Photo of Prof J Roy Sambles

Prof J Roy Sambles

Professor of Experimental Physics

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Extension: 4103

Telephone: 01392 724103

Surface Waves and Microwave Photonics

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 (metmaterials) 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.The  group made the key observation of confirming Sir John Pendry’s prediction of ‘spoof’ surface plasmons on structured metal surfaces [1]. Since then they have moved on to a range of studies of novel metamaterial surfaces [2]. One of the other key observations in this area has been the experimental confirmation of ‘squeezing’ of microwaves into micron sized gaps and the consequences for thin absorbing structures which then arise. We have also gone on to show that one may combine gratings, liquid crystals and microwaves to make thin tuneable microwave devices.The research projects actively being pursued at present are:

  • Structured thin metal film for microwave control (James Edmunds [3, 4])

  • Novel microwave mesh structures (Celia Butler [5])

  • Low signature metamaterial surfaces for microwaves (Matthew Biginton [6])

  • Modelling metamaterial layered structures (Melita Taylor [7])

  • Zig-zag metal grating structures for microwaves (Helen Rance)

    Zig-zag gratings and surface  plasmons in the visible (Tom Constance)

    Structures for absorbing surface waves (Simon Berry)

     

 

 Acoustic metamaterials

The lessons learnt from studies of microwave mematerials are now beeing extended into the acoustic domain with original studies of acoustic propagation in structured layered materials. This work is in collaboration with EU scientists and with industrialists interested in underwater acoustics.

  • Acoustic metamaterials (Alasdair Murray)

RFID, anticounterfeiting and other Knowledge Transfer activity

A wide range of development work is also being undertaken in the broad area of the use of microwaves, metamaterials and structured surfaces. Much of this work is in collaboration with QinetiQ under an EPSRC KTA grant of over £3m to take the original ideas which have arisen from the research within the group on to demonstrators and then to spin-out companies. Two specific areas being looked into at present are RFID portals and improved RFID capabilities, and antifcounterfeiting using patterned metals developed from our novel reseach of photonic structures in nature.

Liquid Crystals

Liquid crystal displays (LCDs) now outnumber the population of the world and are a multi-billion dollar a year industry. Understanding the fundamental physics behind the operation of such structures is essential to allow the development of the next generation of such devices. Many liquid crystals are composed of rod-shaped molecules which tend to align roughly parallel to each other. Dictating their alignment by applying a voltage across a thin layer of the liquid crystal material allows the amount of light transmitted through it to be controlled. This has been the underlying physical principle in the operation of pixels in LCDs since their invention in the 1970s. To enable us to develop this technology to meet the high speed-switching, low power consumption and miniaturized pixel sizes required for today’s market we need to be able to understand the behaviour of the liquid crystal layer on a sub-millisecond timescale with a spatial resolution on the sub-micron scale.

The liquid crystal research in the Electromagnetic Materials Group, lead by Prof. Roy Sambles in the School of Physics at Exeter is primarily concerned with the use of optical techniques to characterise the alignment properties of a range of liquid crystals. Through the use of rigorous multilayer optics models and liquid crystal continuum theories fundamental physical and dielectric properties of these materials are determined. In the 1990’s the group pioneered the use of the “half-leaky” and “fully-leaky” optical guided mode technique to determine the director profile (average molecular orientation) in thin (~10 micron thick) liquid crystal layers. This technique was then developed further in the early 2000’s to allow the electrically induced director reorientation dynamics to be recorded on a sub-millisecond timescale. More recently, work has been undertaken to take the study of liquid crystals in the microwave and THz [8] regions of the electromagnetic spectrum. We are also pursuing original studies of the viscodynamics of liquid crystals [9].Further information relating to these previous studies can be found in the publications section of this site. The research projects that are currently active in the group are as follows:  

 [1] Hibbins A P, Evans B R and Sambles J R (2005) Science 308, pp 670-672‘Experimental verification of designer surface plasmons.’

[2] Lockyear M J,  Hibbins A P and Sambles J R (2009) Phys Rev Lett, 102, 073901‘Microwave surface-plasmon-like modes on thin metamaterials’

[3] Edmunds J D, Taylor M C, Hibbins A P, Sambles J R and Youngs I J (2010) J Appl Phys, 107, 103108 ‘Babinet’s principle, and the band structure of surface waves on patterned metal arrays.'

[4] Edmunds J D, Hibbins A P, Sambles J R and Youngs I J (2010) New J Phys, 12, 063007 ‘Resonantly inverted microwave transmissivity threshold of metal grids.''

[5] Butler C A M, Parsons J, Sambles J R, Hibbins A P and Hobson P A (2009) Appl Phys Lett, 95, 174101 ‘Microwave transmissivity of a metamaterial-dielectric stack.'

[6] Biginton M P, Hibbins A P, Sambles J R and Youngs I J (2010) Optics Express 18,23916-23 ‘Microwave transmission through capped hole arrays.’

[7] Taylor M C, Edmunds J D, Hendry E, Hibbins A P and Sambles J R (2010) Phys Rev B, 82, 155105‘Microwave response of hole and patch arrays.’

[8] Jewell S A, Hendry E, Isaac T and Sambles J R (2008) New J Phys 10, 033012 ‘Tuneable Fabry-Perot etalon for terahertz radiation.’

[9] Holmes C J, Cornford S L and Sambles J R (2010) Phys Rev Lett 104, 248301 ‘Small surface pre-tilt strikingly affects the director profile during Poiseuille flow of a nematic liquid crystal.’