Optics and photonics:Physics enhancing our lives

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Contents

Foreword

New insights into interactions between light and matterOptics and photonics research is underpinning a revolution in technological applications of light

The perfect imageAdaptive optics offers a range of techniques to bring light waves to the sharpest focus possible

What lies beneathOptical coherence tomography can provide three-dimensional microscopic images of living tissue

Photonic waterfallsA novel type of semiconductor device called a quantum cascade laser has a bright commercial future

Microstructured optical fibresFibre optics is undergoing a revolution thanks to the development of new types of structures fabricated at the scale of the wavelength of light

Plasmonics lights upA new approach to manipulating light at the nanoscale offers exciting prospects for optical technologies

Slow lightIngenious methods of slowing down the passage of light through a medium could revolutionise optical communications

Contacts

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Optics and photonics: Physics enhancing our lives Foreword

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ForewordPhysics and physicists play a vital role in underpinning our way of life, improving its quality and contributing in a major way to wealth creation. Innovations such as lasers and liquid crystal displays, which have become part of everyday life in a diverse range of applications from telecommunications to TVs, emerged from studies in basic physics, becoming crucial technologies in just a few decades. It is, therefore, a worthwhile challenge to speculate where similar explorations today will lead us in the future.

This booklet on optics and photonics, the second in a series covering the main areas of physics, attempts to do just that: highlight and showcase examples of world-leading research in the UK, which has a strong potential for commercial exploitation. It is jointly sponsored by the Institute of Physics – the professional body and learned society for physics and physicists – and the Engineering and Physical Sciences Research Council (EPSRC) – the largest UK funder of physics research.

We hope that this booklet illustrates how the research investment made by the EPSRC, and the support provided by the Institute to its membership, will enable the UK’s economy, and society at large, to benefit from the discoveries and advances being made in leading-edge physics research.

Dr Robert Kirby-HarrisChief ExecutiveThe Institute of Physics

Professor David DelpyChief ExecutiveEPSRC

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Light provides one of the major ways in which we gather information about the world around

us. Over the centuries, people have learnt how to manipulate light using mirrors and lenses to focus and magnify distant and microscopic objects. An early understanding of the wave-like nature of light led to the development of optical devices such as diffraction gratings, which depend upon constructive and destructive interference between light waves to split white light into its component wavelengths. Today, optical interference effects are applied in many technologies such as communications, imaging and in ultra-fine measurement, and perhaps find their cleverest use in holography. Optics research combined with novel engineering continues to generate useful imaging techniques, for example, adaptive optics (p6) and optical coherence tomography (p8), which are benefiting fields as varied as medicine, astronomy and defence.

Let there be lightElucidation of the nature of light at the most fundamental level has had a profound effect on how we describe the Universe, and is central to the two cornerstones of modern physics: relativity and quantum theory. Relativity established that the speed of light is a fundamental constant of Nature, while quantum mechanics ascribed combined particle and wave-like characteristics to both matter and light, and revealed their deep relationship. Light is described as propagating waves composed of oscillating electric and magnetic fields – electromagnetic radiation – which can interact with the electric and magnetic fields associated with matter particles such as electrons, leading to classical optical phenomena such as reflection and refraction. Light is also thought of as packets or ‘quanta’ of electromagnetic energy – photons – which are emitted or absorbed by the electrons in atoms, for example, as they

lose or gain energy. It is this understanding of the energy-transfer process between light and matter at the quantum level – now called photonics – that led to the development of spectroscopy, whereby materials are characterised by their interactions with specific wavelengths of light. Spectroscopy is a powerful analytical tool now used routinely in astronomy, the chemical industry, forensics and in biomedicine.

Photonics research also produced new and powerful light-emitting devices, of which the prime example is the laser. Although discovered in the 1950s, it was not until the 1980s that laser applications really took off. One important physics driver was that the high intensity of laser light allowed researchers to study extreme optical responses – so-called nonlinear effects; exploring and harnessing this behaviour for future optical technologies continues to be a major goal in photonics research.

A key advance that brought lasers into the everyday world was the development of semiconductor-chip technology. This led to the fabrication of tiny, robust semiconductor lasers that are now ubiquitous – found in supermarket barcode readers, CD players and computer printers. Laser technology and applications continue to progress and broaden, as physicists and device engineers design ever more sophisticated semiconductor structures. Quantum cascade lasers (p10) are among recent significant developments that look set to have a variety of uses.

In parallel, the late 20th century saw another major development – the use of glass fibres to guide light beams via total internal reflection. Combined with laser amplifiers, fibre optics revolutionised global telecommunications and ultimately made the internet possible. The combination of optical communications and semiconductor devices suggested that light could be used not only to send signals but also to

Optics and photonics research is underpinning a revolution in technological applications of light

New insights into interactions between light and matter

Optics and photonics: Physics enhancing our lives Introduction

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process them on fast optical chips; optoelectronics and the concept of the all-optical computer became buzz-words in the 1980s. However, progress in optoelectronics stalled because the wavelengths used in optical communications are very much longer than the scale of chip components, which, over the decades, has been steadily shrinking towards the nanometre-scale.

New paradigmsNevertheless, in recent years, new ideas have emerged – based on a revisiting of the basic physics of how light interacts with matter – which show great promise. The result is that the field of optics and photonics has been re-invigorated. The first breakthrough came in the late 1980s when US physicists showed that devices with regular arrays of features on the scale of the wavelength of light – photonic crystals – could block or guide specific bands of wavelengths via interference, to create behaviour analogous to that of electrons in semiconductors. ‘Photonic band-gap’ crystals offered the prospect of manipulating light at least at the micrometre, if not the nanometre-scale. Soon afterwards, UK physicists usefully demonstrated similar behaviour in microstructured optical fibres, sometimes called photonic crystal fibres (p12). These fibres also act as a nonlinear medium, producing intense white light. In general, designer fibre lasers are seen as providing the next generation of high-power light sources.

The next novel development, which also originated largely in the UK, was that of ‘metamaterials’ – composite materials designed with features which show electric and magnetic responses to light analogous to those of atoms to produce bulk optical effects such as refraction. The remarkable property of metamaterials is that they can exhibit negative refraction, and so have the potential to control light in unexpected ways. Negative refraction is the practical demonstration of the concept of transformation optics, brilliantly developed by theorist Sir John Pendry at Imperial

College London. Employing similar mathematical reasoning to that underpinning General Relativity (in which gravity is the result of a mass causing space to curve round it), the behaviour of a path of light is the result of space apparently being warped in defined ways by a metamaterial. The idea is compelling and opens the way to totally novel applications such as the ‘invisibility cloak’, whereby light beams curve right around an object to create a kind of ‘hole’ in space, in which the now-invisible object resides.

