Microwave metamaterials

  • Our research work is built around developing and studying structured metallic surfaces and metal-dielectric or ferrite-dielectric composites that exhibit novel and valuable phenomena.
  • The vast majority of our studies are based around 3D metamaterials, or 2D metasurfaces.
  • Compact and directional antennas, signature control, imaging and sensing, beam steering, frequency selective wallpaper, electronic tagging, and energy harvesting are just a few of the application areas that benefit from our research.

The term ‘metamaterial’ was originally coined at the turn of the 21st century, and the earliest studies were conceived in the microwave domain, with arrays of small and closely spaced resonant elements embedded in a dielectric host. Since then the concept has focussed on both 2D (metasurface) as well as 3D-crystal (metamaterial) implementations. The precise shape, geometry, size, orientation and arrangement of a metamaterial’s building blocks (termed ‘meta-atoms’) defines its electromagnetic response. This is comparable to the way electron transitions and vibration modes contribute to the characteristic response of conventional materials.

Our work on 2D metasurfaces is associated with the control of surface wave propagation through local control of the surface impedance boundary condition. This is achieved either with surface patterning, through regular, random or spatially-graded geometries, or via the use of overlayers. We are interested in the manipulation of the energy so that it propagates along channels or edges, or becomes localised or absorbed on the surface or gets scattered out (e.g. via defects). Further activities are focused on tuneable metasurfaces and those that can efficiently couple plane waves into surface bound energy.

Our work with 3D metamaterials creates bulk materials that have novel electromagnetic properties than can be chosen at the point of design using the shape, size or spacing of resonant inclusions within a passive host. We are particularly interested in materials that have a high refractive index, or an index that is near zero. The former are extremely valuable if they can be impedance matched to free-space, which is achieved by strengthening their response to the incident magnetic fields (permeability) to the levels similar to that of their response to electric fields (permittivity). A key objective of our work is therefore focused on the enhancement of magnetic properties.

Research expertise

We have established a set of facilities for exploring and characterising the response of materials and structures across the radio frequency (RF) and microwave regimes. These include vector and scalar network analyzers, signal generators and horns/waveguides etc. They are capable of undertaking experiments from 70 kHz to 125 GHz.

We can study the free-space, angle and frequency dependent reflectivity and transmissivity of samples above 800 MHz, and are also able to undertake stripline and near-field coupling measurements from the low-frequency band (tens of kHz) up to ~50 GHz.

A cornerstone of our experimental apparatus is a three-dimensional near-field scanner that allows the mapping out of fields over a 1m3 volume. We have developed a state-of-the-art broadband material characterisation technique that enables us to determine the permittivity and permeability of materials across a similar frequency range. Our microwave waveform generator (65 GSa/s) and oscilloscope (80 GSa/s) allows for pulse-measurements up to 25 GHz.

In addition we have all the computer codes required to model the electromagnetic response functions of multilayers, diffraction gratings and arbitrary metamaterial structures and devices. We use frequency and time domain techniques, and are advanced users of commercial software such as Comsol, CST and HFSS.

To fabricate experimental samples, we use 3D printing technology, as well as conventional workshop techniques and Printed Circuit Board methods.

Rapid Prototyping 

  • We have an Ultimaker S5 3D-printer in our labs, which we use for every-day manufacture of cm- and mm- sized photonic and phononic crystal and metamaterial designs for microwave and acoustic experimentation. It has a build volume of 330 x 240 x 300mm with layer resolution < 100 microns.
  • We also have use of further college facilities in Exeter Technologies Group.

Microwave Lab 

  • Capability from 70kHz – 110 GHz using multiple Vector Network Analysers (VNAs).
  • Horn antennas that can be utilised from 800-1000 MHz and 5 to 110 GHz.
  • Microwave benches with rotating stages that provide a collimated (distance source) beam to determine the response of test samples as a function of angle of incidence.
  • A computer controlled xyz-scanning stage and near-field probes provides the ability to measure the intensity and phase of the electromagnetic fields scattered from objects under test, and across surfaces.
  • We also have a 65 GSa/s Arbitrary Waveform Generator coupled to a 4 channel scilloscope with 25 GHz of real-time bandwidth.

Numerical and analytical modelling 

  • We are advanced users of Ansoft HFSS, CST Microwave studio and Comsol Multiphysics for numerical modelling of electromagnetic and acoustic systems. Differential methods for predicting the response of multilayered corrugated surfaces (diffraction gratings) have been developed, together with modal matching analytical techniques for predicting the response of well-defined photonic structures.
  • Our team have expertise in the development of finite difference time domain (FDTD) methods for predicting the response of advanced photonic materials, including the incorporation of the Landau-Lifshitz-Gilbert equation for the study of magnetic materials.

Micromagnetic modelling of magnonic phenomena

  • The ability to accurately predict the properties of magnetic nano-structures and devices is undertaken using the mumax simulation software, and permits the study of dynamical phenomena, such as spin waves in magnetic nanostructures (magnonics) and the kinetics of magnons.

Funded research projects

Current projects include:

  • SYMETA: Synthesizing 3D Metamaterials for RF, Microwave and THz Applications. (EPSRC, EP/N010493/1).
  • TEAM-A: The tailored electromagnetic and acoustic materials accelerator. (EPSRC, EP/R004781/1).
  • XM2: EPSRC Centre for Doctoral Training in Metamaterials (EPSRC, EP/L015331/1)

Studentship projects:

Studentship projects are part of the Centre for Doctoral Training in Metamaterials, and include:

Project title

Researcher(s)

Funder

High frequency magnetics materials Cameron Gallagher and Conor McKeever Part-funded by DSTL
Localisation and topological protection of Microwave Surface and Interface Waves Yulia Dautova Part-funded by QinetiQ
Microwave surface waveguides and antennas Julia De Pineda Gutiérrez Part-funded by Flann Microwave
Design and characterisation of 3D metamaterials and superdirective antennas Pavel Petrov Part-funded by Leonardo
Chiral metamaterials Lauren Barr  
Analytical modelling of surface waves metasurfaces Miguel Camacho Aguilar Part-funded by QinetiQ
Microwave resonances in spin-wave nanometre magnetic patches David Osuna Ruiz  
PT symmetry breaking in active metamaterials Milo Baraclough  
Magnetic metamaterials for innovative solutions to the inductive tagging problem Emily Glover  
Advanced Magnetic Materials for Radio, Microwave and Millimetre-wave applications Katie Lewis Funded by DSTL