The University of Exeter's Centre for Metamaterial Research and Innovation is one of the UK's largest hubs of metamaterial research. Our academic expertise spans electromagnetism (from visible and infra-red through to THz and microwave), acoustics and fluidics, and the materials we work with have wide application, e.g., imaging, sensing and spectroscopy, acoustic and RF signature reduction, energy storage and harvesting.
Our academics have suggested some further exciting projects that we hope to run next year. Please review these if you have, or plan to apply for, PhD funding, or can self-fund your programme of study. There are a number of routes to secure funding for PhD study - see here.
If you are yet to secure funding, you are nevertheless encouraged to look through the project below, and make an application - we will eneadvor to source funding for excellent candidates. Funding for studentships can become available at any point during the year, therefore please apply early and we will contact you with opportunities as they arise.
All potential applicants are strongly encouraged to contact the named supervisors to discuss the projects in more detail, via firstname.lastname@example.org
Optical and Photonic Metamaterials
Metamaterials are artificially structured materials with properties beyond those found in nature.1 These properties (electromagnetic, acoustic or mechanical) arise due to engineered sub-wavelength elements. Large-area manufacturing of metamaterials is challenging. The complex nature of metamaterials presents demanding process requirements such as: accurate sub-wavelength geometries which are heterogeneous and sub-wavelength, multi-material, large-area and in produced in rapid timeframes. A plethora of lithographic and printing technologies exist (Fig 1, left), yet no existing approach provides an all-encompassing solution.2 For example, there are often significant tradeoffs: ultrahigh resolution (<10 nm) direct-write patterning is often exorbitantly expensive and low throughput (i.e. e-beam lithography); or, high throughput, low cost and high resolution patterning is often pattern inflexible (i.e. roll-to-roll nanoimprint lithography) such that only one design is imparted.
Two photon polymerisation lithography (TPL) is an attractive technique used to accurately direct-write 3D structures in polymers through the intrinsic nonlinearity of multiphoton absorption—near-infrared femtosecond pulses trigger solidification confined to only the focal volume (voxel). In this way, it is often considered to be a 3D printer on the nano-and-micro scale.3 Unfortunately, it’s low throughput (‘one voxel at a time’), thereby limiting its use to low volume manufacturing. In contrast, holographic interference lithography (HIL) can create periodic (long range order) over large areas through the interference of two or more wavefronts. The photoresist is exposed in the 3D volumes of constructive interference over large areas, however the patterning can be inflexible.4
In this project, we will combine the advantages of TPL and HIL to develop two-photon holographic interference lithography (TP-HIL) for large area manufacturing of 3D metamaterials. We will exploit multi-beam interference lithography—with one or more wavefronts controllable through a high resolution spatial light modulator—and two-photon absorption with a tailored photoresist (Fig.1, right). This will look to increase throughput (parallelisation) while maintaining high resolution pattern complexity. Further, we will investigate novel multi-material polymers /resists, for both resolution enhancement and non-polymeric final structures, for example: metal-nanostructure-loaded polymeric structures for two-photon initiated metal salt reduction, which has been shown to produce 3D metallic nanostructures. The envisaged system will have high resolution (<100 nm) direct-write 3D manufacturing capability but at the throughput comparable to parallelized lithographic systems.
The research spans fundamental optical physics, materials science, through to applications, and the student will develop a diverse skillset during the PhD project, including: computational optics, electromagnetic simulation (incl. Lumerical FDTD and COMSOL), nanofabrication within a state-of-the-art cleanroom, systems construction, electro-optic systems characterisation, and advanced data analysis
Read the full project description Abstract for 3D optical mm using two photon
- Please contact Dr Calum Williams for more information
References Kadic, M. et al. Nat Rev Phys 1, 198–210 (2019)
Biosensors are vital across a wealth of industries, from healthcare diagnostics, pharmaceuticals to food safety. Multiple complex techniques are currently needed in order to and monitor, and differentiate, specific analytes in real-time. A label-free, compact and highly sensitive biosensing platform capable of detecting major application-specific analytes in real-time would be highly desirable. For example, a dynamic signaling biosensor functionalized to detect major human diseases would be invaluable as a point-of-care diagnostic tool in 21st century patient-focused healthcare.
In this project, we will develop a new class of biosensor based on space-folded all-dielectric metaphotonics (Faraji-Dana, M., Arbabi, E., Arbabi, A. et al. Nat Commun 9, 4196 (2018)) integrated atop a CMOS image sensor (CIS). Leveraging design concepts in planar integrated optics, metasurface design and spectral sensing, the proposed platform will consist of a multitude of metasurface optical elements (MOEs) lithographically patterned on a thin glass substrate which in-turn is built upon a CIS. MOEs can be designed to engineer light-matter interactions in order to locally control the spectral, amplitude, polarization and phase of light—along with enhancing fluorescence and Raman signals. Top-down lithographic patterning means all MOEs can be fabricated in a single / double-step (two sides of substrate), reducing cost while permitting design complexity. Integrating all optics either side of a single glass substrate folds the optical path, greatly reducing size, and the CIS provides cost effective (<£25) electro-optic readout capability.
Read the full project description Space-folded metaphotonics for multiplexed biosensing on a single chip.