Recently, metamaterials have been designed at the nanoscale, offering huge potential for optoelectronics and other technologies. A contributing phenomenon in manipulating light via structured materials is the coupling of light with the oscillations of electrons at metal surfaces to form ‘plasmons’. Plasmonics (p14) encompasses behaviour that allows light to be influenced by structures that are smaller than its wavelength, and so offers the prospect of bridging the ‘scale-gap’ between photonics and electronics. Finally, as well as new approaches to steering light in optical systems and opto-electronic devices, physicists have also been investigating ingenious ways of slowing or even halting the passage of light through a material (p16), an enabling technology for controlling optical data transmission and storage.

All these areas are moving rapidly, in a highly synergetic way. However, none of them would have progressed without the development of novel fabrication techniques, often at the nanoscale, combined with a deep theoretical understanding of the underlying physics. The UK is a world-leader well-placed in all these aspects to contribute to optics and photonics research, and to benefit from the resulting applications. These include not only ultrafast optical communications and new imaging techniques, but also energy conversion and transmission, and further into the future, quantum information technology relying on controlling single photons of light. Optics and photonics are truly coming of age.

New insights into interactions between light and matter

Research in optics and photonics has contributed to:

New imaging and analysis techniques for biomedicine

Safer, simpler medical treatments and diagnoses

Higher-resolution microscopes and telescopes

Laser eye surgery

Accurate measurement

Micro-fabrication and cutting techniques

Laser welding

Fast telecommunications and the internet

Flat-panel TVs, computer and mobile-phone displays

Better cameras

Cheaper, more efficient light sources

Supermarket barcode readers

CD and DVD development

Improved national security and defence

Chemical and biological sensors

Improved environmental and climate monitoring

Power transmission

Solar cells

Secure banknotes and credit cards

A typical AO system used to image the retina: a sensor measures the wavefront aberration; a linked control system calculates the required movement of a deformable mirror which corrects the aberration

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major problem in collecting information carried directly by light in air – whether an image from

the remotest regions of the Universe or intelligence transmitted via laser across a desert – is that the light waves must travel through a turbulent medium before they are detected. Variations in density and composition in the atmosphere change its local refractive index, which distorts the shape of the normally flat ‘wavefronts’ of light, and reduces the quality of the image or data being transmitted. The analogous effect is seen by looking through the rippling surface of a swimming pool at the tiles below.

To counter these effects, physicists and engineers have developed ingenious techniques – collectively known as adaptive optics (AO). Although it was the military that first employed AO in the 1980s to image satellites, astronomers

soon saw its potential to improve images in ground-based telescopes, which otherwise can look 50 times as blurred as would be seen from space. Today, all the world’s largest telescopes are fitted with AO systems to produce remarkable images of distant galaxies seen previously only with the Hubble Space Telescope. What is even more exciting is that the AO technologies developed are now rapidly expanding into a host of other applications, from microscopy and eye surgery to biometrics, DVD players, and even outside TV broadcasting.

How AO worksAO aims to compensate for rapidly-changing wavefront distortions (aberrations) in real time. Conventional AO systems consist of three parts: a wavefront sensor, a corrector and a linking control system. The most commonly used sensor is the Shack-Hartmann device, an array of lenslets which each focus a portion of the light into a spot on a charge coupled device (CCD) camera. The degree of aberration is then calculated from how the spots move relative to each other. Another type of sensor measures any wavefront curvature due to aberration by comparing the light intensity just short of the location where the image is focused and also just after (as in a standard eye test). The information is then passed via an electronic control system to the corrector, which is usually a very thin mirror whose surface can be rapidly deformed by arrays of piezoelectric or electromechanical actuators to correct the aberrations.

Light from retina

Aberratedwavefront

Beamsplitter

Eye

Correctedwavefront

Wavefrontsensor

Deformablemirror

Control system

High-resolutionretinal image

The first astronomical AO system on the UK’s William Herschel Telescope using an artificial laser ‘star’

The planet Uranus seen without (left) and with AO

The perfect imageAdaptive optics offers a range of techniques to bring light waves to the sharpest focus possible

APPLICATIONS

Lasers

Large telescopes

Eye diagnosis and surgery

Microfabrication

Image recognition

Security

Microscopy and bioimaging

Free-space communications

Cameras

Optoelectronic equipment

Optics and photonics: Physics enhancing our lives Adaptive optics

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The UK is extremely active in developing AO. One of the first telescopes to be fitted with an AO system was the UK’s 4.2-metre William Herschel Telescope on La Palma in the Canary Islands. To image a faint galaxy for example, the AO system relied on observations of a bright star that appears close by on the sky as a guide. Now, an artificial guide-star in the form of a laser beam focused high above the atmosphere offers a more practical option. Gordon Love and Richard Myers at Durham University, who have been developing these systems, are now designing new approaches for the next generation of Extremely Large Telescopes with diameters of 40 metres or more. These include systems employing multiple laser guide-stars with multiple mirrors to image wide areas of the sky in 3-D. AO is also being incorporated in individual astronomical instruments such as spectroscopes.

Applications to biomedicineThe AO systems used by astronomers are bulky, but Chris Dainty – while at Imperial College London – and his team there, developed more compact, low-cost systems suitable for studies of the eye. Accurate diagnosis of eye disease depends on being able to image the retina at high resolution to reveal any cellular changes. Combining an AO set-up with a laser-scanning ophthalmoscope can compensate for imperfections in the intervening tissues of the eye to give retinal images that are several-fold clearer. Luis Diaz, an ex-student of Dainty’s, working at City University London, is aiming to improve the system design, while Dainty himself, now at the National University of Ireland, Galway, is concentrating on simulating vision aberrations that will help in the design of replacement intraocular lenses. A more familiar spin-off is the introduction of wavefront sensing techniques in laser refractive surgery (LASIK); they help in correcting for subtle aberrations in eyesight.

An even more surprising application has been developed by AOptix – a company in Hawaii co-founded by another of the Dainty’s students, Malcolm Northcott. Using AO, AOptix has designed an iris image-recognition system ideal for security and border control; it can quickly and accurately identify someone’s iris from a distance of 2 metres, even when moving. The company has also developed AO-based laser communication

systems with an ultra-high bandwidth to give fast wireless transmission of high-definition film from remote locations over distances of 5 kilometres.

An obvious area that can benefit from AO is laser microscopy. Martin Booth at the University of Oxford employs a novel type of ‘sensorless’ AO to compensate for aberrations when imaging deep inside biological specimens. A mathematical method is used to cycle a deformable mirror through a series of shapes to find the most perfect image. This is then selected in less than a second. The development of mouse embryos (100 micrometres thick) can be studied in this way (right). He is also applying AO to sharpen the laser-etching of high-refractive-index materials used in optoelectronic devices. John Girkin, now at Durham University, has developed a computational scheme to remove aberrations when imaging deep into living tissue such as gut and blood vessels, while Alan Greenaway and his team at Heriot-Watt University take a different approach using a version of curvature-sensing. They have developed a portable system that can be bolted on to any microscope and used to image living tissue such as human sperm.

