Microwave photonics

Key personnel: Prof J. Roy Sambles , Dr Alastair P. Hibbins, Dr Matt J. Lockyear

Our research work is primarily built around the development and subsequent study of subwavelength structured metallic surfaces that exhibit novel and valuable phenomena. A list of our current research projects can be found here.

The exceptional properties of these structures enables new functionality in a range of applications including; stealth materials, light-weight flexible ultra-thin radar absorbers, cloaking, beam steering, smart antennas, electronic tagging and frequency selective wallpaper.

We predominantly undertake our work at microwave frequencies: this typically involves the use of computational modelling software as a design and investigative tool, overseeing the manufacture of the samples, carrying out the experimental studies, analysis of data and dissemination of findings via the scientific literature.

There are eight microwave researchers in the Electromagnetic Materials group: Prof. Roy Sambles FRS and Dr. Alastair Hibbins (supervisors), Dr Matthew Lockyear, six PhD students (Lizzy Brock, James Edmunds, Celia Butler, Helen Rance, Melita Taylor and Matthew Biginton) and MPhil student (Toby Campbell). We currently receive funding from EPSRC, University of Exeter, Dstl, QinetiQ and BAE Systems.

Engineering microwave plasmonics: "Spoof" surface plasmons

Surface plasmons (SPs) are transverse magnetic (TM) modes that propagate at metal-dielectric interfaces and constitute an electromagnetic field coupled to oscillations of the conduction electrons. In the visible domain there is a very short penetration of the field into both the metal and the dielectric, thus allowing one to localise the light at, and influence the power flow across an interface. In the microwave regime metals have a very large conductivity thereby almost entirely excluding the fields from the metal, and the SP mode is then no more than a loosely bound surface current. This then appears to prevent the application of plasmonic structures at microwave wavelengths. However we have recently experimentally verified that even a perfectly conducting metal, perforated with an array of subwavelength holes, can support strongly localized SP-like waves. This is because the holes in the metal have a cut-off frequency below which no propagating modes are allowed. Below this cut-off, only evanescent fields exist on the metal side of the interface, and it is exactly this field characteristic that is required for a surface mode.

We are studying the reflection and transmission characteristics of structured metal substrates as a function of hole geometry, hole depth and the properties of the material which is used to fill the holes. We are also investigating the use of gratings of zigzag-shaped grooves to offer the possibility of obtaining very high Q resonances for filtering, absorption or field enhancement devices. Furthermore an EM response independent of angle and polarisation may even be achieved via diffractive folding of the SP-like bands. Arrays of metal patches closely spaced from a ground plane can also support surface modes and we are exploring the possibility of obtaining ultra thin (wavelength/100) structured metal layers that provide the required boundary conditions for applications that may include slow light and surface waveguides.

<

To find out more about our microwave plasmonics work, see for example,

• "Experimental verification of designer surface plasmons" SCIENCE 308, 670, 2005
• "Waveguide arrays as plasmonic metamaterials: Transmission below cutoff" PHYSICAL REVIEW LETTERS 96, 073904, 2006
• "Prism coupling to 'designer' surface plasmons" OPTICS EXPRESS 20, 20441, 2008

Metamaterials

Our ability to manipulate electromagnetic (EM) radiation is dependent on the range of materials available to us. However, the response of metals becomes dominated by their large conductivity at microwave frequencies and it becomes increasingly difficult to confine and manipulate long wavelength radiation. However our choice has been widened by the realisation that one can carefully structure composite materials to engineer artificial EM properties, as radar engineers have long known. The term ‘metamaterial’ has recently been coined to describe fabricated structures and composites that either mimic known EM responses or have EM character not found in naturally occurring materials. The exciting metamaterial concepts of negative refraction, perfect focusing and EM cloaking have recently received much attention.

At Exeter, we have a number of projects considering the behaviour of 2D metamaterials (or 'metasurfaces'). For example, we are exploring the EM properties of individual nm-thick layers of regular and random arrays of small metal patches, near their percolation threshold. We are considering novel tuneable metamaterial surfaces for applications such as switchable filters, phase-shifters, reconfigurable EM shields, and beam steerers. The utilization of metasurfaces as novel boundary conditions is also being considered in waveguide and RF-ID technologies. Conventionally the extraordinary EM properties of metamaterials rely on a resonant phenomenon, however we are also investigating methods to extend the bandwidth over which they offer useful characteristics. The development of analytical descriptions of the effective permittivity and permeability of random two- and three-dimensional metal-dielectric composites is an important goal, since it will potentially lead to a metamaterial 'recipe' for engineering any required set of EM material parameters.

Microwave absorbers and filters

Electromagnetic radiation absorbing materials (EMRAMs) are useful in a range of applications, e.g., to reduce the radar clutter and associated ‘dead-zones’ induced by the growing number of wind-farms. Furthermore the major thrust in EMRAM for many applications has been towards the development of thin, broadband absorbers.  However satisfactory broadband EMRAM performance is reliant upon getting EM energy into the structure, and then providing sufficient loss to absorb the necessary energy within the allowed structure thickness.  These two requirements often conflict because high-loss materials typically suffer from high front-face reflections (impedance mismatch).  At Exeter our primary aim of our projects is to investigate potential development routes that can be utilised to help overcome this problem, while minimising weight, thickness, fragility and complexity, while optimising flexibility and operational bandwidth.

One of our first ultra thin microwave absorbing surfaces developed at Exeter is shown. It comprises of circuit board (300 microns of dielectric clad by 20 microns of copper on both sides), with very narrow (300 micron) slits etched into one face. Despite the structure being less than 500 microns thick, it absorbs 7 mm microwave radiation! Full details of this, and subsequent studies can be found in Physical Review Letters, and Journal of Applied Physics. We are continuing to study new microwave absorbers, as part of our projects "An exploration of patterned thin layers of metal-dielectric composite materials for microwave absorption" and "Microwave characteristics of tessellated surfaces".

We are also interested in novel microwave transmission devices. These have included the extraordinary Fabry-Perot-like transmission through subwavelength slits in metal substrates (e.g. see this paper). The transmission through more complex structures involving slits with internal structure to produce very low frequency resonances, and so-called "compound gratings" that support very additional high-Q resonances have also been studied. In addition, the "one-way diffraction grating", is quite remarkable, since it transmits more radiation in one orientation than the other!