Colour cameras utilize absorptive filter arrays atop the image sensor to spectrally discriminate light into red, green and blue (RGB) bands. These colour filter arrays (CFAs) are typically arranged in 2x2 unit cells (RGGB Bayer kernel) and tessellated across the image sensor (Fig.1A). Albeit providing spectral sensitivity, this spatial arrangement means only 50% of the total incident light reaches the green pixels, 25% the blue pixels and 25% the green pixels. Further, as the absorptive dyes themselves only transmit ~40% light, it means in combination ~70% of incident light upon the sensor is lost.
In recent years, nanophotonic colour routers (light sorters) have been proposed as an alternative filtering approach to absorptive CFAs.1,3 Colour routers split the incident light into separate colours (wavelengths) and route the energy to specific pixels (Fig.1B). This meta-optic is composed of many sub-wavelength scatterers, with a designed distribution such that light is routed to different output positions depending on its wavelength. 4 Theoretical efficiencies as high as ~95% have been reported3 yet experimental realization is challenging.
In this project, we will develop generalized light sorters based on inverse-designed meta-optics to efficiently route different wavelengths to different spatial positions. Our approach will employ 2D and ~2.5D meta-optic approaches (Fig.1C) in order to increase manufacturability while maintaining high optical performance (transmission efficiency, spectral sensitivity, angular sensitivity). We will extend our light sorter design scheme to: (1) longer waveband imaging (i.e. short-wave and mid-wave infrared); (2) polarimetric imaging, and (3) plenoptic (depth) imaging.
The research spans fundamental optical physics through to applications, and the student will develop a diverse skillset during the PhD project, including: computational optics, electromagnetic simulation (incl. Lumerical FDTD and COMSOL), nanofabrication within a state-of-the-art cleanroom (incl. e-beam lithography, physical vapour deposition etc.), electro-optic systems characterisation, validation of performance, and advanced data analysis.
Read the full project description Abstract for Inverse-designed meta-optics for light sorting.
Please contact the supervisor, Dr Calum Williams for more information.
Mid-infrared (MIR) spectroscopy reveals nearly all chemical/biological substances through identification of their unique vibrational absorption signatures (molecular fingerprints)—essential for biomedical diagnostics, remote sensing and environmental monitoring. Within the broadband MIR spectral region (wavelengths ~2-20 μm), the mid-wave infrared (~3-5 μm) and long-wave infrared (~7-14 μm) are both vital for application-agnostic molecular fingerprinting, yet no miniaturised sensing technology encompasses the entire waveband.1 Unfortunately, MIR spectroscopic sensing is considered a cumbersome, bulky technique confined to the laboratory. Significant technological challenges remain for scaling-down traditional spectroscopy systems.2,3
Metasurfaces are the 2D equivalent of 3D metamaterials: artificially engineered materials with properties impossible to find in nature. Integrated optical metasurfaces utilise nanoscale light-matter interactions within arrays of sub-wavelength structures (meta-atoms) to manipulate electromagnetic waves, all within the format of photonic integrated circuit architectures.4-6
This interdisciplinary project will develop integrated metasurfaces within photonic microsystems, for unconventional multifunctional and miniaturised MIR spectroscopy. We will investigate inverse-designed metasurfaces for: (1) the enhancement of targeted vibrational absorption modes; (2) transduction of multimodal phenomena such as the ultrasensitive photothermal effect;7 and (3) the integration of alternate sensing architectures, for example near-field standing-wave spectroscopy.8 The resulting suite of technologies will enable miniaturised, ultrasensitive, receptor-free in-situ (near-field) and remote (far-field) sensing capability of gaseous / liquid analytes.
The research spans fundamental optical physics through to applications, and the student will develop a diverse skillset during the PhD project, including: computational optics, electromagnetic simulation (incl. Lumerical FDTD and COMSOL), nanofabrication within a state-of-the-art cleanroom (incl. e-beam lithography, physical vapour deposition etc.), electro-optic systems characterisation, validation of sensing performance, and advanced data analysis.
- Please contact Dr Calum Williams for more information
References Haas, J. & Mizaikoff, B. Advances in Mid-Infrared Spectroscopy for Chemical Analysis. Annual Review of Analytical Chemistry 9, 45–68 (2016)
Photocatalysis plays a pivotal role in addressing environmental concerns and advancing the quest for achieving a net-zero future. However, one of the major bottlenecks in photocatalytic processes is the limited optical activity of the catalytic metals employed. This project aims to revolutionize photocatalysis by leveraging the power of Artificial Intelligence (AI) to design novel catalytic metal-based nanostructures that significantly enhance the interaction between light and matter. By doing so, we strive to elevate energy efficiency in photocatalytic reactions, with a specific focus on CO2 reduction and hydrogen production.
Read the full project description Two-photon holographic interference lithography
Please contact the supervisor, Dr Changxu Liu for more information.
Colour cameras utilize absorptive filter arrays atop the image sensor to spectrally discriminate light into red, green and blue (RGB) bands. These colour filter arrays (CFAs) are typically arranged in 2x2 unit cells and tessellated across the image sensor. Albeit providing spectral sensitivity, this spatial arrangement means only 50% of the total incident light reaches the green pixels, 25% the blue pixels and 25% the green pixels. Further, as the absorptive dyes themselves only transmit ~40% light, it means in combination ~70% of incident light upon the sensor is lost.