AO is also being applied to lasers. Their output and beam quality is affected by the gradual heating up of the optical elements. Researchers at the University of Strathclyde, Imperial College and elsewhere are investigating methods of introducing AO into laser architecture to sharpen the beams of lasers used in free-space communications, atmospheric sensing, and even of very high-power lasers for nuclear fusion.

AO now covers a wide range of techniques that involve controlling wavefronts. As well as relying on deformable mirrors as controllers, researchers such as the Durham group are investigating lenses made of electrically controllable arrays of liquid crystals (as found in flat-panel TV screens). The Japanese company Pioneer already incorporates liquid-crystal correctors in its DVD players which, when laser-scanning a disk, can optically adjust for any manufacturing imperfections. The widespread uses of AO in mobile-phone cameras is just around the corner.

D. Debarre/University of Oxford

Micrograph of a mouse embryo without (top) and with (bottom) AO

A. Gurney/J. Girkin/University of Strathclyde

Living blood–vessel sections imaged with laser fluorescence microscopy

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echniques such as ultrasound and magnetic resonance imaging

(MRI) are used routinely in hospitals to image inside the body safely and non-invasively. Now, physicists are developing a new method of imaging subsurface living tissue that depends on the wave properties of light. Optical coherence tomography (OCT) works a little like ultrasound but builds up two or three-dimensional images by detecting light reflected from layers of tissue up to 2 millimetres thick – and much deeper if the tissues are transparent, as in the eye. OCT is already being employed in clinics to image the retina. The resolution is now good enough – 1 to 2 micrometres – to study cellular processes related to cancer or tissue repair, for example. OCT provides an instant ‘optical biopsy’ of otherwise inaccessible tissues, and avoids the usual, time-consuming process of excising and preparing samples for examination with a microscope.

The technique emerged as a direct result of modern physics-based technologies – fibre optics, lasers, light-emitting diodes (superluminescent diodes) and photo-detectors – and is based on optical interference effects. Light from a

light-emitting diode (LED), fibre-amplifier or laser, covering a range of frequencies with wavelengths of between 800 and 1500 nanometres, is split and channelled down two optical fibres. The light from one fibre goes to the sample-object being imaged, while the light

transmitted in the other fibre is reflected off a ‘reference’ mirror, and then recombined with the light reflected from the sample. The combined light is detected and its intensity recorded (see diagram bottom left). The light source is chosen such that the constituent

waves remain ‘in step’ – that is, coherent – over only a very short distance (the coherence length) of a few micrometres. Then, if the light in both fibres travel optical paths, which differ by less than the coherence length, they will produce interference fringes as the waves reinforce or cancel each other. Path differences greater than the coherence length will not produce fringes. However, light ‘echoes’ from the sample will be delayed by an amount depending on the depth, resulting in a path difference that can then be measured by moving the mirror enough to observe the fringes.

A new scanning technique This approach was first used in the 1980s by a research group in Vienna, Austria to measure eye-lengths, from cornea to retina. Then in 1991, a team at the Massachusetts Institute of Technology in the US realised its potential as an imaging technique. The reflectivity profiles obtained by scanning with the mirror also give information on the intensity of reflected light, which depends on the optical properties at a particular location in the sample. Images can be mapped by scanning laterally to produce cross-sections.

Since then, OCT has progressed rapidly. The scanning time has been shortened a hundred-fold by using a slightly different approach in which either the mirror remains fixed and the light is split

What lies beneathOptical coherence tomography can provide three-dimensional microscopic images of living tissue

APPLICATIONS

Ophthalmology

In vivo imaging of subsurface tissues in skin, gut, and nervous and vascular systems

Real-time microscopy of cellular processes

Diagnosis of cancer, skin damage and burns

Monitoring of engineered tissue growth

Alternative to animal testing

Non-destructive analysis of paintings, ancient stained glass, parchment, archaeological objects

Materials development and quality control

Animal conservation and climate change

Broad bandwidth source (LED)

Optical delay lineDetector

Fibre-opticbeam splitter

Sample

Computer

Movingreference mirror

Single reflection site Scan (envelope)

varnish underdrawings red lake priming

OCT reveals underdrawings in paintings, as shown in the red drapery of The Magdalen by the face-on image (a) and the cross section (b)

National Gallery/H. Liang/A. Podoleanu et al

Principle of optical coherence tomography

Optics and photonics: Physics enhancing our lives Optical coherence tomography

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into its constituent frequencies with a grating, and measured using a charge coupled device (CCD) camera or a high-speed tuneable light source is used. Interference patterns for different frequencies happen for specific depths in the sample. The information gathered can quickly be translated with a computer back into imaging data.

Wolfgang Drexler, who leads a biomedical imaging group at the University of Cardiff, worked in both the Austrian and US teams on early developments of OCT. In particular, he developed ultra-high resolution OCT, using a titanium-sapphire laser as a light source, to image and monitor cells in the human retina in real time.

OCT has become a routine clinical tool for ophthalmology, particularly for diagnosing diseases of the retina. To obtain even better images, Drexler has been combining adaptive optics (p6) with OCT – an approach also being explored by another pioneering team led by Adrian Podoleanu at the University of Kent. In collaboration with a Canadian company, Ophthalmic Technologies Inc, they have developed commercial OCT systems that produce ‘face-on’ images and can be combined with simultaneous conventional laser imaging to create a powerful diagnostic tool for studying eye disease.

Biological imagingOCT technology is now being applied to study many different kinds of biological structures, such as skin and nerve cells, blood vessels and epithelial tissues. To improve resolution further, Pete Tomlins at the National Physical Laboratory near London, has been analysing the physics behind what OCT actually reveals – in terms of the patterns of light scattering and absorption by tissues, and the detection systems used. The goal is to extract as much information as possible from the optical data and relate it to tissue health. Working with the biophotonics research group, led by Nick Stone at the Gloucestershire Royal Hospital, Tomlins is mapping the refractive index of pre-cancerous and cancerous tissues in the oesophagus with the aim of correlating it with significant chemical changes. The group is also using OCT to measure the curing dynamics of dental composite materials and producing reference ‘phantoms’ for clinical OCT.

