XM2 thesis submitted by Lauren Barr: Giving Metamaterials a Hand – Electromagnetic interactions in chiral metamaterials

Very many congratulations to the EPSRC CDT in Metamaterials PGR Lauren Barr for submitting her PhD thesis on “Giving Metamaterials a Hand – Electromagnetic interactions in chiral metamaterials”, supervised by Prof. Euan Hendry and Prof. Alastair Hibbins.

(The thesis title is a play-on-words, as the term chiral comes from the Greek word for hand, and describes things that have no mirror symmetry – like hands!)

Lauren previously published various articles on twisted-cross metamaterials (Scientific Reports, 2015) and near-field chiral interactions (Physical Review B, 2018),  on direct measurements of topological states (Nature Communications, 2017) and ideal Weyl points in microwave metamaterials (Science, 2018), and presented her results at many conferences, e.g. at Metamorphose 2015 (Ruka, Finland), NanoMeta2017 (Seefeld, Austria), CIMTEC 2016 (Perugia, Italy) and Metamaterials 2017 (Marseille, France).

Lauren moved to Exeter in 2014 to join the CDT after completing a Master’s degree in physics at Queen’s University Belfast. From day one, Lauren engaged fully with the opportunities the CDT offered to develop her portfolio of experiences beyond her research project by joining the student advisory group and the SPIE student chapter (now Exeter University Optics and Photonics Society (EUOPS)). Together with her peers, she successfully applied for an $8,000 grant to host an international OSA (Optical Society) network of students (IONS) conference at Exeter in 2019.

Lauren as SoapBox Science speaker in June 2017

Her passion for sharing knowledge and insipring the wider community is outstanding. Lauren coordinated and presented in two videos, highlighting her work to the general public in one case, and to a scientific audience in another; demonstrated light-based experiments at a Big Bang event (2016),  acted as a “Metabuddy” for Honiton College students (2107), and worked with children at the Girls into STEMM / Girls into Physics days (2017 and 2015, resp), was selected as a presenter for Soapbox Science” (2017) and “Pint of Science” (2018), as well as developing and running several unique outreach events such as the “Lighting up RAMM” event at Exeter’s Royal Albert Memorial Museum (2018).

Apart from the time spent working on my research, I also spent many happy hours outside the office. I have fantastic memories of Erick’s (often late-night) house-parties, Sathya’s (often spicy) lunches and Sam’s (slightly competitive) games nights. The CDT has given me the chance to learn much more about many different aspects of physics and research, and meet very cool people from all over the world. After completing my viva I hope to stay in Exeter a little longer, to work on some new projects and pass on some of my experiences to new students. Long-term, I just plan to keep doing the things I find most interesting. (Lauren Barr, August 2018)

Well done, Lauren! We’re very confident that you’ll pass your viva with excellent results.

PhD thesis abstract “Giving Metamaterials a Hand – Electromagnetic interactions in chiral metamaterials”:

The focus of this thesis is the interaction of electromagnetic fields with chiral structures in the microwave regime. Through this study, which focuses on three regimes of electromagnetic interactions, I aim to develop a deeper understanding of the consequences and manifestations of chiral interactions The structures are on the order of, or smaller than, the wavelength of the probing radiation. As the structures are chiral, they have broken inversion symmetry, and exist in two states where one is the mirror image of the other. The results in this thesis can have impacts on future optical communications technologies and methods of sensing biological molecules.

To begin with, the manipulation of the circular polarisation of a propagating beam by bilayer chiral metasurfaces is investigated. The metasurfaces consist of two layers of stacked crosses with a twist between top and bottom layers, forming chiral metamolecules. A broad frequency region of dispersionless polarisation rotation appears between two resonances, due to alignment between electric and magnetic dipoles. The dependence of this effect on the layer separation is studied for two similar metasurfaces.

Evanescent chiral electromagnetic fields are the focus of the next chapter. An array of chiral antennas produces chiral near-fields at their resonant frequency. Aligned and subwavelength helices placed within this field interact differently depending on the handedness of the field with respect to the handedness of the helices. This difference in interaction strength is measured for the helices and an effective medium model where multipolar interactions are forbidden. Comparison of these two systems leads to the conclusion that the contribution to a chiral interaction from multipolar modes is minimal, in contrast to previous publications.