In recent years, nanophotonic colour routers (light sorters) have been proposed as an alternative filtering approach to absorptive CFAs. Rather than absorb, colour routers split the incident light into separate colours (wavelengths) and route the energy to specific pixels. Optical efficiencies as high as ~95% have been reported.
In this project, we will develop generalized light sorters based on inverse-designed meta-optics to efficiently route different wavelengths to different spatial positions. Our approach will employ 2D and ~2.5D meta-optics in order to increase manufacturability while maintaining high optical performance We envisage this project may expand to other imaging and sensing architectures which could benefit from light sorting, including plenoptic imaging and chiral sensing.
Read the full project description Inverse-designed meta-optics for light sorting.
Acoustics, phononic and mechanical metamaterials
Our civilization is defined by the functionalities delivered by devices, ranging from smartphones to airplanes. These functionalities are however constrained by materials available to engineers, when constructing and indeed even conceiving a device. Radically new dynamical properties and advanced functionalities can be created by tailor-tuning the spectra of wave excitations in structured media – so-called metamaterials. Among other waves, ‘surface acoustic waves’ have been investigated for over one hundred years and currently used e.g. for a wide and diverse range of functions, e.g. analogue signal processing in mobile phones. Recently, the field of metamaterials research has expanded to acoustic waves. To date, however, there have been very few suggested ways of designing acoustic metamaterials that can be dynamically reconfigured and tuned. Integration with magnetic materials, well known for their ability to store information e.g. in magnetic hard disk drives, offers an exciting route for achieving non-volatile tuning of acoustic metamaterials. We strive to develop a new class of magneto-acoustic metamaterials in which the role of their building blocks (“meta-atoms”) is played by magneto-acoustic resonators [1,2]. Such metamaterials will add magnetic field tunability to structures aimed to control the propagation of surface acoustic waves, opening intriguing opportunities both in fundamental science and technology. The memory phenomenon inherent to magnetism will enable significant energy savings in non-volatile magneto-acoustic data and signal processing devices. For instance, they would be instantly bootable and could be more easily integrated with the existing magnetic data storage devices. From the point of view of fundamental science, the magneto-acoustic metamaterials will serve as an excellent test bed for studying the physics of wave propagation in non-uniform and non-stationary media.
- Please contact Prof Volodymyr Kruglyak, Prof Geoff Nash, Dr. A. V. Shytov, and / or Dr. S. A. R. Horsley (depending on whether an experimental, theoretical or computational project is preferred) via email@example.com for more information.
 O. S. Latcham, et al, “Controlling acoustic waves using magneto-elastic Fano resonances”, Appl. Phys. Lett. 115, 082403 (2019).
 O. S. Latcham, et al, “Hybrid magnetoacoustic metamaterials for ultrasound control”, Appl. Phys. Lett. 117, 102402 (2020).
This project aims to understand the mechanical and dynamic properties of a novel composite made of recycled rubber and graphene to be used for vibration isolation of structures or parts of them. We aim to design, manufacture, and test novel low-carbon lightweight devices. Dynamic tests will be performed to determine an optimal formula for graphene coated recycled rubber pads; curing conditions will be investigated. Experimental results will inform the development of a theoretical model that will be used to design the devices.
We envisage applications in various sectors including seismic isolation, oil and gas industries, railway industry and sound and vibration sectors.
Please contact Prof. Maria Rosaria Marsico or Dr. J. Londono via firstname.lastname@example.org for more information.
• Rivera ED, Londoño Monsalve J, Craciun MF, Marsico MR. (2023) Experimental Assessment of the Mechanical Performance of Graphene Nanoplatelets Coated Polymers, Advanced Engineering Materials, DOI:10.1002/adem.202300830.
RF, microwave and mm-wave metamaterials
Waves propagating in quasicrystals are at a curious point where neither Bloch’s theorem (applicable to periodic media), nor the diffusion approximation (applicable to random media) are appropriate. While quasicrystals can be constructed from a deterministic rule, they do not exhibit the translational invariance that allows propagation to be understood in terms of a single unit cell. However, it has been known for some time that diffraction from photonic quasicrystals exhibits similar sharp peaks as are observed for true periodic crystals, due to the presence of long range order. This project explores the propagation of electromagnetic waves on the surface of 2D quasicrystals. Such lattices (e.g. those generated using the Fibonacci sequence in particular) have an interesting link to topology via Chern numbers, and fractals. Although little experimental work has been done to explore these aperiodic structures, our fabrication and characterisation techniques for exploration in the microwave domain naturally lend themselves to this project, and to accompany this experimental strand, there will be a challenging programme of numerical and analytical modelling work for the student to undertake.
Metamaterial structures have been designed at Exeter to strongly scatter electromagnetic radiation used for radar detection, telecommunications and the internet of things. These can be used for making small, hard to detect objects like quadcopter drones appear more visible to radar, which is useful for e.g. airport security. Alternatively if they are driven electrically they can be used to produce fake signals which can confuse radar and disguise where and what an object is. Finally these can be attached to sensors and dispersed across an environment to provide data over certain environmental conditions across a large area.