Several variations of OCT have evolved. Taking advantage of the Doppler effect (change in frequency

with movement towards or from a detector), OCT can also be exploited to monitor the micro-circulation in blood vessels less than a millimetre across. Mark Dickinson at the University of Manchester has been working with a network of local hospitals to develop Doppler OCT for studying blood flow in the skin in order to understand diseases like scleroderma (hardening of the skin) and Reynaud’s disease (reduced blood flow in the skin), while Steven Matcher at the University of Sheffield uses polarisation OCT in which polarised light (light waves all oriented in the same direction), through refraction effects, reveals structural changes in collagen associated with diseases like osteoarthritis.

An advantage of OCT is that it lends itself to endoscopy, which uses a fibre-optic probe. The Manchester network has employed this approach to monitor any pathological changes in the otherwise inaccessible peritoneum in patients who rely on kidney dialysis. Ralph Tatam’s group at Cranfield University has gone further, in devising an OCT probe comprising a bundle of thousands of fibres which view an array of positions in the tissue simultaneously, thus avoiding awkward mechanical scanning.

A potentially exciting application of OCT is in tissue engineering. The Sheffield group is investigating if OCT can be used to monitor the invasion of melanoma cells in a tissue-engineered skin construct. This could not only help clinicians to determine the optimum treatment regime for a patient but also offer an alternative to animal testing in modelling disease. Pierre Bagnaninchi at the University of Edinburgh is also planning to monitor optical signatures, via OCT, related to the multiplication and differentiation of stem cells for use in the regeneration of tissues such as bone and liver.

OCT imaging is also ideal for visualising other delicate materials. In collaboration with the British Museum, the National Gallery and Nottingham Trent University, the Podoleanu group has built a portable system for examining paint layers and underdrawings in works of art (top left). So far, UK researchers have been assembling their own OCT equipment for specific applications, and there is still much more work to be done in developing new light sources and detectors suitable for commercial systems. Nevertheless OCT could soon become a routine tool in industry, medicine and research.

An OCT image mapping the refractive index of healthy oesophageal mucosa tissue

M. Tedaldi/P. Tom

lins/NPL

A 3-D OCT retinal image of a single photoreceptor in vivo

W. Drexler/Cardiff University

Mark Dickinson has been using OCT to detect changes in the skin of a Costa Rican tree frog, caused by climate change

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Since their invention more than 40 years ago, lasers have evolved into a profusion of

varieties – from huge, high-power instruments to tiny semiconductor structures used for information processing. The latter are now found everywhere – in CD players and computers for example – and are mass-produced in their millions. Physicists and engineers are continually exploring new ways to make semiconductor devices emit light more efficiently; one of the most exciting recent developments is the quantum cascade laser (QCL). QCLs can also be designed to produce light over a wide range of wavelengths that were previously inaccessible. They have many potential applications, including environmental sensing, security and optical communications.

The most common semiconductor laser is a two-component, diode system based on alloys such as gallium arsenide or indium phosphide. When a voltage is applied, negatively charged electrons are excited from one set, or band, of quantum energy levels to a higher energy band, leaving behind positively charged ‘holes’. When the electrons and holes recombine, they emit photons of light with a wavelength that depends on the band-gap. The QCL, however, works in a more subtle way. It consists of a periodic sequence of very thin layers of semiconductors, arranged to give a stepwise cascade of quantum ‘mini-bands’, down which electrons can drop without recombination, like an electronic waterfall (diagram above left). A photon is emitted at each step, leading to a power output up to 1000 times greater than for a laser diode – in which recombination produces only one photon.

The QCL concept was invented by a team led by Federico Capasso and Jerome Faist at

Bell Laboratories in the US in 1994, and has since become one of the most active areas of semiconductor-laser research. The first lasers were based on indium gallium arsenide combined with aluminium indium arsenide and worked in the mid-infrared region of the spectrum at wavelengths between 3.85 and 17 micrometres. This is the wavelength range in which many molecules absorb energy via their characteristic vibrations and rotations, and their infrared absorption spectra act as identifying molecular fingerprints. One of the first applications of QCLs has thus been in spectroscopy, in detecting trace amounts (less than one part per billion) of environmentally significant and polluting atmospheric gases.

Identifying trace compoundsUsing similar semiconductors, Charlie Ironside’s group at the University of Glasgow has produced QCLs in the UK. In conjunction with Miles Padgett’s team, also at Glasgow, they collaborated with Shell in designing an instrument deployed to spot ground-level whiffs of the hydrocarbon ethane (which absorbs at 3.34 micrometres) as a marker of oil reserves below. The compact and robust system will replace the cumbersome lead-salt lasers which oil companies employ when prospecting in the desert for example, but which work only at very low temperatures, so require cooling.

Geoff Duxbury and colleagues at the University of Strathclyde have developed a new type of spectrometer based on a pulsed QCL in which the wavelength can be changed by the rapid heating induced by the current pulse. In this way, a complete spectrum can be recorded for each pulse. By bolting the spectrometer onto the bottom of a

APPLICATIONS

Pollution monitoring and environmental sensing

Detection of drugs and explosives

Monitoring and analysis of pharmaceuticals

Detection of disease

Breath analysis

Process control

Optical and free-space communications

Laser-based imaging and spectroscopy

Collision-avoidance radar

Car cruise-control

Compact high-resolution microscopes

A novel type of semiconductor device called a quantum cascade laser has a bright commercial future

Photonic waterfalls

C. Phillips/Imperial College London

G. Duxbury and K. Hay/ University of Strathclyde

In-flight measurements of air quality with a QCL

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Optics and photonics: Physics enhancing our lives Quantum cascade lasers

plane, initial work was geared to simultaneously detecting water, nitrous oxide and the potent greenhouse gas methane. They have also explored the detection of excess nitrous oxide produced from fertilised fields, as well as car-exhaust gases and cigarette smoke. Another investigation involved analysing the chemical processes used to synthesise diamond films. The Strathclyde research has led to a spin-out company, Cascade Technologies, which designs and markets QCL-based spectrometers used for monitoring emissions from ships.

In the meantime, John Cockburn’s team at the University of Sheffield, which also has a highly successful research programme on mid-infrared QCLs, has been experimenting with semiconductor compositions to improve performance and extend the accessible wavelength range. Current material combinations work well down to about 4 micrometres, so the goal at Sheffield has been to make shorter-wavelength devices. By introducing antimony into the semiconductor mix, the researchers have demonstrated QCL operation down to 3 micrometres at room temperature. The ultimate challenge will be to build a QCL to operate at wavelengths of 1.3 or 1.4 micrometres for fibre-optic telecommunications.

Longer wavelengthsThe spectral range is also expanding on the longer wavelength side. Edmund Linfield and Giles Davies at the University of Leeds are developing QCLs based on gallium arsenide/aluminium gallium arsenide that work in the 70 to 140-micrometre range. The terahertz radiation emitted is currently of great interest for security applications because it can penetrate many materials. The researchers have a wide range of projects in hand, including a QCL-based imaging system in which the diffuse light reflected from an object’s interior structure is identified and analysed.

