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.

Our work utilises novel geometric designs, alongside new 3D printable methods to develop small, lightweight and intricately structured ‘meta-atoms’ that reflect, scatter or direct electromagnetic radiation and energy in a controlled manner.  For example, these meta-atoms can appear orders of magnitude greater than their physical size to radar, and can be compact (~1 cm) and lightweight.  Through careful design the resonant modes of individual, or clusters of meta-atoms (meta-molecules) can be tuned to give sharp, frequency and polarization-specific signatures, and highly controllable electromagnetic reponse.  They are finding utlisation in applications where there is a requirement to enhance radar visibility (RCS) or to act as passive antennas to direct wireless signals to communciation 'not-spots'.

This subtheme is led by Professor Alastair Hibbins, working alongside Professor Bill Barnes, Dr Cameron Gallagher, Professor Rob Hicken, Dr Ian Hooper, Dr Paul Keatley, Prof Feodor Ogrin, Dr Alex Powell, and Professor Roy Sambles.

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., with capabilities 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 70 kHz to 50 GHz.

A cornerstone of our experimental apparatus is a three-dimensional near-field scanner that allows the mapping of fields over a 1m3 volume. We have also developed a state-of-the-art broadband material characterisation technique that enables us to determine the permittivity and permeability of materials up to ~50 GHz, and have the capability of undertaking time-resolved measurements with bandwidths of up to 25 GHz using an arbitrary waveform generator (65 GSa/s) and oscilloscope (80 GSa/s).

In addition we have developed computer codes to model the electromagnetic response 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 crystals 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 fully-lined anechoic chamber for antenna and/or scattering measurements.
• 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.
• We can predict the properties of magnetic nano-structures and devices 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.
• Our team have expertise in the development of finite difference time domain (FDTD) methods for predicting the MHz and GHz response of metamaterials, including the incorporation of the Landau-Lifshitz-Gilbert equation for the study of magnetic materials. This work is a University spin-out company, www.MaxLLG.com.

• Petrov et al., Microwave Superdirectivity with Dimers of Helical Elements, Phys. Rev. Appl. (2020) [PDF]
• De Pineda et al., Metasurface bilayer for slow microwave surface waves, Phys. Rev. B (2019) [PDF]
• Baraclough et al., Metamaterial Analogues of Molecular Aggregates, ACS Photonics (2019) [PDF]
• Camacho et al., Extraordinary Transmission and Radiation from Finite by Infinite Arrays of Slots, IEEE Trans. Antenna Propag. (2019) [PDF]
• Keatley et al., A Ferrite-Filled Cavity Resonator for Electronic Article Surveillance on Metallic Packaging, IEEE Trans. Mag. (2019) [PDF]
• Gallagher et al., A Broadband Stripline Technique for Characterizing Relative Permittivity and Permeability, IEEE Trans. Microv. Theory Tech. (2019) [PDF]
• Gao et al., Experimental observation of photonic nodal line degeneracies in metacrystals, Nat. Commun. (2018) [PDF] 
• Seetharaman et al., Realizing an ultra-wideband backward-wave metamaterial waveguide, Phys. Rev. B (2018) [PDF]
• Dautova et al., Mimicking graphene physics with a plane hexagonal wire mesh, Applied Physics Letters (2018) [PDF]
• Tremain B et al. Isotropic Backward Waves Supported by a Spiral Array Metasurface, Sci. Rep. (2018) [PDF]
• Barr et al., Investigating the nature of chiral near-field interactions, Phys. Rev. B (2018)[PDF] 
• Yang et al., Ideal Weyl points and helicoid surface states in artificial photonic crystal, Science (2018) [PDF]
• Camacho et al., Theoretical and experimental exploration of finite sample size effects on the propagation of surface waves supported by slot arrays, Physical Review B (2017) [PDF] 
• de Pineda et al., A broadband metasurface Luneburg lens for microwave surface waves, Applied Physics Letters (2017) [PDF] 
• Yang et al., Direct observation of topological surface-state arcs in photonic metamaterials, Nat. Commun., (2017) [PDF]
• Camacho et al., Designer surface plasmon dispersion on a one-dimensional periodic slot metasurface with glide symmetry, Optics Letters (2017) [PDF] 
• Bychanok et al., Fully carbon metasurface: Absorbing coating in microwaves, Journal of Applied Physics (2017) [PDF]
• Camacho et al., Resonantly induced transparency for metals with low angular dependence, Applied Physics Letters (2016) [PDF] 
• Mitchell-Thomas et al. Omnidirectional surface wave cloak using an isotropic homogeneous dielectric coating, Scientific Reports (2016) [PDF]
• Atherton et al., Topological Modes in One Dimensional Solids and Photonic Crystals, Physical Review B - Condensed Matter and Materials Physics (2016) [PDF]
• Lansey et al., Measurement of Photon Sorting at Microwave Frequencies in a Cavity Array Metasurface, IEEE Trans. Ant. Propag. (2015)[PDF]
• Parke et al., Independently controlling permittivity and diamagnetism in broadband, low-loss, isotropic metamaterials at microwave frequencies, Applied Physics Letters (2015) [PDF]
• Hooper et al., Massively sub-wavelength guiding of electromagnetic waves, Sci Rep (2014) [PDF] 
• Parke et al., Broadband impedance-matched electromagnetic structured ferrite composite in the megahertz range, Applied Physics Letters (2014) [PDF]
• Keatley PS et al., Ferrite-filled cavities for compact planar resonators, Applied Physics Letters (2014) [PDF]