This applied project will combine metamaterial physics, electrical engineering and potentially the design of various flier structures in order to design, create and test the various scattering systems, both in an isolated form, and connected to
- Please contact Dr Alex Powell via email@example.com for more information
- Read the full project description: AP2 Truly Broadband 3D metamaterials
3D printed metaparticles based on platonic solids for isotropic, multimode microwave scattering, AW Powell et. al. EuCAP 2022 proceedings, 1-4
Broadband radar invisibility with time-dependent metasurfaces, V. Kozlov, D. Vovchuk & P. Ginzburg Scientific Reports, 11, 14187 (2021)
Three-dimensional electronic microfliers inspired by wind-dispersed seeds, B. H. Kim et al., Nat. 2021 5977877, vol. 597, no. 7877, pp. 503–510, Sep. 2021.
Electromagnetic metamaterials have revolutionised the design of antennas, reflectarrays and many other electromagnetic components over the last decade. Currently however, once a material is designed and fabricated, its properties are fixed - unless each component is fitted with a complex array of electronics, which adds significant weight, bulk and cost. A family of materials that could be fabricated, and then readily altered in situ, would therefore have a myriad of applications in fields as diverse as communications, healthcare or environmental monitoring.
One route to achieving this is to use surfaces based on origami or kirigami techniques that can be folded or manipulated at will: These materials alter their geometries in a controlled way on the application of a stimulus, typically heat, light or moisture. This is generally achieved via the stimulus causing greater expansion in one part of the material (or composite material) than another, leading to bending. This process can be reversible or produce permanent changes, can create highly complex structures and has already led to innovations in microfluidics, energy storage and soft robotics.
This project will explore the possibilities of hybridising electromagnetic metamaterial components and origami/kirigami materials, to create a new family of reconfigurable structures, with a view to applications in telecommunications and beyond, and the expectation of the discovery of much new physics to drive future research.
The project will pull together elements of electromagnetic physics, mechanical engineering, and advanced materials and manufacturing. There are many exciting areas to explore, but the nature of the work will involve expanding your knowledge outside that of a single undergraduate discipline. Therefore this project would be ideal for an enthusiastic, highly motivated candidate looking to pursue exciting multidisciplinary, curiosity-driven research with real-world applications.
Magnonics, spintronics and magnetic metamaterials
Magnetic Random Access Memory based upon nanoscale Magnetic Tunnel Junctions (MTJs) has been proposed as an alternative to CMOS-based memory devices such as DRAM. Within the MTJ, the magnetization of a soft magnetic layer is switched by Spin-Orbit Torque (SOT). However, switching a “bit” stored in MRAM currently requires ~1000 times more energy than a bit stored in a CMOS device. In this project we will use Surface Acoustic Wave (SAW)-induced Ferromagnetic Resonance (FMR) in conjunction with SOT to reduce the switching energy of MRAM. An interdigitated transducer (IDT) defined on a piezoelectric substrate generates a SAW that propagates towards and strains the soft CoFeB magnetic layer of the MTJ. Due to magnetoelastic coupling, a few tens of cycles of SAW at the FMR frequency (~10 GHz) will cause the magnetization to precess at large amplitude so that a much reduced SOT is needed for the magnetization to switch. Test devices will be fabricated at Virginia Commonwealth University and the Massachusetts Institute of Technology before the time dependent magnetization dynamics are studied in Exeter by means of time resolved scanning Kerr microscopy.
View the abstract for Abstract for Energy efficient Switching of Spin-Orbit Torque Memory Devices
- Please contact Prof Robert Hicken via firstname.lastname@example.org for more information.
Spin waves (elementary excitations of magnetically ordered materials) boast extreme nonlinearity and modest loss while having micrometre to nanometre wavelengths at GHz frequencies. This presents a unique path towards miniature and powerful yet energy efficient devices for unconventional computing. In this project, you will seek to combine two inherently energy-efficient technology paradigms: (i) magnonics (using spin waves to process signals and data) and (ii) neuromorphic computing (using large-scale integrated systems and analog circuits to solve data-driven problems in a brain-like manner). Going well beyond existing paradigms, we will use nanoscale chiral magnonic resonators  as building blocks of artificial neural networks. The power of the networks will be demonstrated by creating magnonics versions of field programmable gate arrays, reservoir computers, and recurrent neural networks. The ultimate efficiency of the devices will be achieved by (a) maximising their magnetic nonlinearity (via spin wave power focusing within chiral magnonic resonators of minimal intrinsic loss); (b) using epitaxial yttrium iron garnet (YIG), which has the lowest known magnetic damping allowed by physics, for thin film magnonic media and resonators; and (c) using wireless delivery of power (minimising Ohmic loss in interconnects). Sensitive to the resonators’ micromagnetic states, such artificial neural networks will be conveniently programmable and trainable within existing paradigms of magnetic data storage. The latter includes magnetic random-access memory (MRAM), which is already compatible with CMOS, while compatibility with other technology paradigms of spintronics will also be sought, explored, and exploited.
- Please contact Prof Volodymyr Kruglyak via email@example.com for more information.
 V. V. Kruglyak, “Chiral magnonic resonators: Rediscovering the basic magnetic chirality in magnonics”, Appl. Phys. Lett. 119, 200502 (2021).
