FORESIGHT

Exploiting the electromagnetic spectrum:State of the scienceoverviews

OFFICE OF SCIENCE AND TECHNOLOGY

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Contents

Introduction 1

Switching to light: 3

all-optical data handling

Manufacturing with light: 7

photonics at the molecular level

Inside the wavelength: 11

electromagnetics in the near field

Picturing people: 15

non-intrusive imaging

1

State of the Science Overviews

The aims of the Foresight ‘Exploiting theelectromagnetic spectrum’ (EEMS) project wereto identify key areas of long-term opportunityacross the spectrum, assess these against UKcapabilities and agree a plan of action to help theUK exploit these areas. Four topic areas wereselected through a rigorous scoping process,involving the academic, business, and usercommunities, along with representatives fromother government departments and fundingbodies. As part of the detailed study of thesefour topics, state of the science reviews werecommissioned for each.

Written by experts in the field, these reviews aretechnical documents that have been used to helpinform the drawing up of technology timelinesand plans for action. The reviews look at newtechnological advances, assess their likelyimpacts in 10–20 years’ time and consider theUK’s relative strengths in these areas. This report provides short and accessible overviewsof the state of the science reviews that havebeen written for the project by Judy Redfern, an experienced science journalist.

Introduction

Many technologies already make good use of some part ofthe electromagnetic spectrum: telecommunications networkscarry signals as light waves down optical fibres; mobilephones send microwave signals through the atmosphere;and x-rays have been used for more than a century inmedical imaging. Now, however, new methods ofmanipulating electromagnetic waves are on the brink ofrevolutionising many established technologies and makingnew ones possible.

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Switching to light: all-optical data handling

Over the past 20 years, optical fibre has takenover from copper cable as the medium by whichdata is transmitted over the telecommunicationsnetwork. Only the final link to your home remainsvia copper cable. Instead of travelling as pulses ofelectrons down a wire, the data is encoded onto alight wave that travels along an optical fibre.However, some of the properties that make lightso superior for carrying data make it difficult toroute to ensure the data reaches the rightdestination. Light is good for transmission, butelectricity is easier to manipulate and so is betterfor switching and routing.

Current networks get around this problem byconverting the optical signal to an electrical onefor routing, and then back to an optical signal foronward transmission. However, the conversionprocess introduces a bottleneck because it isslow and limits the information transfer rate. Atcurrent volumes of telecommunications traffic,such bottlenecks are not an issue, but they willbecome significant as we move into abroadband future.

The problem would be solved if the signal couldbe kept in optical form throughout the switchingand routing process. To achieve this, there aremajor science and engineering challenges to betackled. For example, a ‘memory’ is a keyfeature of any router. Electronic memory iseasily available, but an optical memory is veryhard to make – light signals are just too fast andslippery to be held in one place for more than amoment. Nonlinear optical techniques, madeaccessible by confining light in tiny spaces, offersome prospects, as do fast-tuning lasers. Butthe ‘Holy Grail’ of an ultra-fast all-optical networkmay have to wait for new approaches to reachgreater maturity. The most promising appear tobe photonic bandgap structures, new materialsthat can manipulate light in novel ways.

In the meantime, hybrid systems are underdevelopment that route and switch an opticalsignal electronically, without converting it.Research is also going on into network

Introduction

Bottlenecks occur in communications networks becauseoptical signals must be converted to electrical signals forrouting and processing. This review looks at the prospects of speeding up networks by developing all-optical methods of signal processing.

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architecture since the structure of a network canhave a considerable effect on the amount ofrouting and switching needed.

The following summarises research on routingand switching, networks, emerging technologiesand finally, as a postscript, the synergy betweenresearch into optical communications andquantum computing and cryptography. First,however, we take a look at the basics of optical transmission and the state of the art in fibre development.

Optical transmission

The increase in the transmission capacity of

optical fibre has more than kept up with the

growth in data traffic.

An optical fibre consists of a glass coresurrounded by cladding made from a slightlydifferent type of glass. The careful choice ofmaterials ensures that light is confined to travelalong the core by a process known as totalinternal reflection: whenever the light beam hitsthe interface with the cladding it is reflectedback towards the core.

Optical fibres can carry far more data thancopper wires. As each fibre is no more than ahair's breadth thick, many can be bundled intoone cable. Light also has another advantage:because different wavelengths do not interactwith each other, data can be transmitted onclosely spaced wavelengths without risk ofcross-talk. This means that data can be carriedon several hundred different channels down one fibre, a process called wavelength division multiplexing.

In designing the optimum optical fibre,researchers need to address three mainlimitations: attenuation, dispersion and nonlinearimpairments. Attenuation causes the strength ofthe signal to drop off rapidly after travelling afew kilometres. Dispersion causes the differentwavelengths of light in the signal to spread out.