The third study concentrates on the electromagnetic wave bound to an “infinitely long” metal helix. The helix has infinite-fold screw symmetry, and this leads to interesting features in the energy-dispersion of the waves it supports. The broad frequency range of high, tunable, dispersionless index is interpreted using a geometrical approach, and the factors that limit the bandwidth explained. A modified geometry is suggested for increased bandwidth.

The final part of the thesis is dedicated to future work, based on the results presented thus far. Three suggestions for future study are presented, including chiroptical signals from higher-order chiral arrangements, the effect of reflecting surfaces next to chiral objects and the possible use of orbital angular momentum for chiroptical measurements.

Honorary Fellow of IOP – Highest honour awarded to Prof Roy Sambles

Roy Sambles at his Christmas lecture in 2017: “The beauty of Science & Art”. The lecture was given jointly with with artist David Batchelor and was hosted by the Cornubian Arts & Science Trust (CAST) at Helston College.

Professor Roy Sambles (FRS) has been made an Honorary Fellow of the Institute of Physics (IOP) for his contributions in pure and applied physics, and as a superb ambassador for physics through outreach and an outstanding President of the IOP. He has made significant contributions to our understanding of the melting process, spin waves in metals, resistivity of thin metal films, molecular rectification, liquid crystal optics, plasmonics and microwave and acoustic metamaterials. Most recently, Roy has focused his research interests on metamaterials and their application to microwaves and sound, and he has led the EPSRC Doctoral Training Centre in Metamaterials from 2014 to 2017.

Roy has served on the Engineering and Physical Sciences Research Council (EPSRC), the Defence Science Advisory Committee and as the only academic scientist on the board of the Counter Terrorism Centre. He was a member of the 2014 Research Excellence Framework Panel for Physics and was recently appointed a Distinguished Visitor to the National Physical Laboratory. His outreach activities over the last seven years have reached more than 2,500 people and in 2013–17, when he was IOP President Elect, then President, he made some invaluable changes to the way the IOP works and interacts with its community.

Roy was made Honorary Fellow alongside professors Sheila Rowan and Sir Peter Williams – please see the IOP news for more information: http://www.iop.org/news/18/august/page_71876.html

Very many congratulations to Roy for this achievement. It reflects the impact his life-long commitment and passion for science has made. He has inspired generations of new scientists, and created an incredibly strong network across academia, industry, and governmental institutions. Roy’s dedication to understanding, discovering, and finding solutions has contributed significantly to the national and international research progression in his field and beyond. The EPSRC CDT in Metamaterials exists thanks to his vision and leadership and allows the University of Exeter to be at the forefront of developing the next generation of leaders in academia and industry.

Stargazing on Dartmoor

The recent nice weather spell in England’s Southwest allowed our the CDT in Metamaterials PGRs and Exeter University Optical Society (EUOPS) members to finally set out with telescopes and binoculars on the long-planned camping trip to the nearby National Park, Dartmoor, for a stargazing night.  They  teamed up with non-CDT colleagues to use this amazing opportunity to share knowledge about astronomy, and to enjoy some time away from the office with each other.

Thanks to Henry Fernandez for organising the event – and to Elizabeth Martin (image 1 & 2) and Rosamund Herapath (image 3) for the night shots displayed below.

Image 1 (courtesy of Elizabeth Martin)
Image 2 (courtesy of Elizabeth Martin)
Image 3 (courtesy of Rosamund Herapath)


 

XM2 thesis submitted by Chris King: Designing non-scattering graded-index media

Very many congratulations to Chris King for submitting his PhD thesis on Designing non-scattering graded-index media”, supervised by Dr Simon Horsley and Dr Tom Philbin.

Chris joined the CDT in 2014, following his Masters degree in Mathematics at the University of Oxford.  As Student Advisory Group (SAG) chair (2015/16 & 2016/17) he successfully linked the postgraduate reseachers  and the CDT Management and Oversight Boards, managing differing viewpoints and expectations and ensuring the PGRs’ voice is represented appropriately across the cohorts.

His work was previously published in Physical Review B (Electromagnetic interactions in a pair of coupled split-ring resonators), Physical Review Letters (Perfect Transmission through Disordered Media), and the Journal of Optics (Zero reflection and transmission in graded index media, Wave Propagation in complex coordinates). Chris also presented his research at the Met Office Academic Partnership (Exeter, MOAP) event (2018), the Defence and Security Doctoral Symposium (Swindon, 2017), and the Metamaterials conferences Metamaterials’2017 (Marseille, 2017) and Meta 2016 (Torremolinos, Spain).