Magnetic steels and alloys are widely used in infrastructure systems and industrial equipment and processes, including coiled-tubing pipes for well intervention and drilling in the oil and gas industry, and in bridge and crane cables, to name a few. These structures experience extreme forms of stresses and mechanical damage during manufacturing, deployment and operation. It is critical to regularly inspect these structures to ensure safety of operations, for maintenance, and to predict their remaining life to avoid the risks of unexpected failures.
This project involves the development of high-sensitivity, novel non-destructive electro-magnetic/acoustic sensors and methods for real-time inspection and early warning detection of defects and anomalies in steel structures. The research involves multi-physics modelling and simulation of the electromagnetic sensors and materials. The research can also involve the experimental development of sensor prototypes and their characterisation using steel samples provided by industrial partners.
- Please contact Dr Mustafa Aziz via firstname.lastname@example.org for more information.
Antiferromagnetic materials have generated great excitement due to their ability to conduct pure spin currents over micron scale distances. In materials such as NiO, that have biaxial magnetic anisotropy, it has been proposed that spin amplification may also be possible, paving the way to more energy efficient operation of magnetic random access memory (MRAM) devices. In this project, the relationship between the propagation of spin current and the underlying antiferromagnetic spin wave excitations will be explored so that the optimum conditions for spin amplification can be determined and realised. Ultimately, it may be possible to realise multi-stage spin amplification through the creation of multi-layered antiferromagnetic metamaterials. Antiferromagnetic spin waves will be detected by means of ultrafast magneto-optical measurements, while spin current propagation will be detected by x-ray detected ferromagnetic resonance measurements at a synchrotron source as shown schematically within the figure.
View the abstract for Spin current propagation through antiferromagnetic thin film metamaterials
- Please contact Prof Robert Hicken via email@example.com for more information.
Current computing systems are based on the manipulation and transfer of information using electric charge or voltage in semiconductor devices and circuits such as transistors and digital logic gates and circuits, with copper based interconnects. With the increased integration of semiconductor devices and threats of reaching device limits and increased power densities, alternative computing technologies are in demand to address current challenges in semiconductor devices and memories.
Spin-waves are oscillatory excitations of magnetic moments in magnetic material with wave-like nature that can propagate along magnetic waveguides or interconnects with intrinsically low energy and at high speeds, thus providing an alternative to semiconductor based devices. Spin-wave magnetic devices have thus been successfully developed and demonstrated at laboratory level to realise Boolean logic and circuits such as NOT, NOR and NAND logic gates, and multiplexer and majority functions.
Spin-wave computing technology, however, is still at its infancy and face a number of challenges to enable their practicable device realisation and commercialisation, such as scaling of device dimensions, fan-out and energy efficiency of the generation and detection of spin-waves. This project investigates, using multi-physics modelling and simulation, the fundamentals of the spin-wave propagation in magnetic devices and the effects of the waveguide materials and transducer topology on device performance to address the above challenges.
- Please contact Prof Mustafa M Aziz via firstname.lastname@example.org for more information.
Ferromagnetic nano-structures (for example nanowires) and metamaterials exhibit high magnetic moments and offer high operating frequencies and magnetic permeabilities. These unique properties make them prime candidates for applications in the next generation of communication systems, electromagnetic wave absorption, noise suppression and in microwave devices. Their compatibility with semiconductor fabrication methods also make them attractive for the next generation of low-power spintronic devices.
The frequency response and scattering properties of the magnetic metamaterial can be tailored and tuned by the shape, size and volume fraction of the nano-scale magnetic constituents, and by using external fields. This project involves the use and further development of a novel numerical algorithm, based on the finite-difference time-domain method, to simulate and study the interaction of electromagnetic waves with single and arrays of magnetic nano-structures for the purpose of designing high-frequency, tuneable metamaterials and communication devices.
- Please contact Dr Mustafa Aziz via email@example.com for more information.
Theoretical concepts in Metamaterials
Reciprocity in the animal kingdom gave rise to the evolution of reciprocal altruism: “you scratch my back, and I will scratch yours”. Aside from mere grooming, the consequences of reciprocity for the sharing of food, medicine and knowledge are profound. However, the breakdown of reciprocity, perhaps fuelled by a lack of affinity or obligation, can also lead to certain benefits for the non-reciprocator, who can profit from the nonreciprocal interaction.
Introducing the concept of nonreciprocity into metamaterials research also allows one to profit from nonreciprocal interactions, with immediate technological applications. Nonreciprocal devices, such as optical circulators and isolators, rely on the directional transfer of energy and information at the nanoscale. Furthermore, the realization of nonreciprocal waveguides will lead to extraordinary propagation lengths, being immune to backscattering.
In this project, we will construct theoretical models inducing nonreciprocity in metamaterials, for example those built from nanoscopic lattices of meta-atoms. We will consider how topology, dissipation and various symmetries can be employed to create a new class of nonreciprocal metamaterials with extraordinary transport and directional properties. Our work will be done in close collaboration with the leading experimentalists at the CMRI, where the novel phenomena that we discover can be simulated by, for example, acoustic waves or microwaves. The results of this project should guarantee future applications in wave physics, metamaterials and nanotechnology, particularly via the exploitation of the unidirectional flow of excitations.