Nonlinear impairments result from weakinteractions between wavelengths in neighbouringchannels and cause signal distortion.

Minimising these limitations depends on anoptimum choice among a number of factors: thewavelength of light and the wavelength range;power per wavelength; channel spacing; thecomposition of the optical fibre and its cladding;the diameter of the core; and how the lightsignal is packaged and introduced into the fibre.Attenuation, however, can never be entirelyeliminated, so amplifiers are required to boostsignals over more than about 80 km. Theerbium-doped fibre amplifier was the first devicecapable of doing this entirely optically for manydifferent wavelengths simultaneously.

The current state-of-the-art fibre, a so-called'standard' single-mode fibre, operates over abroad spectral range from 1,260–1,625nanometres (10-9 m), is impervious to water(which if allowed to penetrate a fibre will absorbsignals in the 1,300–1,400-nanometre range) andhas a very narrow core diameter (9 micrometres)to eliminate dispersion. The fibre's spectralrange gives it a frequency bandwidth of 35 THz,which is more than enough to carry all the trafficon the US internet. At present, fibre capacityoutstrips demand and hence the drive for furtherimprovement is minimal.

Routing and switching

All-optical routers and switches require

optical processing which is at the cutting

edge of scientific research. Hybrid systems

that process electronically tagged optical

signals could offer an interim solution.

Research in optical processing has centred ondeveloping a method based on a loop of opticalfibre. Light entering the loop can leave by one oftwo exits depending on whether a laser beam isalso flashed into the loop. The decision 'on' or'off' is very fast, but the device is physically too

5

Switching to light: all-optical data handling

big for use in most networks and there is littlescope for miniaturisation because of the limit onhow far optical fibre can be bent.

Hybrid devices, in which a signal remains opticalthroughout but has an electronic label forswitching and routing, show more medium- tolong-term promise. Several have beendeveloped. One of the most promising is aswitch based on microelectromechanicalsystems (MEMS) technology. It consists of tinymirrors etched onto silicon in much the sameway that transistors are etched onto anintegrated circuit. The mirrors move in responseto an electrical signal and can be directed tosend an incoming wavelength to a specificoutput port. MEMS switches with thousands oftiny mirrors, offering more than 10 terabits persecond of switching capacity, have beendemonstrated, but their practical use is limitedbecause they allow the signal to degrade.However, simple MEMS switches, with no morethan 16 x 16 input x output channels, are undertest in networks in Japan.

The problem of degradation is a serious one forall-optical networks. It is avoided in today'snetworks because the optical signal goesthrough regular electrical conversion, a processthat regenerates a perfect copy of the originaloptical signal. An all-optical method ofregenerating hundreds of wavelengthssimultaneously is one of the most pressingneeds for any system in which the optical signalis not regularly converted.

Networks

Bottlenecks can be circumvented by

redesigning networks to minimise the

need for processing at network nodes.

At present, when a message, say e-mail, entersa telecommunications network, it typicallyconsists of several packets, each of which islabelled with the recipient's address. Packets are

routed through the network individually vianodes that decide on the next leg of the route,given the density of network traffic. When thepackets are near their destination, they are re-assembled into the original message for delivery.Packets travel between nodes as optical signals,but they have to be converted to electronicsignals for processing at nodes.

Just as it can be quicker to take a long route viabackstreets to avoid a hold-up at traffic lights, soconventional packet-switched data networksroute packets to avoid bottlenecks at nodes.However, they under-use the bandwidthavailable in the optical cable between nodes.Network architectures are under developmentthat could reduce the need for processing atnodes in order to concentrate on making thebest use of this bandwidth. The simplest is awavelength-routed optical network (WRON) inwhich lightpaths are kept 'alive' across thenetwork irrespective of the amount of databeing sent. In more sophisticated architectures,such as optical burst or optical packet-switchingnetworks, messages are broken up into packetsand are either aggregated into wavelength'bursts' before being sent across the network tothe exit point or are transmitted as individualwavelength packets.

These architectures work more like the old-fashioned Royal Mail than today's packet-switched data network. For example, just as thesorting office puts all parcels for Glasgow into avan bound for that city, so an entry point to anall-optical network, say an internet serviceprovider, assigns all e-mail messages bound forGlasgow to the same wavelength. In a WRON,each e-mail will go immediately whether thewavelength is full or not, equivalent tosometimes sending a van with only one letter init. WRONs will be efficient only when trafficvolumes are so high that, in practice,wavelengths are always nearly full. In an opticalburst network, the wavelength, just like theGlasgow-bound van, will set off at certain times

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or when enough messages or parcels haveaccumulated. When it reaches its destination(the Glasgow sorting office or exit point) theparcel or e-mail is delivered to the rightdestination via a local service or network. Themore sophisticated the architecture, the moreefficient the use of the network capacity, but thegreater the demands on data storage andprocessing in routers at the network edges.