Another intriguing application of terahertz QCLs is as optical switches in telecommunication systems. Chris Phillips of Imperial College London, working with the Massachusetts Institute of Technology in the US, has demonstrated that shining near-infrared laser light into the active region of a QCL results in emissions at two new wavelengths, thus providing an all-optical way of

changing the wavelength. Small wavelength shifts are necessary to encode a series of signals so that they can travel down an optical fibre simultaneously (multiplexing).

At the moment, terahertz QCLs operate only at cryogenic temperatures, but the Leeds group, in collaboration with Harvard University in the US, has engineered a design that now holds the world record for the highest operating temperature of 178 K. An alternative approach is to move to silicon-based devices which should operate at much higher temperatures and would have the advantage of being compatible with standard silicon-chip technology. Douglas Paul at Glasgow, has been working on a silicon/silicon germanium system, which does not yet give enough gain to lase, but shows promise.

QCLs show great potential as tunable sources with all the advantages of semiconductor lasers. One approach to tunability is to change the laser wavelength by applying a voltage across the device (as in a transistor) to alter the energy levels in the semiconductor layers, which is what the Glasgow team is experimenting with. The emission of a QCL can also be directed by embedding the device in a photonic crystal (a periodic microstructure designed to steer light waves) which reflects and concentrates the laser light. UK researchers are working on many different aspects that will eventually make these novel devices as ubiquitous as laser diodes.

A mounted QCL

A QCL grating

Equipment used to fabricate QCL semiconductor

structures at the University of Leeds

J. Cockburn/University of Sheffield

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Optical fibres – hair-like strands of silica glass – provide the medium for today’s

telecommunications, transmitting vast amounts of data around the world as fleeting pulses of light. Although the technology has been well-established since the 1980s, over the past decade or so physicists have taken another look at the optical fibre and have developed new types, structured at the microscale, which enable light to be manipulated in novel ways. Microstructured fibres were pioneered in the UK in the 1990s, and the field has since exploded. New structures and optical phenomena are still being explored, with applications including laser-power transmission, sensing and imaging.

A conventional optical fibre consists of a core of glass doped with germanium, with a covering of pure glass, which has a slightly lower refractive index than the core. Light beams are then confined and guided down the core by total internal reflection. In the 1990s, Philip Russell with colleagues Jonathan Knight and Tim Birks – first based at the University of Southampton and then at the University of Bath – decided to look at trapping light in a fibre more efficiently by creating a so-called photonic band-gap across it. Physicists based in the US had shown that interference effects in transparent three-dimensional materials peppered with periodic arrays of holes (photonic crystals) would block the transmission of certain wavelengths. The challenge was to make an analogous photonic-crystal fibre pierced with longitudinal air holes. The ingenious technique that the researchers developed – based on a method first used by the Egyptians to make containers from coarse glass fibres – was to stack a hexagonal bundle of glass tubes and rods, and draw it out

under heat to make fibres with channels just a few micrometres across – a ‘holey’ fibre.

The first structures fabricated consisted of a small solid core, down which light was forced to travel – imprisoned by the total internal reflection resulting from the lower overall refractive index of the surrounding holey structure. Later, as the fabrication technique improved, the researchers were able to confine light in fibres with an air-filled hollow core, via a true photonic band gap.

Both solid and hollow-core fibres have turned out to have a plethora of interesting properties, which are now being investigated by several research groups in the UK. Teams at both Bath and Southampton (which has one of the largest fibre-optics research groups in the world) have continued to develop new structural variations by altering the hole or core size, or the geometry, to achieve novel optical effects for specific applications. For example, the first fibres made carried just a single light mode at all wavelengths even through a relatively large core – a useful property for the transmission of high-power laser light. BAE Systems is now developing high-power laser systems based on photonic-crystal fibres for avionic optical sensors and communications.

Microstructured fibres allow two important characteristics of light propagation to be manipulated: the first is the speed at which different frequencies in a pulse travel down them – the dispersion over time into different colours;

APPLICATIONS

White-light emission

Frequency combs for metrology

Optical amplifiers and switches

High-power laser delivery for welding, micro-machining, and anti-corrosion surface treatment

High-harmonic generation for X-ray production

Power distribution for aerospace

Fibre-optic sensors for jet fighters

Ultra-compact gas micro-cells

Chemical and environmental sensors

Quantum information processing

Particle guidance

Ultra-short pulse lasers

Fluorescence imaging

Optical coherence tomography (p8)

Photodynamic therapy

Flow cytometry

Fibre-based spectrographs for telescopes

Microstructured optical fibres

Fibre optics is undergoing a revolution thanks to the development of new types of structures fabricated

at the scale of the wavelength of light

A hollow Kagome structured optical fibre

F. Benabid/University of Bath

Brilliant ‘supercontinuum’ light generation using a microstructured fibre

R. Taylor/Imperial College London

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Optics and photonics: Physics enhancing our lives Microstructured optical fibres

and the second is the ‘nonlinear’ response resulting from the refractive index of silica changing slightly with light intensity in a confined but extended medium. Light passing through a fibre can thus be tuned to emit different frequencies, often in short, intense pulses. The most spectacular example is when infrared light, transmitted through a fibre with a small core and large air holes, is transformed into an intense burst of white light 10 000 times brighter than the Sun. Roy Taylor and his team at Imperial College London were the first to demonstrate this supercontinuum generation at high powers using both pulsed and continuous-wave fibre lasers.

The first application of supercontinuum sources was to generate trains of very short pulses of light equivalent to a ‘comb’ of equally spaced frequencies, which can be used to measure frequency very accurately. Another important goal has been to enhance supercontinuum generation in the blue and ultraviolet spectral regions for biomedical microscopy and analysis (p8). Many biological imaging techniques rely on fluorescence at these wavelengths. The Imperial College team, in collaboration with colleagues at Bath, has been developing specially tapered fibres for this purpose, and Gail McConnell and colleagues in the Centre for Biophotonics at the University of Strathclyde have been tailoring a supercontinuum laser for fluorescence microscopy at several wavelengths.

Hollow-core fibresOriginally, the aim was that hollow-core fibres would have sufficiently low losses to be used in long-distance telecommunications. So far, they have not been able to compete with conventional fibres. However, they have the potential to carry extremely high-power laser light in air without damaging the surrounding silica, and are being developed by the Bath team for welding and precision micro-machining applications in the car and aerospace industries.