- Please contact Prof Charles Downing at C.A.Downing@exeter.ac.uk for more information
Supervisors: Prof Mikhail Portnoi and Dr Eros Mariani
This theoretical project aims at unveiling the electronic and optical properties of novel Weyl metasurfaces hosting exotic quasiparticles with unusual “designer” dispersion, including tilted Dirac cones in the band structure.
Developing novel materials and metamaterials with “designer” spectra for electrons, photons, magnons and phonons is the growing trend in contemporary condensed matter physics. The tide of research is turning towards theoretical predictions followed by experimental observations of exotic quasiparticles in solid state systems such as Weyl and Majorana fermions. Since the 1980’s, two-dimensional (2D) systems provided a particularly rich playground for the discovery of new quasiparticles, including fractionally charged anyons in quantum Hall systems and massless Dirac fermions in graphene and topological insulators as the most spectacular examples. Simultaneously, the most recent advances in nanotechnology include the development of van der Waals heterostructures (artificial few-layer materials held by van der Waals forces) and finding a way of producing carbon nanotubes of selected chirality.
The proposed theoretical PhD research will start by addressing the electronic and optical properties of novel designer 2D van der Waals materials formed by a planar arrays of single-walled carbon nanotubes. We will then extend the core ideas from this setup to other 2D Weyl metasurfaces (two-dimensional metamaterials with energy spectrum described by Weyl equation), including Borophene and spatially modulated graphene layers.
Single-walled carbon nanotubes are long cylindrical molecules with electronic properties defined entirely by the way they are rolled from a graphene sheet. Namely, they can be semiconducting with a band gap up to several electron-volts, metallic without any band gap or quasi-metallic with a tiny band gap of a few meV induced by curvature effects. Combining the tubes in a regular planar array should result in a strongly anisotropic dispersion of the emerging 2D quasiparticles. The most interesting results are expected for arrays of metallic and quasi-metallic nanotubes for which the motion normal to the nanotube axis will result in the most dramatic changes in the electronic properties. In particular, a band gap may be opened in an array of metallic tubes or collapse for quasi-metallic nanotubes. The motion normal to the tube axis will also result in lifting the valley degeneracy in highly-symmetric zigzag nanotubes leading to tilted Dirac cones supporting a new type of 2D Weyl fermions with hyperbolic equipotential lines.
An analytical description of the peculiar low-energy electronic dispersion in this system will be at the core of the proposed research. We will study a plethora of unique physical effects stemming from this dispersion including unusual magneto-transport phenomena and inter-band dipole transitions, which are expected to be in the highly sought-after terahertz frequency range.
The investigation of topological states of matter has recently become one of the hottest topics of research in physics, leading to the award of the Nobel Prize in Physics to Haldane, Kosterlitz and Thouless in 2016. Topological states of matter exhibit fundamental properties that are protected by their symmetries and cannot be easily altered by small perturbations. This feature makes them the ideal candidates for hosting excitations able to carry information in a topologically protected way. While this seems ideal in view of potential applications in information technology, it can also represent a drawback as the topological phases, once imprinted in the design of a system by breaking specific symmetries, lack the tunability expected from conventional computing and IT devices.
There is a mismatch between astrophysical observations and theory. Observations of galaxy dynamics and gravitational lensing are consistent with there being considerably more mass in galaxies than we can see. The prevailing theory postulates new species of “dark matter” particles, which have significant mass but insignificant interaction with the electromagnetic field , thus being invisible to close to earth observations of astrophysical objects. This project builds on recent proposals to use resonant wire-medium based metamaterials [2,3,4] as the basis for detectors that enhance the interaction between candidate dark matter particles and the electromagnetic field.
One candidate dark matter particle is the axion, a field introduced in 1977 to explain the observation of zero (or at least very weak) CP violation by the strong force . This theory modifies the Lagrangian density for the standard model to contain both our usual massless electromagnetic field, and an axion field with non-zero mass. The interaction between the two is parametrized by an interaction constant χ.
In infinite free space, conservation laws prevent photon creation from the axion field, hence this matter being “dark”. However, within a plasma subject to a strong magnetic field, the interaction can be greatly increased [2-4], becoming resonant when the axion rest mass frequency equals the plasma frequency. The theoretically expected axion mass puts this resonant frequency in the terahertz or lower. In this frequency range, artificial plasma wire-based metamaterials have been developed , with a tunable plasma frequency. This makes it feasible to develop metamaterial based detectors for axionic dark matter particles. This theoretical project will investigate the design and feasibility of these detectors.
In this project we will:
(1) Examine the theory of electromagnetism coupled to axions and understand the general conditions for enhancing their interaction with materials, as well as how to numerically simulate these effects through e.g. modifying COMSOL multiphysics. We will consider a broader class of materials than the wire media in , including both dielectrics and metasurfaces.
(2) Search for candidate metamaterials for enhancing the rate of axion to photon conversion. This will be done using both analytic theory and numerical simulations using e.g. adjoint optimization to develop inhomogeneous tunable structures where the coupling to axions is maximized within some region, analogous to increasing the local density of states for an antenna .
(3) Compare metamaterial-based axion detectors to existing cavity-based designs.
(4) More broadly explore the topic of axion electrodynamics and the similarity between bianisotropic metamaterials and coupling to the proposed axion field. This may allow us to also use the designs developed in (1-2) to enhance the usually small resonant bianisotropic response of metamaterials.
