND CLINICAL SCIENCE, UNIVERSITY OF EXETER MEDICAL SCHOOL, EX1 2LU, UK

A novel and efficient therapy platform based on biocompatible graphene nanocrystal (GNC) has been established. This GNC based combined chemo-photodynamic treatment also revealed reduced drug side effects compared to that of monotherapy. It showed excellent

corrosion resistance in terms of passivity and low toxicity both in vivo and in vitro systems.

Graphene nanocrystals (GNC) are nanomaterials with one atomic thick hybridized carbon layer1. GNCs are promising candidate for medicine and biological applications (e.g. DNA sequencing, cancer diagnosis and photodynamic antibacterial properties2,3) due to their chemical inertness and photoluminescence (PL) properties4. Although GNCs have low toxicity and excellent biocompatibility5 it is unclear whether the therapy efficacy of GNC can satisfy the requirement of practical clinical treatment in terms of sensitive, selective, rapid and cost effectiveness features. Previous studies suggest that graphene oxide (GO) if used with the combination of photodynamic and chemotherapy reduces the drug side effects and anticancer activity6. The major challenges in such kind of therapies are to augment the cytotoxicity of therapeutic agents to decrease the singlet oxygen quantum yield of the encapsulated/associated photosensitiser2 and the ability of dispersion in aqueous solution7. Therefore, optimisation of such therapy applications requires complete understanding of the electrochemical electron transfer properties of GNC.

This work feeds in to the considerable debate about the relative electrochemical activity and toxicity of GNC to overcome the problems existing in therapy applications. This study involves the synthesis and assessment of its corrosion resistance, complete blood count and serum biochemistry, histological studies of vital organs of treated animals and cellular response to the mouse fibroblast cell line, photodynamic therapy, chemotherapy and combination of these two therapies for drug loading. Accordingly the first report of toxicity and electrochemical activity with the therapeutic responses on a GNC are presented.

1. A. C. Ferrari et al., Nanoscale, 11, 4587–4810 (2015)2. J. Ge et al., Nature commun, 5, 1-8 (2014)3. Z. S. Qian et al., Biosens Bioelectron, 60, 64-70 (2014)4. J. Shen et al., Chem Commun, 31, 3686-3699 (2012)5. C. Yu et al., Biomaterials, 19, 5041-5048 (2014)6. W. Zhang et al., Biomaterials, 32, 8555-8561 (2011)7. L. Zhou et al., J. Photochem and Photobiol B: Biol, 135, 7-16 (2014)

Figure 1 Electrochemical behaviour of GNCs in 0.9 % NaCl solution (a) CV graph (b) OCP trends (c) Potentiodynamic polarisation curve.

Figure 2 Histological evaluation of the vital organs of the rabbits at 7 weeks after intravenous injection of the GNCs. Tissues of 5 and 15 mg/kg GNCs treated rabbits are similar with that of tissues of control group.

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Beaming of microwave surface waves EMILY YOUNG, ALASTAIR P. HIBBINS, J. ROY SAMBLES

A thin copper sheet, populated by an array of complementary split ring resonators (CSSRs), presents very strong surface wave beaming in orthogonal directions at two distinct frequencies, a phenomenon previously unreported in the literature.

Figure 1 shows the typical geometry used. Samples are etched onto 18 μm thick copper sheets backed by 50 μm thick Mylar in a square lattice of pitch d = 2 mm. The array is significantly thinner than existing single frequency beaming surfaces1,2,3.

Near-field stripped coaxial probes are used to both excite and detect surfaces waves. On one side is an excitation probe sweeping through frequency from 5 GHz to 70 GHz, on the other side a second probe is used to scan across the surface in the xy plane to measure both the magnitude and phase of the normal component of the electric field, Ez. Both probes were arranged normal to the sample, at a distance of ~ 500 μm from the surface.

Two frequencies display strong surface wave beaming, orthogonal to each other, as shown in figures 2 at 20.00 GHz and 49.15 GHz. The observed beaming frequencies are associated with the two lowest resonances of the CSSRs4,5, and are both sub-wavelength in width and approximately non-diverging.

This very narrow surface wave beaming on such thin, and easy to fabricate, samples may prove useful for engineering applications in the microwave frequency regime.

1. A. F. Matthews, A. F., Optics Communications 282, 1789 (2009)2. S. H. Kim et al., Physical Review B 83, 165109 (2011)3. K. J. Kim et al., Optics Express 22, 4050 (2014)4. F. Falcone et al., IEEE Microwave and Wireless Components Letters

14, 280 (2004)5. M. Navarro-Ca et al., Optics Express 17, 18184 (2009)

Figure 1 Schematic showing the experimental set up.

Figure 2 Experimental near-field scan results showing beaming at 20.00 GHz, and 49.15 GHz with the excitation probe at the centre of the plot.