Photonic bandgap structures

Photonic bandgap structures offer the best

prospects for creating all-optical routers

and switches.

Fully satisfactory all-optical routers and switchesneed optical memory and processing thatexceeds the performance of their electronicequivalents. Recently developed photonicbandgap structures offer the best prospects for creating dense and potentially nonlinearintegrated optical circuits on which can begrown tiny components and waveguides.

A photonic bandgap structure consists of amaterial (such as glass) into which a periodicpattern of holes has been etched. The pattern ofholes determines the path of the light (typically,it travels through the glass avoiding the holes)and can be designed to deflect light round sharpbends. Tiny waveguides, unconstrained by thebending-limit disadvantage of optical fibre, canbe created.

Photonic bandgap devices could even be madefrom fibre. Photonic crystal fibre consists ofglass fibre penetrated longitudinally by a numberof hollow tubes. The positioning and size of thetubes affects the fibre's properties, opening upthe possibility of designing optical switcheswithin a fibre itself.

Quantum technologies

Many of the principles underlying the design ofan optical network will also apply to the designof a quantum computer and there will besynergy between research in the two areas.Developments in quantum cryptography,however, will have a more immediate impact.Quantum cryptography detects the presence ofan eavesdropper on the line from thedisturbance caused to the quantum state of asingle photon. It is only effective over opticalchannels and has so far been tested overdistances of 40–60 km.

Further details

This topic overview draws on the state of thescience review for the ‘Switching to light: all-optical data handling’ topic written by PolinaBayvel, Michael Dueser and John E. Midwinterfor the Foresight ‘Exploiting the electromagneticspectrum’ project. This document, the otherthree state of the science reviews and details ofthis project, are available on the CD-ROM thataccompanies the Foresight EEMS launch packand on our website at:http://www.foresight.gov.uk/emspec.html

7

Manufacturing with light:photonics at the molecular level

The four key technologies identified within thisreview are:

Optical tweezers: Shine a focused laserbeam on a microscopic particle and you can holdit in position, or move it alongside anotherparticle to see how the two interact. Laserbeams that perform such feats are known asoptical tweezers. Of enormous use in research,optical tweezers could find more widespreaduse in routine chemical testing.

Probing and controlling chemicalreactions: Light's ability to initiate and thenprovide information on chemical and biologicalreactions is also already widely used, but in thelab rather than in industry. However, this topicraises the tantalising possibility that light couldbe harnessed to produce tailor-made drugs andchemicals on an industrial scale.

Laser micromachining: Lasers arealready used as industrial tools to machine bulkmaterials, such as steel. Laser micromachiningis being developed to perform much smaller-scale tasks such as sculpting the tiny cogs andwheels of micromachines. Lasers provide aremarkably gentle and precise way of sculptingalmost any material.

Photonic crystals: Photonic crystals cansqueeze light into tiny (sub-wavelength) spaces,reducing it to a size suitable for probing ormoving molecules. Photonic crystals areartificially created materials that manipulate lightin much the same way as a semiconductormanipulates electrons, enabling photons to beredirected and stored. Just as developments insemiconductor technology forged the electronicsrevolution, so developments in photonic crystalscould pave the way for all-opticaltelecommunications networks and opticalcomputing. Photonic crystal fibres (optical fibres

Introduction

New techniques are becoming available that expand the waysthat light can be used to manipulate and examine objects atthe molecular scale. This review looks at these and relatedapplications, including biomolecular manipulation, personaliseddrug production and micro/nano-scale fabrication.

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that incorporate photonic crystals) are alreadycommercially available, but there is enormouseconomic potential in the development of awide range of photonic crystal devices. Of all thetechnologies investigated in this study, photoniccrystals show perhaps the greatest promise forfuture exploitation.

These four technologies are underpinned bylaser technology, which has undergone rapiddevelopment over the past 5–10 years. Cheap,small, high-power lasers are now available thatcan focus beams to a spot less than awavelength in diameter. The shortest possiblelaser pulse has also decreased to just a fewfemtoseconds (10-15 s), with attosecond (10-18 s)pulses on the horizon. Light travels just 0.0003 millimetres in one femtosecond and 0.3 nanometres (10-9 m) in one attosecond. Such short laser pulses can manipulate matteron these molecular scales. It is this shrinking ofboth size and pulse, together with finer controlover the beam, that has opened up new usesfor lasers. These advances are being matchedby the development of new materials capable of exploiting such finely tuned light.

Optical tweezers

Lasers are already used like tweezers to hold

and manipulate microscopic particles. The

challenge for the future is to manipulate

more than one particle simultaneously.