- Contact Dr Simon Horsley via firstname.lastname@example.org for more information
 J. L. Feng “Dark Matter Candidates from Particle Physics and Methods of Detection”, Annu. Rev. Astron. Astrophys. 48, 495 (2010).
 M. Lawson, A. J. Millar, M. Pancaldi, E. Vitagliano, and F. Wilczek, “Tunable Axion Plasma Haloscopes”, Phys. Rev. Lett. 123, 141802 (2019)
 R. Balafendie, C. Simovski, A. Millar, P. Belov, “Wire metamaterial use for dark matter detection”, 2022 Sixteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 1-3 (2022).
 R. Cervantes et al/. “Search for 70 μeV Dark Photon Dark Matter with a Dielectrically Loaded Multiwavelength Microwave Cavity” Phys. Rev. Lett. 129, 201301 (2022).
 R. D. Peccei and H. R. Quinn, “CP Conservation in the Presence of Instantons”, Phys. Rev. Lett. 38, 1440 (1977).
 S. Mignuzzi, S. Vezzoli, S. A. R. Horsley, W. L. Barnes, S. A. Maier, and R. Sapienza “Nanoscale Design of the Local Density of Optical States”, Nano Lett. 19, 113-117 (2019).
Metamaterials for quantum technologies
Precision measurement underpins science and technology, and novel sensors that push the fundamental limits of accuracy and precision are required for applications ranging from nano-electronics to medical imaging. Colour centres have atom-like electronic transitions that can be probed with optical and microwave techniques (Fig. 1(b)), and thanks to a spatial extension on the scale of the atomic lattice, they can provide an exquisite probe of their local environment. In this project, you will develop an integrated microwave and photonic platform to control and investigate spins in 2D materials (Fig. 1(a)), with the ultimate aim of building a new generation of sensors with the highest possible sensitivity and spatial resolution.
- Please contact Dr Isaac Luxmoore via email@example.com for more information.
Two-dimensional (2D) van der Waals materials feature exotic electrical, magnetic, optical, and structural properties, providing energy and area efficiencies far exceeding what is possible with conventional electronics. Particularly promising are atomically thin transition metal dichalcogenides (TMDCs) because they allow us to utilise their local extrema in the electronic band structure called valleys for quantum computation with valley-based qubits. This project explores the design of metamaterials composed of TMDCs and 2D magnetic materials with the ultimate goal of building a new generation quantum platform exploiting both valley and spin degree of freedom. The focus will be on time-resolved measurements using ultrafast laser pulses, in order to probe the dynamics of valleys and spins. The interdisciplinary nature of this project will allow a student to gain expertise in our state-of the-art laser facilities and microscopes [1,2], be involved in the fabrication of 2D metamaterials at the Graphene Centre in Exeter and collaborate with external academic and industrial partners.
- Please contact Dr Maciej Dąbrowski via firstname.lastname@example.org for more information.
 M. Dąbrowski et al. All-optical control of spin in a 2D van der Waals magnet, Nat. Commun. vol. 13, 5976 (2022).
 M. Khela, M. Dąbrowski et al. Laser-induced topological spin switching in a 2D van der Waals magnet, Nat. Commun. vol. 14, 1378 (2023).
Metamaterials for healthcare
Optogenetics is a powerful and controlled neuromodulation technique, which mostly used to study the brain and treat brain diseases by using neural implant containing light to stimulate genetically modified neurons. Traditional brain implants are made of metals like platinum and iridium, which severely limit miniaturisation and signal resolution and, as a result, cause major adverse effects. Furthermore, optogenetics methods for powering the neural implants relies on stiff and tethered (e.g. optical fibres) systems. Due to the remarkable qualities of graphene, including its light weight, biocompatibility, flexibility, and exceptional conductivity, can be used to create considerably smaller devices that are safer to implant and that can be wirelessly powered.
This aim of this research is to design, fabricate and characterize a wireless graphene based neural implant for optogenetics.
Please contact Dr Rupam Das or Dr Ana Neves via email@example.com for more information.
McGlynn E, Nabaei V, Ren E, Galeote-Checa G, Das R, Curia G, Heidari H. (2021) The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics, ADVANCED SCIENCE, volume 8, no. 10, article no. ARTN2002693, DOI:10.1002/advs.202002693.
Das R, McGlynn E, Yuan M, Heidari H, IEEE. (2021) Serpentine-Shaped Metamaterial Energy Harvester for Wearable and Implantable Medical Systems, 2021 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS (ISCAS), DOI:10.1109/ISCAS51556.2021.9401288.
Das R, Moradi F, Heidari H. (2020) Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review, IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, volume 14, no. 2, pages 343-358, DOI:10.1109/TBCAS.2020.2966920.