Over the past three years, Chris shared his passion for science with the wider public on a number of occasions, such as the Sidmouth Science Festival, the Big Bang South West and the first PGR-led International Day of Light event at the University of Exeter. In addition, he acted as a role model for the next generation of potential scientists through his engagement with Exeter’s Maths School.

Chris will continue his career as a theoretical physicist with a keener focus on commercial applications at QinetiQ, Farnborough, starting 3rd September 2018.

Well done Chris! There is no doubt you will pass the viva swimmingly. We look forward to welcoming you back at graduation day and for future collaborations.

“Two particular highlights for me as a scientist during my time in the CDT were 1) publishing papers – there was no better feeling than seeing my name at the top of journal article, and 2) presenting at conferences – the CDT program gave me the confidence for this.  I’d advise future PGRs to take every opportunity to practise talking about your work, be it formally in conferences and group meetings, or informally to colleagues in the office. Also, don’t stop learning- this probably sounds slightly strange, what I mean is when you’re stuck on something, don’t continue banging your head against the wall- learn and try something new, then maybe return to the original problem with a clear head and wider knowledge.” (Chris King, 6 August 2018)

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PhD thesis abstract: Designing non-scattering graded-index media

With recent advances in metamaterials research, scientists are increasingly trying to understand how interesting wave phenomena emerge from new materials. A key branch of this subject is transformation optics: using coordinate transformations to design materials which don’t scatter light, used, for example, in the design of cloaking devices. This thesis is a study of various mathematical techniques, such as transformation optics, for designing non-scattering materials.

The materials studied in this thesis are characterised by their macroscopic electromagnetic material properties; their permittivity and permeability. These quantities will be assumed isotropic, and to vary smoothly in space (such media being described as `graded-index’). These inhomogeneous media can be understood as the limit of a stack of different infinitesimally thin homogeneous blocks. However, the theory of graded-index media is analytically more tractable than the theory of piecewise media than the piecewise homogeneous permittivity and permeability profiles that arise from placing blocks of different materials side by side.

The first part of the thesis introduces the necessary background for the rest of the thesis. Chapter 1 introduces the background electromagnetic theory used throughout the rest of the work and chapter 2 is a review of some of the existing literature on designing non-scattering media including an introduction to metamaterials, which are one route to realising graded-index media.

The second part of the thesis concerns the use of phase-integral methods for designing planar, reflectionless media, inhomogeneous in one dimension. Chapter 3 introduces the phase-integral method and describes how it can be used to understand reflection in the complex position plane. Chapter 4 uses the phase-integral method to derive a large family of index profiles reflectionless from one side for all frequencies of light and angles of incidence. Chapter 5 uses the phase-integral method to calculate exact reflection coefficients for some example index profiles. Chapter 6 is concerned with designing media which, in addition to being reflectionless, transmit all incident light. It is found that this perfect transmission property is exhibited by even very highly disordered media. Chapter 7 looks further at the reflectionless media designed in chapter 4, deriving a subfamily which, in addition to being non-reflecting, are also perfectly absorbing.

 The third and final part of the thesis deals with the design of planar, non-scattering media which are inhomogeneous in two dimensions. One way to handle this problem is to write the electromagnetic field in terms of its amplitude and phase, and use local conservation of energy in a lossless medium to understand how the amplitude and phase are related to each other. Chapter 8 uses this idea to find explicit expressions for non-scattering index profiles, including a generalisation of an existing version of transformation optics, and the design of a `beam-shifter’. Chapter 9 uses the characteristic method to solve the relationship between amplitude and phase and how it can be used to design non-scattering profiles, such as periodic media which don’t diffract. Finally, chapter 10 discusses nodes of the electromagnetic field and how they can arise from sending a wave onto a medium, in particular looking into the possibility of diffraction of a plane wave from a periodic structure into a pair of complementary modes in transmission; the corresponding transmitted field of which contains nodes.