Like a moth attracted by a candle, a microscopicparticle is trapped where the light intensity ishighest, in the middle of a highly focused laserbeam. The particle can be moved around inthree dimensions and even rotated rapidly bymoving the beam or by giving it an intrinsic‘twist’. Conversely, by monitoring the particle’sposition, it is possible to sense the forces actingon it, for example, when it is brought close toanother particle.

One way of trapping several particles at once isto time-share the laser beam between them. An alternative is to use an array of focusedbeams produced by a hologram to makemultiple 'optical traps' to hold many particles inplace. The best and most adaptable way ofcreating such holograms is with spatial lightmodulators, which consist of a matrix of liquid-crystal cells that can be manipulated individuallyto create the desired hologram. Spatial lightmodulators can be reprogrammed rapidly tochange the hologram, making them ideal for thereal-time, three-dimensional manipulation of anarray of optical traps.

As well as measuring forces between particles,arrays of traps can also be used to sort particlesaccording to size, for example, to separatechopped-up pieces of DNA for geneticsequencing. However, optical traps are too largeto pin down most DNA fragments, which aresmaller than the wavelength of light. Newtechniques under investigation for trappingnanometre-sized particles include attachingmicrometre-sized ‘handles’ to them to makethem larger, and making smaller traps byexploiting new methods of squeezing light tosub-wavelength sizes. These methods includeusing photonic crystals and exploiting near fieldeffects, which occur when light interacts with asurface (see the ‘Inside the wavelength:electromagnetics in the near field’ review).

Alone, optical tweezers and traps are researchtools of enormous potential, mainly because of their ability to handle micro- and even nano-sized particles. Integrated onto a chip with other techniques, such as microscopy andspectroscopy, their use could extend to routineanalysis of biological and chemical samples, for example, in the doctor's surgery, forenvironmental monitoring or even for forensic testing.

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Manufacturing with light: photonics at the molecular level

Probing and controlling

chemical reactions

Using lasers pulsed for fractions of a second

it is already possible to watch chemical

reactions taking place. The future holds

possibilities of using laser pulses to watch

the movement of electrons within atoms.

Typically, chemical reactions at the molecularlevel occur on femtosecond timescales. Shine a femtosecond pulse of laser light of the rightwavelength at a molecule and you can initiatethe reaction. Shine further pulses a fewfemtoseconds later and you can 'watch' thereaction as it occurs, rather than having to inferwhat happened from the end products. Liketaking a series of snapshots to make a movie,femtosecond laser pulses record chemicalreactions, as they take place.

Many spectroscopic techniques are available orunder development for working out the natureof the chemical reaction from how it changesthe incident light. Conversely, it is possible toinfluence the outcome of a chemical reaction bychoosing an appropriate laser pulse shape. Thepulse shape can be modified until the desiredreaction is achieved.

Although much remains to be done withfemtosecond laser pulses, work is underway witheven shorter pulses that could probe processesthat occur on attosecond timescales, such as themovement of electrons within atoms. Attosecondpulses can only be produced indirectly by shininga femtosecond pulse into a gas of atoms such as deuterium. The femtosecond pulse exciteselectrons in the gas, leading to the emission ofattosecond pulses of x-rays and extremeultraviolet light. The short wavelength, as well as pulse length, of attosecond sources will openup new applications, in particular in studyingbiological processes.

Finally, femtosecond laser pulses shining on a nonlinear crystal can generate terahertz

radiation, which lies between the microwaveand infrared bands of the electromagneticspectrum and is otherwise difficult to generate.Terahertz radiation could be used for safemedical or security imaging because, unlike x-rays, it can penetrate materials withoutdamaging them.

Laser micromachining

Pulsed lasers can also be used

for micromachining and for three-

dimensional imaging.

The shorter the laser pulse, the less the thermaldamage around the machining site. The heatingis so fast and extreme that only a localisedsurface layer of material is vaporised, leavingunderlying layers unheated and undamaged.

The ability of lasers to machine almost anymaterial makes the technique particularlyattractive to industry. Nanosecond (10-9 s) pulses are short enough for most applications.Research to increase laser power, repetition rateand develop optics for highly focused beams willresult in greater uptake by industry over the nextfew years. However, shorter femtosecondpulses could be useful for niche applicationssuch as the machining of microscopic devices(for example, microelectromechanical systems –MEMS) out of silicon-based materials.

Femtosecond laser pulses are also finding usesin biomedicine, for example, to build up a three-dimensional image of brain-tissue samples. The first pulse images the tissue surface and the next ablates it to expose a fresh surface forimaging – and so the process repeats until theimage is built up. Femtosecond lasers mightalso find new uses in medicine, for example, ineye and ear surgery, dentistry, and in angioplastywhere they could be used to ablate plaque fromblocked artery walls.