Radiofrequency techniques are the dominant wireless technology used for bioelectronic applications due to their relative safety and maturity. These systems use components such as antennas, waveguides and phased arrays to control the propagation of electromagnetic fields, which are usually the largest and most energy-demanding part of a bioelectronic device and thus determine the safety and efficacy of the system. However, the human body is a lossy, heterogeneous and dispersive medium, presenting major challenges for wireless technologies. Biological tissues, in particular, absorb electromagnetic radiation, which must be within safety limits to prevent adverse thermal or stimulatory effects. Because tissue absorption increases with higher electromagnetic field frequencies, an operating frequency of less than 5 GHz is required to access regions deep in the body. However, this requirement also limits the miniaturization of the components and the ability to focus the electromagnetic field because the wavelength in biological tissues exceeds a centimetre at such frequencies. Furthermore, the human body is in constant motion and its size and composition greatly vary between individuals. These features present formidable challenges for the design of miniaturized, robust and high-performance wireless bioelectronic components for sensing and therapy.
This objective of this research will be to explore and study the metamaterials/metasurface, which can be engineered to control electromagnetic fields around the human body and could be used to overcome the current limitations of bioelectronic interfaces.
- Please contact Dr Rupam Das or Prof Alastair Hibbins via firstname.lastname@example.org for more information.
 Das R, Yoo H. (2017) A Multiband Antenna Associating Wireless Monitoring and Nonleaky Wireless Power Transfer System for Biomedical Implants, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, volume 65, no. 7, pages 2485-2495
 Das R, Basir A, Yoo H. (2019) A Metamaterial-Coupled Wireless Power Transfer System Based on Cubic High-Dielectric Resonators, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, volume 66, no. 9
Due to improvements in MRI techniques, 7 Tesla (T) magnets are now utilised to produce images with higher spatial resolution and signal-to-noise ratios (SNR) than 1.5 T and 3 T systems. The wavelength grows smaller than the human head at ultra-high-field (UHF) MRI, causing considerable interference effects in the B1+ field that can significantly deteriorate image quality. Additionally, the interaction of the RF field with the human body may provide new difficulties due to factors like greater RF transmitter power, RF power deposition, or Specific Absorption Rates (SAR).
The UHF MRI employs a variety of coil types, including surface coils, microstrip transmission lines, and birdcage coils. However, the use of metasoleniod coils inspired by metamaterials has yet to be investigated.
This The aim of this research is to electromagnetic modelling, designing (using electromagnetic modelling), fabricateion and characterisezation of metamaterial-based coils to improve the B1+ field homogeneity in high field MRI.
Das R, Yoo H. (2013) Innovative design of implanted medical lead to reduce MRI‐induced scattered electric fields, Electronics Letters, volume 49, no. 5, pages 323-324, DOI:10.1049/el.2012.4033.
Das R, Yoo H. (2017) RF Heating Study of a New Medical Implant Lead for 1.5 T, 3 T, and 7 T MRI Systems, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, volume 59, no. 2, pages 360-366, DOI:10.1109/TEMC.2016.2614894.
Electrical or focal brain stimulation based on implantable devices remains a therapeutic strategy of interest for people with medication-resistant forms of brain diseases such as epilepsy and who are not candidates for surgery. However, uncontrolled electrical stimulation of undesired neurons introduces shortness of breath, cough, throat pain, thereby restricting the extent of this approach. Recently, Optogenetics provides controlled stimulation in genetically modified neurons to allow optical stimulation (470 nm blue light) or inhibition (580 nm yellow light). Nevertheless, scientists pursuing optogenetics treatments for brain diseases still face some technical challenges, for example, traditional optogenetics methods for powering the neural implants relies on stiff and tethered (e.g. optical fibres) systems.
The objective of this research is to explore and develop a metamaterial-based wirelessly powered system for optogenetics.
- Please contact Dr Rupam Das or Prof David Wright via email@example.com for more information.
 McGlynn E, Nabaei V, Ren E, Galeote-Checa G, Das R, Curia G, Heidari H. (2021) The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics, ADVANCED SCIENCE, volume 8, no. 10, article no. ARTN2002693, DOI:10.1002/advs.202002693. [PDF]
 Das R, McGlynn E, Yuan M, Heidari H, IEEE. (2021) Serpentine-Shaped Metamaterial Energy Harvester for Wearable and Implantable Medical Systems, 2021 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS (ISCAS), DOI:10.1109/ISCAS51556.2021.9401288. [PDF]
 Das R, Moradi F, Heidari H. (2020) Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review, IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, volume 14, no. 2, pages 343-358, DOI:10.1109/TBCAS.2020.2966920. [PDF]
This project aims to optimise sorting of biological cells according to their mechanical properties by utilising novel microfluidic devices.
Various diseases lead to alteration in the physical properties of the cell plasma membrane such as membrane elasticity, viscosity and electrostatics. This could compromise biological functions hosted by the plasma membrane. Sorting of cells according to their physical properties is therefore an important step in understanding the effects of disease on membrane properties and cell function. This project will explore novel strategies for cell sorting based on their viscoelastic properties, using structured microfluidic devices with carefully controlled geometries. The concept will be validated on subpopulations of human red blood cells with different viscoelastic properties (membrane bending and shear moduli, membrane and cytoplasmic viscosity) and geometry (volume-to-area ratio), all of which are important for cell deformability. Later stages of the project will focus on designing and implementing adaptive microfluidic devices capable of optimising cell sorting depending on particular cell properties in the subpopulations. This is an exciting interdisciplinary project suitable for a student interested to work on the boundary between physics and biology.
- Please contact Dr Peter Petrov or Dr Stefano Pagliara via firstname.lastname@example.org for more information.