Hollow-core fibres also have the advantage that other materials can be incorporated in them, so they can act as cheap, robust and portable microcells. One application being investigated is gas sensing, for example, in detecting methane in landfill sites. Fetah Benabid at Bath is extending the optical capabilities of these microcells to low powers. Using a fibre built with a woven basket-like structure called a Kagome

lattice and filled with a gas such as hydrogen, nonlinear effects can efficiently convert a weak continuous light source into ultra-short light pulses, normally achievable only with high-power lasers. One aim is to generate a broad-band frequency comb. Another intriguing application is to use the fibre to confine gases such as acetylene in experiments to study the quantum effects associated with electromagnetically induced transparency (p16).

The manipulation of single photons in a fibre is of great interest for quantum information technology since this approach would be suitable for long-distance communications. John Rarity and Jeremy O’Brien at the University of Bristol, working with William Wadsworth at Bath, have been building small-scale quantum logic processors based on pairs of correlated photons created by nonlinear processes in a solid-core fibre. They are also developing an all-fibre source of single photons for very secure quantum cryptography, used to scramble sensitive information sent between organisations such as banks, large companies and government departments.

Microstructured fibres made from materials other than silica offer higher nonlinearities, which would enable optical processing to operate at lower powers. David Richardson and his group at Southampton are fabricating fibres from compound glasses such as lead silicates, bismuthates, tellurites and chalcogenides, which can have nonlinearities of more than 100 times higher than silica. Because the glasses melt at much lower temperatures, the fibre structures can be made by extruding the material through a mould. The ultimate goal is to make highly efficient all-optical switches, as well as fibres that operate at extended wavelengths in the mid-infrared range.

Introducing structure into optical fibres at the microscale has provided a new vehicle for photonic effects. The basic physics is still being studied, but start-up companies are already selling products based on this exciting technology. We can expect many more exciting developments in this field.

Cross-section of a complex microstructured fibre

A micrograph of guinea-pig muscle taken using a supercontinuum laser

B. McConnell/University of Strathclyde

T. Birks/University of Bath

14

Gold and silver are prized for their shiny appearance – a result of light being reflected

by the ‘sea’, or plasma, of electrons loosely bound within their bulk. However, light can be made to interact with the electron plasma in more subtle ways, which physicists are now eagerly exploring with a range of applications in mind, such as biosensors, optical circuits and solar cells.

The plasma oscillates with a characteristic frequency, giving rise to waves of electron density – plasmons – that span a few nanometres. Light impinging on the interface between a metal and a non-conducting material (dielectric) such as air, if of the same frequency, may resonate with the electrons, forming electromagnetic modes called surface plasmon polaritons (SPPs). These modes provide a useful way of coupling light and electrons, and introduce the intriguing possibility of manipulating light at scales much smaller than its wavelength. SPPs could provide a way to integrate optical and electronic components, not easily achieved because of the mismatch of scale: electronic-chip components can now be patterned with features down to 50 nanometres, while optical fibres used in telecommunications must have a diameter at least half the wavelength of the light they carry (1.3 to 1.5 micrometres).

The potential for this emerging field of ‘plasmonics’ has come about as a result of techniques available to pattern metal surfaces on scales down to the nano-level, as well as theoretical developments. The behaviour of SPPs is affected not only by the nature of the metal and the dielectric but also by the shape of the interface – with the result that the plasmons can

be tuned over a wide range of wavelengths, and ‘squashed’ into a very small volume or steered in a particular direction.

Although the existence of plasmons has been known for about a century, it is only recently that their significance has been widely recognised. In 1989, Thomas Ebbesen, then at the NEC Research Institute in Japan, found that a gold film perforated with millions of tiny holes transmitted much more light than expected. It seemed that the holes accumulated the light as SPPs, which then travelled through to the other side of the film before re-releasing the light. This startling result triggered renewed interest in plasmons, and other new developments quickly followed. One of these was the prediction by the theorist Sir John Pendry at Imperial College London who showed that light could, indeed, be controlled in unexpected ways by microstructured materials – metamaterials, which give rise to so-called designer plasmons, a prediction confirmed by Alastair Hibbins, Roy Sambles and colleagues at the University of Exeter. Today, several UK research groups have world-leading programmes investigating the underlying science and technological potential of a wide variety of plasmonic structures made in ingenious ways.

Concentrating lightSurface plasmons are highly localised near the surface of the nanostructure supporting them. For some years, chemists have been using this property in analysis to enhance, by more than a billion-fold, the characteristic scattered light from sample molecules attached to a roughened silver or gold surface (surface enhanced Raman scattering, SERS). Jeremy Baumberg at the University of

A new approach to manipulating light at the nanoscale offers exciting prospects

for optical technologiesAPPLICATIONS

Chemical and biological sensors

Drug delivery and cancer treatment

Spectroscopy

Lighting

Solar cells

Nano-lasers

Terahertz radiation emission and transmission

All-optical circuits and logic elements

Wireless power charging

Superlenses and near-field microscopy

Invisibility cloaksS. Noble/University of Southam

pton

Plasmonics lights up

The Lycurgus cup normally appears green, but when Illuminated from within, it glows red due to surface plasmon excitation

resulting from gold and silver nanoparticles in the glass

A plasmonic gold surface can emit terahertz radiation

British Museum

K. Wynne/University of Strathclyde

15

Optics and photonics: Physics enhancing our lives Plasmonics

Cambridge has taken the concept further, creating regular dimpled plasmonic surfaces via chemical templating with plastic microspheres. These then act as more efficient SERS biosensors when ‘biorecognition’ molecules are attached. The team’s ‘plasmonic pits’ have recently been commercialised by D3 Technologies, a subsidiary of Renishaw.

In contrast, Bill Barnes, Andrew Shaw and colleagues at Exeter plan to commercialise a biosensor based on an array of metal nanoparticles designed to detect hundreds of different molecular biomarkers simultaneously by measuring the change in the plasmon resonance. Anatoly Zayats at Queen’s University Belfast is developing plasmonic crystals for detecting molecules via their optical response, while Stefan Maier at Imperial College London has also been exploring enhanced optical sensing, designing elaborate ring-and-disc cavities that trap and intensify the plasmonic resonances. Intriguingly, these structures also show classical analogues of phenomena such as electromagnetically induced transparency (p16).

Another application of plasmonic field-enhancement is in improving the efficiency of light-emitting diodes – or the reverse, absorption of light in solar cells – by coupling the plasmons into the optically-active organic molecules or semiconductor structures involved in light emission or absorption. Baumberg is looking at plasmonic structures to improve the efficiency of solar-energy conversion.