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Photonic crystals

New materials are under development that

can manipulate and control light. The most

promising are photonic crystals. They will

have a major impact on many of the areas

considered by this Foresight project.

Photonic crystals can manipulate light to performmany functions that would otherwise bedifficult, if not impossible. For example, they canbe designed to reduce the diameter of a laserbeam to much less than a wavelength, openingup the possibility of trapping molecules whichare too small to be trapped by an ordinary laser.They can also be designed to transmit just onewavelength, to spread a narrow band ofwavelengths into a broad band of intense whitelight, to change the direction of incident light, oreven to slow it down. These and other abilitiesare opening up an enormous number of newapplications, including the ‘Holy Grail’ of opticalprocessing and computing.

Photonic crystals consist of a material into whicha three-dimensional periodic pattern of holes hasbeen etched. By careful choice of the material,the size and spatial arrangement of the holes,and the placing of defects that interrupt theperiodicity of the hole arrangement, photoniccrystal structures can be created that performfor light all the functions that semiconductorintegrated circuits perform for electrons. A lot ofresearch is underway worldwide into the designof photonic crystal circuits and integrated chips,the types of material they could be made from,and efficient methods of manufacture.

Materials under investigation include organicsemiconductors, liquid crystals and evenassemblies of colloids (suspensions of smallsolid particles in a liquid) made with the aid of optical tweezers. Manufacturing methodsinclude new methods of lithography such as embossing, microcontact printing and micromoulding.

Photonic crystals can also be made into fibres.Many different fibre designs already exist, thetwo most promising for near-term applicationbeing single-mode and hollow-core fibres. In asingle-mode fibre, light travels though thetransparent material, avoiding the holes. In ahollow-core fibre, light travels through a centralhole, which is typically filled with air, allowingvery high powers to be carried without the fibre disintegrating.

Various effects, known as strong nonlinearoptical effects, make single-mode fibresparticularly promising for use intelecommunications, but more research needsto be done on reducing losses before they cancompete with traditional optical fibres. Thenonlinear effects can also turn a narrowbandwidth laser beam into a broad bandwidth,but intensely bright, white light source withmany potential uses, for example, in coherencetomography, a medical imaging technique.Hollow-core fibres could be useful gas sensors,as the laser will determine the composition ofthe gas in the hole.

Research into photonic crystal fibres is feedinginto the other three key technologies identified inthis study by aiding the development of lasersthemselves. The fibres are at the cutting edge ofresearch to improve laser beam delivery systems.

Further details

This topic overview draws on the state of thescience review for the ‘Manufacturing with light:all-optical data handling’ topic written by KishanDholakia and David McGloin for the Foresight‘Exploiting the electromagnetic spectrum’project. This document, the other three state ofthe science reviews and details of this project,are available on the CD-ROM that accompaniesthe Foresight EEMS launch pack and on ourwebsite at:http://www.foresight.gov.uk/emspec.html

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Inside the wavelength:electromagnetics in the near field

A new class of artificially created materials,known as metamaterials, is set to have a majorimpact on a broad range of technologies,including mobile communications, medicalimaging, communications between and withincomputer chips and flat-panel displays. Negativeindex materials, one type of metamaterial,exploit the so-called near field that always existsclose to a source of electromagnetic radiation,but tails off rapidly at distances longer than awavelength. Unlike the more familiar far field,which propagates away from a source, the near

field transports no energy from the sourceunless it is intercepted. Negative indexmaterials, however, are able to manipulate thenear field to dramatic effect: they display somenovel and surprising electromagnetic properties.

An electromagnetic source can be thought of as a place where radiation emerges from onemedium into another. Familiar sources include a glass lens, a light bulb, a radio mast forbroadcasting TV signals and a waveguide hornwhich emits microwaves. For visible light,wavelengths are very short (4–7x10-7 m) so thenear field decays too close to the interface to be intercepted casually. But microwaves andradiowaves have wavelengths ranging from afew centimetres to hundreds of metres. Whenyou use a mobile phone, for example, your headis in the near field of the microwaves emitted bythe phone's antenna, which have wavelengthsof tens of centimetres.

At optical wavelengths, near fields have littleimpact and so have been largely ignored. Atlonger wavelengths, their impact needs to bemanaged. Antennas and radio transmitters, forexample, should be designed to divert orminimise them, so that they do not drain energyunnecessarily from the power source, such as abattery, or from the propagating far field. With afew notable exceptions, the near field has often

Introduction

Mobile communications, imaging and flat-panel displays couldbe revolutionised by new materials that can control previouslyunused physical properties of light and electromagneticradiation (the near field).

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therefore been seen as a mild irritant that isdifficult to manage. With the development of thenew negative index materials, however, thatattitude is changing.

Capturing near fields

Materials are being developed to control the

near field for new applications.