 

Zahid Hussain’s research visit at the TU Munich: MOF-Derived Bimetal Oxide/Carbon (M-MO/C) Nanocomposites for Solar Light Driven Applications

Zahid Hussain started his PhD in the EPSRC CDT in Metamaterials in 2016 and was invited to join the Catalysis Research Center at the Technical University, Munich, from January to December 2018.

This fantastic opportunity aims to derive bi-metal oxide nanocomposites from metal organic frameworks (MOFs) for high efficient solar light driven applications. Zahid got in touch recently to provide us with an update on his experiences which we’d like to share with you:

“During my stay at Technical University in Munich, I divided my project in three phases. Firstly, I synthesized titanium based metal organic framework (MOF) NH2-MIL-125 and studied the structural and textual properties. For this purpose, basic characterization techniques were employed. Once I confirmed that the MOF structure is stable under ambient conditions, I used it for photocatalytic dye degradation of methylene blue (MB) and H2 evolution reaction (HER). In the second part, I derived TiO2/C composites from the above mentioned MOF upon calcination at high temperature. I used different temperatures and gaseous atmospheres to optimize the crystal structure, morphologies and energy band gaps. Once the MOF derived TiO2/C composites were synthesized, I used them for visible light photocatalysis and found that the derived composites show much higher photocatalytic dye degradation and H2 production efficiency as compared to the precursor MOF. Moreover, the MOF structure was not stable upon chemical interaction with MB dye molecules.

In the second phase of the project, I studied the synergistic effect of bi-metallic MOF and derived bimetal oxide/Carbon composites. For this work, I selected a transition metal such as Copper (Cu) and introduced it during the synthesis of Ti-MOF to achieve bimetallic MOFs with different molar ratios. For comparison, I followed two synthesis routes: 1) direct mixing of two metal ions during synthesis and 2) synthesis with loading of second metal ion after the synthesis of Ti-MOF. The synthesized bimetallic MOFs were carbonized to produce TiO2/CuO/C nanocomposites via high temperature pyrolysis under different conditions. The obtained nanocomposites were fully characterized to understand the underlying synthesis mechanisms and the correlation of the MOF precursors and the derived bimetal oxide composites. This is an ongoing project and the final research findings will be published in a peer reviewed journal very soon.

The third phase of my research visit at the TU Munich is to use these MOF derived nanocomposites for photovoltaic applications. This work is expected to be completed by the end of 2018.

During my stay, I also participate in public outreach activities. We organized a show lecture: “The Crazy Chemist’s Guide to the Universe” for general audience and undergraduate students to demonstrate how our universe is made of chemical elements which we use and experience in our daily life. We are planning to organize another activity to show that being a “concerned scientist’’, how scientists can play a constructive role in the betterment of society.

I am thankful to the CDT in Metamaterials (XM2) and my supervisors to allow me to develop these very useful research collaborations with the TU Munich. It provided me with an excellent opportunity to develop a range of new experimental research skills.”

XM2 thesis submitted by Ben Ash: Locally Resonant Metamaterial for Surface Acoustic Waves

Fig 1: CDT in Metamaterials PGRs Erick Burgos-Parra (left) and Ben Ash (right) submitting their PhD thesis on 2 August 2018.

Very many congratulations to our 4th year PGR Ben Ash, who recently submitted his PhD thesis on “Locally Resonant Metamaterial for Surface Acoustic Waves”. Ben informed us that he has accepted an offer for his dream job as a Microsystems engineer which starts on Monday 13th August with Oxford HighQ, http://www.oxfordhighq.com/, a start-up which is developing ‘microcavities’ to sense, trap and measure the properties of nanoparticles and chemicals. His main focus will be the optimisation of the fabrication of these ‘microcavities’.

Previously, Ben published his work in a Nature Communicatiosn article A highly attenuating and frequency tailorable annular hole phononic crystal for surface acoustic waves, Nat Commun, volume 8, no. 1, DOI:10.1038/s41467-017-00278-0.

“It was a pleasure to be a part of the CDT. I appreciated the various training I received, both technical and otherwise, which have made me a more rounded researcher of physics and engineering. To be one of many PhD students within a cohort was fantastic. I have no doubt that the collaborative discussions I had resulted in my research being of a higher quality, which has led to me landing my dream job.” (Ben Ash, 8 August 2018)

Well done Ben! We have no doubt that you will pass your viva splendidly.