Plasmons can also be harnessed to assist in more extreme ‘nonlinear’ optical processes. They can, for example, be used to help generate intense short-wavelength laser pulses, which could be used for carving out nanoscale chip components. Nikolay Zheludev at the University of Southampton is tackling another intriguing proposal – the ‘lasing spaser’ consisting of an array of plasmonic resonators which could generate coherent light beams from optical wavelengths to the far infrared and terahertz range. Klaas Wynne at the University of Strathclyde has discovered a novel way of generating terahertz radiation by using ultra-short laser pulses to create accelerated surface plasmons on a forest of metallic needles. They act like lightning rods pushing out electrons, emitting terahertz radiation in the process.

Light on a wireSPPs at longer wavelengths may travel up to a few centimetres along an interface before dying out. In appropriately designed structures, they could directly transmit optical data along a metal wire. One application is as interconnects between logic elements on an all-optical chip, which would allow much more data to be processed quickly over a wide bandwidth. Maier makes waveguides working at optical and near-infrared frequencies, using a variety of structures including chains of metallic nanoparticles and multilayer structures, while the Belfast group has been experimenting with nanorods. In addition, Sambles and Hibbins have created thin, flexible structures consisting of a copper-layer sandwich with a plastic filling. The metal has periodic slots designed to produce a resonant plasmonic response, which effectively ‘squeezes’ millimetre waves down to microwave scales within the device. Meanwhile, Chris Philips at Imperial College is taking a new approach that allows plasmons to travel tens of centimetres. Laser light couples into a multi-layer semiconductor nanostructure in a way that depends on the layers’ quantum energy to give a new type of guided plasmon. This type of plasmon coupling could also offer a new route to quantum information processing.

A long-sought-after aim is to make processors with all-optical elements. Zayats is using plasmon-enhanced effects to develop an all-optical transistor. Zheludev’s group has shown that plasmon signals can be modulated using ultra-short optical pulses, and has developed a plasmonic memory element based on single nanoparticles of gallium that change their crystal structure when addressed by a laser pulse.

Plasmonics is proving to be an extremely fertile area in manipulating how light and matter interact and will clearly have many technological spin-offs in the future.

Conceptual view of an all-optical transistor using plasmons

A ‘nanovoid’ plasmonic surface for biosensing

A. Zayats/University of BelfastJ. Baum

berg/University of Cambridge

16

APPLICATIONS

All-optical routers for the internet

Multiplexing optical signals

Optical data storage

Optical shift registers

Quantum information processing

Medical diagnosis

Slow lightIngenious methods of slowing down the passage of light through a medium could revolutionise optical communications

The speed of light, at an unimaginable 300 million metres a second in empty space, is

regarded as an immutable constant that defines our understanding of matter and energy. However, we know that light travels a little more slowly through transparent materials such as air and water. In the past decade, physicists have developed strategies to slow light down much further – to bicycling speeds, even stopping it dead in its tracks.

The principle behind slow light relates to the familiar concept of refraction. When light waves (electromagnetic radiation) pass through a transparent medium, interactions with the electrons cause the waves to spread out, or disperse, over a range of slightly different

frequencies, each of which moves at a different ‘phase’ velocity. The different-frequency waves interfere with each other to create larger groups of waves that form an envelope, which travels more slowly and tends to bend in the directions that the individual slower phase-velocity waves are moving – hence light is slowed down and bent.

It is this overall ‘group’ velocity of light that characterises how the electromagnetic energy is propagated, and is the significant factor in optical communications and processing. Engineering a sluggish group velocity offers the capability to ‘hold back’ and manipulate optical communications signals, leading to faster internet connections and even a route to quantum computing.

Quantum brakesThe first experiments to put the brakes on light propagation were carried out during the 1990s. Lene Vestergaard Hau and her team at Harvard University in the US slowed light down to an unbelievable 17 metres a second in a cloud of sodium atoms cooled down to 50 mK above absolute zero. At this temperature, all the atoms sit within their lowest quantum energy state A (bottom left diagram). A laser beam was tuned such that its wavelength and thus energy was absorbed by the atoms, exciting them to a higher quantum state in the process C. The sodium gas shines bright yellow as the light is re-emitted. A weaker, coupling laser was then introduced, which is tuned to excite the atoms from a state B – of slightly higher energy than A – again to state C. What happens is that states A and B form a long-lived mixed quantum state that cancels the light absorption so that the gas becomes transparent. This effect produces a high dispersion in the light and an accompanying steep change in the refractive index, which results in a radical slowing of the group velocity. In fact, using this principle of electromagnetically induced transparency (EIT), Hau was able to freeze light altogether.

The novel aspect of this set-up is that the light from the first laser passes right through the gas without absorption, yet is still coupled to the atoms by a second, weak control beam. Hau’s demonstration stimulated many research groups to investigate such strong light-matter couplings in other systems. Soon, the same effect was replicated in rubidium atoms at room temperature. Ifan Hughes and colleagues at Durham University have observed EIT in room-temperature rubidium vapour, slowing light by a factor of 650. They have been experimenting with quantum-state transitions

Quantum state A

Laser beam

Second, coupling laser beam

(Light absorbed by atoms)

Mixed quantum state forms, which cancels absorption

Quantum state B

Quantum state C

Mechanism of EIT in very cold sodium atoms

Guided between layers of metamaterial, light slows down and

forms a ‘trapped rainbow’

O. Hess/University of Surrey

Optics and photonics: Physics enhancing our lives Slow light

17

that are easily controlled with a magnetic field, with the idea that such an approach could be used to sense and map tiny magnetic variations associated with heart or brain activity, thus acting as a diagnostic tool. In another experiment, rubidium vapour heated to 150°C was used to rotate the plane of polarisation of a temporally short light pulse, which travelled 50 times slower than in a vacuum. The wide frequency window for the pulse rotation opens up the possibility of switching telecommunication bandwidth pulses.

An obvious long-term application of EIT is in quantum computing which depends on creating ‘entangled’ quantum states. Tim Spiller of HP Labs Bristol has studied EIT’s potential as a ‘bus’ to generate entanglement between two photons of light acting as bits of quantum information (qubits). Slow light media would also offer a means of temporarily buffering qubits as well as conventional optical signals. However, the most practical system would operate in a solid rather than a gas. Chris Phillips of Imperial College London has demonstrated EIT in ultra-thin semiconductor layers, slowing down light by a factor of 40 or 50. He also found that the system could be made to emit light with gain, introducing the possibility of a novel, so-called inversion-less laser.

Less speed, more hasteA more classical (non-quantum) approach, which is more compatible with optical communications technology, is being studied by Valeri Kovalev and Bob Harrison of Heriot-Watt University. Light pulses in an optical fibre are slowed down by effectively converting them into sound waves by the process of electrostriction (the light field squeezes the material). A laser pulse is fed in at one end of the fibre, while a second, control laser of slightly higher frequency is pumped in at the other. The resulting interference effects trigger a high dispersion which slows the passage of light. Although the effect is not as dramatic as in EIT, the light losses are less. This kind of process could allow the transmission of optical data to be buffered thus enabling optimum synchronisation of multiple signals across optical networks.