The scanning near field optical microscope(SNOM) is one of the notable exceptions wherenear fields have already been harnessed and putto good use. It works by placing a sample withina wavelength of visible light of the tip of asharpened optical fibre so that the sample'ssurface is within the near field created at theoptical fibre tip. The sample picks up energyfrom the near field that affects the light thatwould otherwise be reflected at its surface.Detail about the molecular structure of thesurface can then be revealed.

A negative index material, however, placed in a near field would capture almost all of it andreduce the reflected beam to near zero. What is more, it would capture the near field from adistance much further than a wavelength awayfrom a source. Negative index materials havesome very unusual properties. For example, anelectromagnetic wave travelling through such amaterial appears to transmit energy one waywhile travelling in the opposite direction; andrather than decaying within a wavelength, a nearfield entering a negative index material wouldactually be amplified. Their most remarkableproperty, however, is that they have a negativerefractive index.

Refractive index gives a measure of the extentto which a material deflects the path of a lightbeam entering it. The deflection comes aboutbecause light travels through different materialsat different speeds. When it travels through airinto glass, for example, it slows down and thuschanges direction at the interface between thetwo media. All natural materials have a positive

refractive index because they bend light awayfrom the surface. Negative index materials,however, bend it in the opposite (negative)direction – hence their name.

Victor Veselago, a Russian physicist, firstexplored the idea of negative index materials in the 1960s. But it was not until 1999 that John Pendry from Imperial College, London, and colleagues at Marconi UK first describedhow they might be made. They realised thatstructures consisting of a periodic arrangementof certain electronic components could look,from a microwave's point of view, like a materialwith a negative index. In 2000, David Smith fromthe University of California, San Diego, producedthe first true negative refractive index material. It consisted of arrays of fine wires sandwichedbetween sheets of fibreglass onto which hadbeen printed copper, millimetre-sized, electroniccomponents called split-ring resonators.

Research is now centred on developing differenttypes of negative index material withelectromagnetic properties to suit different usesand wavelengths. These materials could be usedto control, route and amplify the near field. Thisability to tailor a material for a use is leading tonew science and applications.

Applications: antennas,

magnetic resonance imaging

and flat-panel displays

The new materials could reduce energy

usage by mobile telephones, produce

lightweight radars, reduce the cost and size

of magnetic resonance imaging, be used in

‘real’ (viewable from all angles) flat-panel

displays and in the next generation of data-

storage devices.

Research on negative index materials has alsogiven a boost to the search for innovative waysof using more conventional materials to capturenear fields. For example, some UK companies

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Inside the wavelength: electromagnetics in the near field

are developing antennas with ceramic cores thattrap the (in this case unwanted) near field.However, such conventional materials can beheavy, expensive and difficult to make, soantenna designers are turning to the newmaterials and coming up with some interestingideas. One application, under development byBoeing in the US, is to use lenses made ofnegative index materials for radar. A lightweightconcave lens made from a negative indexmaterial could do the same job as a muchheavier, more expensive and more difficult tomanufacture convex lens made from aconventional material.

In magnetic resonance imaging (MRI), negativeindex materials could fulfil a role thatconventional materials cannot because they can operate in a high magnetic field withoutdisturbing it. In MRI, a sample, usually a patient,is placed inside a powerful magnetic field. Thehydrogen atoms in the patient's body line up in the direction of the magnetic field. A radiofrequency pulse disturbs this alignment and thepicture is formed by collecting the characteristicradio waves emitted by the atoms as they realign with the magnetic field.

A major drawback with MRI is that theresolution of the image is limited by thewavelength of the radio waves emitted by theatoms. The only way of reducing the wavelengthat present is to increase the magnetic fieldstrength which is difficult and expensive to do. A solution for the longer term, however, couldbe to capture the sub-wavelength details in thenear field using a ‘superlens’ made from a slabof negative index material.

One interesting application for surface media,under development in Japan with supportingresearch in Europe, is in flat-panel displays. Thelight in such a display would emerge as tightbeams from holes in the surface media material.Different colours could be selected by adjustingthe viewing angle. The most important future

applications of surface media, however, arelikely to make use of their ability to focus light toa spot smaller than its wavelength. For example,the amount of information that can be writtenonto a CD or DVD is limited by the diameter of alaser beam, which is limited by the wavelengthof laser light. At present, one feature film can becomfortably recorded onto one DVD. Being ableto focus a laser to a spot much smaller than itswavelength could lead to DVDs with up to 1,000movies recorded on each.