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One of the main drivers for the EPSRC in developing the doctoral training model has been the desire to put industry front and centre in the development of PhD students. To that end, members of our Oversight Board from industry (QinetiQ, Oclaro, BAE Systems, Dstl and Flann Microwave) along with other collaborators from Thales and Sub10 Systems are delivering some of our intensive 2-day workshops (on ‘Sensors and Security’ and ‘Fundamentals of Acoustics’) and giving seminars on the perspectives of employment within industry of materials science postgraduates. To date, the feedback that we have gained from these interactions has been exceptionally positive, with our industrial colleagues strongly praising the quality of the students, their work, and their engagement with the industrial viewpoint.

Within the Centre the concept of joint supervision is stimulating stronger interactions between staff leading to new joint projects, the key example being a Programme Grant submission in magnetic

metamaterials to the EPSRC from three of the XM2 supervisors. These interactions are not simply within a single department, or just between theorists and experimentalists but progressively between materials scientists in Physics and Engineering. ‘Creativity@Home’ events are also engendering new thinking among some of the staff, as well as the XM2 students and post-doctoral researchers who are all invited to attend. Overall the cohort ethos and the associated training is transforming the operation of the functional materials researchers at Exeter.

Internationally, through our extensive recruitment procedure, we have attracted excellent students from Chile, China, Germany, Greece, India, Pakistan, Spain and the Ukraine. Some of our students have already visited Grenoble to use facilities, and existing collaborations in Sweden and Pakistan have been strengthened – all part of the development of an international presence for the Centre.

Impact

CONFERENCES AND PUBLICATIONS FROM YEAR 1

Magneto-optical observation of mutual phase-locking in a pair of spin-torque vortex oscillatorsP. S. Keatley, S. R. Sani, G. Hrkac, E. Burgos, S. M. Mohseni, P. Dürrenfeld, J. Åkerman, and R. J. HickenMagnetism 2015 – Leeds, UK.

Microwave emission characteristics of spin-Hall nano-oscillatorsT. Spicer, P. S. Keatley, P. Durrenfeld, A. Houshang, M. Ranjbar, A. A. Awad, R. K. Dumas, J. Akerman, and R. J. HickenMagnetism 2015 – Leeds, UK.

GO/ZIF Nanocomposites and Their Derivatives for Electrochemical ApplicationsL. Deng, Y. Zhu, and Y. Xia12th International Conference on Materials Chemistry (MC12) – York, UK.

Chiral Phenomena in the near-field of metamaterialsL. Barr, B. Tremain, A. Diaz-Rubio, E. Hendry and A. P. HibbinsRoyal Society International Scientific Meeting, Spatial transformations: from fundamentals to applications, Chicheley Hall. UK.

Investigation of surface wave beaming in complementary split ring resonatorsE. Young, J. A. Dockrey, J. R Sambles, A. P. Hibbins and C. R LawrenceRoyal Society International Scientific Meeting, Spatial transformations: from fundamentals to applications, Chicheley Hall. UK.

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An Industrial PerspectiveIndustrial research in the UK is highly dependent upon the recruitment of high-calibre scientists from academia, but this is rarely a straightforward process. Potential recruits are often poorly prepared for the shift into the commercial world, with only a hazy understanding of the differences between academic and industrial science, and a limited understanding of what an employer is truly looking for. This leads to many good candidates failing at interview – the first hurdle in their careers – and those that pass usually require lengthy on-the-job training in the most basic business skills before employers can unlock the true potential of their recruits.

XM2 is aimed at bridging this gap between industry and academia, providing employers with highly trained technologists who can make an immediate impact to their businesses whilst maintaining the scientific rigour and innovative thinking that UK businesses so desperately require. As a member of the Oversight Board, I can see the excellent progress that is being made. Through a mixture of research projects, classroom training, visits to industry and invited speakers, a vibrant team of researchers has been formed, pursuing fundamental research whilst maintaining an appreciation of its value to the outside world. The calibre of the students has been very impressive, with many publishing their work within months of joining the Centre, and they are exhibiting the love of problem-solving and trenchant questions that will make them excellent scientists and engineers. This is all due to the quality of their education and leadership, provided by some of the UK’s best academic researchers, all of whom are experts in their fields. XM2 is making impressive progress, preparing new blood for UK industry, and I’m looking forward to being involved in the new discoveries that lie ahead.

CHRIS LAWRENCEQinetiQ and Oversight Board Member

The Students’ PerspectiveWhen a student completes their undergraduate degree and steps out through the doors of their university they are faced with a wall of choices, the first of which is often ‘academia or industry?’ The answer to this question can seem to instantly define the rest of their career, but XM2, the Centre for Doctoral Training in Metamaterials in Exeter, opens doors to an array of opportunities, as the expertise gathered during the four years can easily lend itself both to university research and the world of business.

As a group of students in XM2 ourselves, we have found that working closely alongside people from all over the world is an opportunity that should not be missed. Each has their own area of expertise, but when solving particularly difficult problems together it becomes apparent that our combined knowledge is enough to tackle seemingly insurmountable challenges. This is what sets us apart – a broad range of people working independently and together, with the confidence to develop as individuals, and to collectively impact the science we do today and in the future.

STUDENT ADVISORY GROUPToby, Lauren, Tom and Erick

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