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Abstract of PhD thesis Locally Resonant Metamaterial for Surface Acoustic Waves

The control of surface acoustic waves (SAWs) using arrays of annular holes was investigated both experimentally and through numerical modelling. Periodic elastic composites, phononic crystals (PnCs), were designed using these annular holes as constituent elements. Local resonances associated with the annular hole structure were found to induce phonon bandgaps of a highly frequency tailorable nature, at frequencies where radiation of acoustic energy into the bulk of the substrate medium is avoided. These bandgaps are numerically demonstrated to exhibit order-of-magnitude improved extinction ratios for finite numbers of PnC elements, relative to the commonly used cylindrical pillar architecture. Devices fabricated on commercially available lithium niobate SAW delay lines verify the predicted behaviour. Through laser knife-edge detector vibrometry, a bandgap attenuation of 24.5 dB at 97 MHz is measured, in excellent agreement with finite element method (FEM) simulations.

Fig 2: Total displacement surface plots for modes A and B for the entire unit cell and for only the locally resonant structure. See publication A highly attenuating and frequency tailorable annular hole phononic crystal for surface acoustic waves, Nat Commun, volume 8, no. 1, DOI:10.1038/s41467-017-00278-0

The first reported experimental evidence of subwavelength confinement of propagating SAWs was realised using the same annular hole PnC concept. Defect holes of perturbed resonant frequencies are included within the PnC to define waveguides and cavities. Confinement within these defects was demonstrated to occur at subwavelength frequencies which was experimentally observed in fabricated cavities using standard SAW transducers, as measured by laser Doppler vibrometry. The success of this result was attributed to the impedance matching of hybridised modes to Rayleigh SAWs in un-patterned substrates at the defect resonance. The work here has the potential to transform the field by providing a method to enhance SAW interactions, which is a route towards the realisation of many lab-on-chip applications.

Finally, the use of annular hole arrays as negative refraction metamaterials was investigated. The symmetry was broken of the unit cells by alternating either the locally resonant frequencies or the distance separating the constituent elements. Both methods, called the bi-dispersive and bi-periodic methods, were numerically demonstrated to exhibit negative group velocity bands within the first Brillouin zone. Preliminary experimental results show that the design has the potential to be used in superlensing, where a SAW spot was imaged over a subwavelength flat lens. Future research looks to demonstrate that this result can be attributed to negative refraction.

New Publication: Investigation of magnetic droplet solitons using x-ray holography with extended references

Congratulations to Erick Burgos-Parra for his recent Scientific Reports publication on the investigation of magnetic droplet solitons using x-ray holography with extended references: https://www.nature.com/articles/s41598-018-29856-y

A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [Co/Ni]x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at −33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0–50 mT), where it is expected to observe regimes of the unstable droplet formation.

(a) Set of three coplanar waveguide used for HERALDO. The oval shaped region is the Si3N4 membrane. (b) Zoom of the top section a CPW shown in (a). The red arrow depicts the position where the transversal cut shown in (c). (c) Schematic of a transversal cut along the red arrow in (b) where the position of the nano contact is shown. (d) Schematic of a section of the 16 × 8 μm2 mesa layer containing the nano-contact orthogonal pseudo spin-valve, where the Co/Ni multilayer acts as the free layer and the Co layer as the pinned layer. In this work devices with Cu nano-contacs of 90 and 110 nm diameter were studied. The red arrows indicate the magnetization of the magnetic layers after appliying a magnetic field ranging within 20–3000 mT out-of-plane (blue arrow). (e) Au layer covering one side of the Si3N4 membrane. An aperture of 5 μm diameter and a reference slit of 6 μm in length and ∼60 nm width were milled using a focused ion beam. The pseudo spin-valve is located on the opposite side of the Si3N4 membrane. (f) Schematic set up for HERALDO measurements with an external magnetic field. The sample is positioned in the middle of a portable octupole magnet system (POMS) and the coherent x-rays from the synchrotron source pass through the aperture and the reference slit. The resulting diffraction pattern is captured by a CCD camera at a distance ∼60 cm behind the sample, at the end of the beam-line. The coplanar waveguide (CPW) supplies the DC current that passes through the magnetic layers and generates the STT required to form the droplet soliton.

>> See all XM² publications here: http://emps.exeter.ac.uk/metamaterials/research/publications/ >>