Photonic-crystal structures (p12) are another candidate medium for slow-light effects. Thomas Krauss and his team at the University of St Andrews

make planar silicon-on-glass waveguides perforated by arrays of microscale holes. Light reflects off the holes to create interference effects that slow down their propagation. This creates slow light that has allowed the team to demonstrate one of the world’s smallest optical switches. With Graham Reid at the University of Surrey, they are now integrating the switch with microelectronic circuitry.

An exciting alternative is to use metamaterials (p5). At the University of Southampton, Nikolay Zheludev has demonstrated what he believes is the first classical analogue of EIT. He uses a metamaterial consisting of a dielectric slab, coated with ultra-thin layers of copper in a fish-scale pattern at scales smaller than the wavelength of light. Light becomes trapped between surfaces due to cancelling interference effects similar to those in the atomic version of EIT, leading to long optical-pulse delays.

Ortwin Hess and his team, also at Surrey, have delayed the passage of light in a glass waveguide sandwiched between layers of metamaterial with a negative refractive index (which bends light in the opposite direction from normal). At the interfaces, the light is pushed backwards by negative phase-shifts, as it bounces along the waveguide – a bit like walking up a sand-hill. The device is tapered so that the light becomes slower and slower and is eventually trapped at a particular width that depends on the frequency, thereby creating a ‘trapped rainbow’. Such a mechanism might be the basis of an optical capacitor. This forms part of a major joint project between Salford, Surrey and Imperial College led by Allan Boardman at the University of Salford.

These kinds of experiments show the tremendous potential of using microscopically-structured materials to control optical signals for future commercial benefits.

The St Andrews team inspects its slow light optical switch shown in the micrograph

T. Krauss/University of St Andrews

Nikolay Zheludev’s team at Southampton studies a metamaterial that mimics EIT in an anechoic (shielded) chamber

S. Noble/University of Southampton

18

Contacts

The perfect imageDr Martin Booth University of Oxford E-mail: martin.booth@eng.ox.ac.uk

Professor Chris Dainty National University of Ireland, Galway E-mail: c.dainty@nuigalway.ie

Professor John Girkin Durham UniversityE-mail: j.m.girkin@durham.ac.uk

Professor Alan Greenaway Heriot-Watt University E-mail: a.h.greenaway@hw.ac.uk

Dr Gordon Love Durham UniversityE-mail: g.d.love@durham.ac.uk

What lies beneathDr Pierre Bagnaninchi University of EdinburghE-mail: pierre.bagnaninchi@ed.ac.uk

Dr Mark Dickinson University of ManchesterE-mail: mark.dickinson@manchester.ac.uk

Professor Wolfgang Drexler Cardiff University E-mail: drexlerw@cardiff.ac.uk

Dr Steven Matcher University of Sheffield E-mail: s.j.matcher@shef.ac.uk

Professor Adrian Podoleanu University of Kent E-mail: a.g.h.podoleanu@kent.ac.uk

Professor Ralph TatamCranfield UniversityE-mail: r.p.tatam@cranfield.ac.uk

Dr Pete Tomlins National Physical LaboratoryE-mail: pete.tomlins@npl.co.uk

Photonic waterfallsProfessor John Cockburn University of Sheffield E-mail: j.cockburn@sheffield.ac.uk

Professor Giles Davies University of Leeds E-mail: g.davies@leeds.ac.uk

Professor Geoff Duxbury University of StrathclydeE-mail: g.duxbury@strath.ac.uk

Professor Charlie Ironside University of Glasgow E-mail: c.ironside@elec.gla.ac.uk

Professor Douglas Paul University of Glasgow E-mail: d.paul@elec.gla.ac.uk

Professor Chris Phillips Imperial College LondonE-mail: chris.phillips@imperial.ac.uk

Microstructured optical fibresDr Fetah Benabid University of Bath E-mail: pysab@bath.ac.uk

Professor Tim Birks University of Bath E-mail: t.a.birks@bath.ac.uk

Dr Gail McConnell University of Strathclyde E-mail: g.mcconnell@strath.ac.uk

Professor John Rarity University of Bristol E-mail: john.rarity@bristol.ac.uk

Professor David Richardson University of Southampton E-mail: djr@orc.soton.ac.uk

Professor Roy Taylor Imperial College London E-mail: jr.taylor@imperial.ac.uk

Plasmonics lights upProfessor Bill Barnes University of Exeter E-mail: w.l.barnes@exeter.ac.uk

Professor Jeremy Baumberg University of Cambridge E-mail: jjb12@cam.ac.uk

Professor Chris Phillips Imperial College LondonE-mail: chris.phillips@imperial.ac.uk

Professor Stefan Maier Imperial College London E-mail: s.maier@imperial.ac.uk

Professor Roy Sambles University of Exeter E-mail: j.r.sambles@exeter.ac.uk

Professor Anatoly Zayats Queen’s University Belfast E-mail: a.zayats@qub.ac.uk

Professor Nikolay Zheludev University of Southampton E-mail: niz@orc.soton.ac.uk

Slow lightProfessor Allan BoardmanUniversity of SalfordE-mail: a.d.boardman@salford.ac.uk

Professor Ortwin Hess University of SurreyE-mail: o.hess@surrey.ac.uk

Dr Ifan Hughes Durham UniversityE-mail: i.g.hughes@durham.ac.uk

Dr Valeri Kovalev Heriot-Watt UniversityE-mail: v.kovalev@hw.ac.uk

Professor Thomas Krauss University of St AndrewsE-mail: tfk@st-andrews.ac.uk

Professor Chris Phillips Imperial College LondonE-mail: chris.phillips@imperial.ac.uk

Dr Tim Spiller HP Labs BristolE-mail: tim.spiller@hp.com

Professor Nikolay Zheludev University of SouthamptonE-mail: niz@orc.soton.ac.uk

19

For further information, contact:

Tajinder PanesorThe Institute of Physics76 Portland PlaceLondon W1B 1NTUKEmail: tajinder.panesor@iop.orgwww.iop.org

© The Institute of Physics and EPSRC 2009

Writer: Nina HallDesigned and produced by: h2o (www.h2o-creative.com)

Other Institute of Physics publications on optics and photonics include: Visions 12 PhotonicsVisions 13 MegatelescopesVisions 21 MetamaterialsVisions 22 T-rays

http://visions.iop.org

Andrew BourneEngineering and Physical Sciences Research Council (EPSRC)Polaris HouseNorth Star AvenueSwindon SN2 1ET UKEmail: andrew.bourne@epsrc.ac.ukwww.epsrc.ac.uk