Further details

This topic overview draws on the state of thescience review for the ‘Inside the wavelength:electromagnetics in the near field’ topic writtenby Anthony Holden for the Foresight ‘Exploitingthe electromagnetic spectrum’ project. Thisdocument, the other three state of the sciencereviews and details of this project, are availableon the CD-ROM that accompanies the ForesightEEMS launch pack and on the project websiteat: http://www.foresight.gov.uk/emspec.html

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Picturing people: non-intrusive imaging

Go to an airport and you will have to walkthrough a metal detector before boarding theplane. Walk around any city centre and thechances are that your image will be recorded onnumerous CCTV cameras. Go to hospital andyou may well have an ultrasound scan or besubjected to some other imaging technique.Non-intrusive imaging, whether for security ormedical purposes, is widespread and almosteverybody nowadays will encounter it at somestage in their lives. New imaging techniques arebecoming available that will increase thecapabilities and pervasiveness of non-intrusiveimaging in the near-to medium-term future.

Most of the electromagnetic spectrum, fromvery long radio waves to high energy x-rays, isused in some way for imaging. Until veryrecently the notable exception was terahertz

waves, which are shorter than microwaves butlonger than visible light. The problem has beenthe lack of cheap sources. That is now changing,however, with the availability of the firstcommercial terahertz imaging system through aUK company.

Terahertz security imaging

Terahertz and millimetre waves may herald a

new generation of CCTV-type technology to

check for weapons, explosives and drugs.

As terahertz radiation reacts with all molecules,many applications are opening up, ranging fromthe detection of skin cancer to the detection oflandmines. Most significant, however, is likely tobe the identification of people who are a securitythreat, especially suicide bombers who need tobe stopped tens of metres before they reach atarget. At present, the only acceptable way tofind a hidden weapon is to search the suspectby hand. The routine use of x-rays, which canreveal metal objects such as guns, isunacceptable because of the health risks fromlow doses of ionising radiation. Furthermore, x-rays cannot pick out non-metal objects, such asceramic knives or plastic explosives. Terahertzradiation, however, can penetrate clothing from a distance to reveal and identify many types ofhidden material including narcotics as well asexplosives and other weapons.

Microwave radiation is also under developmentfor security imaging. Originally developed to seethrough fog and rain, microwave imaging hasalso been used to detect weapons hidden underclothing and illegal passengers hidden on lorriesthat have non-metallic sides. Lower-frequency

Imaging technology is already widely used for applicationssuch as CCTV and medical imaging. New science promisessignificant advances in these areas.

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microwaves penetrate clothing and theatmosphere better than higher frequencies, but as the wavelength is longer, the resolution is poorer.

Hybrid systems

Improved medical imaging will allow diseases

to be spotted and diagnosed at much earlier

stages, without biopsy and with less use of

damaging ionising radiation.

In medical imaging, research is directed atimproving the resolution of existing techniquesand at developing new technologies to replacethose that use ionising radiation, particularlygamma cameras and positron emissiontomography (PET) and to a lesser extent x-rays.Improved resolution will allow individual cellsand molecules to be imaged so that diseaseprocesses can be detected before they lead togross anatomical changes. At present, however,techniques using ionising radiation, such as PET,are often the only way of providing informationon metabolic or physiological processes andthey are unlikely to be superseded in the near future.

Hybrid imaging systems that combine twotechniques and, in the process, optimise both,are showing promise of improving resolution andreducing exposure to ionising radiation. Forexample, MRI provides good structuralinformation but, even with the best existingmachines, the resolution is limited to about 1millimetre. PET can detect the high metabolicactivity of, say, cancer cells by measuringgamma rays given off by a radioisotopeintroduced into surrounding tissue, but theresolution is poor. By combining the twotechniques, however, the position of cancer cellscan be located within an anatomical structureand the boundaries of the tumour located withgreater precision than with either techniquealone. Such precision could be used to guidesurgery and also to monitor treatment progress.

Improved magnetic

resonance imaging

Smaller, cheaper magnetic resonance

imaging will become available for health

imaging, although ultrasound and infrared

imaging (thermography) may capture

this market.

Magnetic resonance imaging (MRI) is one of thefew medical imaging techniques able to imagealmost any organ in the body, but, because ofthe high cost, it is not available in most UKhospitals. PET is also expensive and even lesswidely available. Research aimed at improvingthe performance of both techniques is unlikelyto lead to cost reductions in the near future. ForMRI, increasing the strength of the machine'smagnetic field will improve resolution, but willinvolve using larger magnets or leading-edgemagnet technology, which will add to costs.Machines operating at the present limits ofresolution, however, are already good enoughfor studies that correlate individual variations insmall anatomical structures with geneticvariation, for example prefrontal neurons withthe genetics of schizophrenia. MRI is also beingdeveloped to image metabolic processes in nearreal time, thus partly replacing PET. Thisfunctional use of MRI, however, also requireshigher magnetic fields.

Small-scale MRI and PET scanners are underdevelopment for use in genetics studies withsmall laboratory animals. The miniaturisation of the technology could be applied to smaller,cheaper machines for human use. However,ultrasound and infrared imaging (thermography)show greater promise of providingcomprehensive imaging systems at reasonablecost that do not rely on ionising radiation.

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Picturing people: non-intrusive imaging

Lower-cost technologies with

potential: ultrasound, infrared

and visible imaging

The use of ultrasound as a treatment tool will

increase. New medical imaging technologies

based on infrared (thermography) and visible

light (optical coherence tomography) are also

likely to increase.

Ultrasound relies on sound waves rather thanelectromagnetic radiation. It has been usedwidely for medical imaging for many years andimprovements in resolution are continuing.However, its use is now expanding to includemore invasive procedures and the provision offunctional information. For example, transverse-only sound waves are being developed that canbe sent down a waveguide to specific locationsin the body where they can perform proceduressuch as the safe breaking up of blockages inarteries. Ultrasound is also being developed toprovide functional information, for example,changes in blood flow using the Doppler effect,which could be used to monitor theeffectiveness of cancer therapies that attackblood vessel formation in tumours.

Thermography is a passive imaging techniquebecause it relies on the detection of heat in theform of infrared radiation given off naturally by abody. Most other imaging techniques are activebecause they irradiate a body with some form ofradiation and measure the response. Infraredtechnology was originally developed for nightimaging for military use, but recent advances,especially in detector technology, are leading tonew applications. For example, thermography isunder development for routine breast cancerscreening as a safer alternative to x-rays.

Visible imaging also has untapped potential forproviding relatively low-cost medical imaging in avariety of settings. The disadvantage, that visiblelight penetrates no more than a few millimetresbelow the skin surface, is offset by the availability

of fibre optic instruments that can deliver light tomany locations within the body. Also, opticalimaging systems can provide much higherresolution than MRI or other imaging techniques.For example, optical coherence tomography(OCT), a technique that provides real-time, three-dimensional images of small surface structures, issensitive enough to guide surgery, or to measurethe response of tissues to drugs. The resolutionis high enough to image individual cells andresearch is underway to identify contrast agentsthat could be used to 'tag' a cell or molecule,enabling its progress to be watched as itundergoes a physiological process.

Photonic crystal optical fibres, which generatevery intense broadband light sources (see the‘Switching to light: all-optical data handling’review), are finding some of their firstapplications in OCT. The range of differentwavelengths delivered by a photonic crystal fibreallows different tissues to be identified in asample through spectroscopy. This is potentiallya very powerful technique that could surpassPET in the provision of high-resolution, functional imaging of structures accessible to a fibre optic instrument.

X-rays

Technology will develop to reduce doses of

x-rays required for imaging; combining x-ray

analysis with other forms of imaging is likely

to be the key way forward.

X-ray imaging is one of the oldest medicalimaging technologies and still the most widelyused. It is also extensively used in securityimaging of baggage and goods. X-ray images areno longer developed on film but are produceddigitally. X-rays are also used to produce high-resolution, three-dimensional images using asystem called computed tomography (CT).Future developments include developing betterx-ray detectors, which will allow a reduction inthe x-ray dose to a patient, and speeding up CT

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so that dynamic processes can be imaged. X-rayCT is also being fused with other techniquessuch as PET and MRI so that high-resolutionanatomical information can be combined withfunctional imaging.

Use of markers to improve imaging

Advances in markers which provide the

‘contrast’ that allows certain imaging

technologies to ‘see’ will lead to improved

medical imaging.

One of the most significant drivers forimprovements in medical imaging could be the development of new contrast agents,substances or techniques that exaggerate thedistinction between different tissues andthereby improve imaging sensitivity. Contrastagents include the use of ultrasound to createtiny bubbles in a liquid, a process known ascavitation, to distinguish it from surroundingmaterial. Advances in genetics and the study of proteins are also expected to lead to thedevelopment of new contrast agents in the form of markers or tags for specific genes orproteins that can be picked out during imaging.

The need for better

software analysis

Improved resolution and new imagingtechniques will produce more data that needs to be analysed and stored. The development ofnew software to enhance images, extract extrainformation or identify specific structuresthrough pattern recognition is thus essential tointerpret these data, and realise the full potentialof new technologies.

Further details

This topic overview draws on the state of thescience review for the ‘Picturing people: non-intrusive imaging’ topic written by Douglas Paulfor the Foresight ‘Exploiting the electromagneticspectrum’ project. This document, the otherthree state of the science reviews and details of this project, are available on the CD-ROM thataccompanies the Foresight EEMS launch packand on the project website at:http://www.foresight.gov.uk/emspec.html

Printed in the UK on recycled paper with a minimum HMSO score of 75. Department of Trade and Industry. © Crown Copyright. www.dti.gov.uk DTI/Pub 7248/1k/04/04/. URN 04/856