Exploiting the Electromagnetic Spectrum - CiteSeerX

Foresight Exploiting the Electromagnetic Spectrum State of the Science Review

Inside the wavelength: electromagnetics in the near field Anthony Holden Consultant Physicist

This review does not represent the view of the DTI or Government policy, but is an account of the state of the art in the field by the commissioned author(s). This document is one of four state of the science reviews produced for the four topics selected for detailed study in the Foresight Exploiting the Electromagnetic Spectrum project. Further details are available at the Foresight web site: http://www.foresight.gov.uk/ Contact the Foresight Exploiting the Electromagnetic Spectrum team at: Foresight Directorate Office of Science and Technology 1 Victoria Street London SW1H 0ET Fax: E-mail:

020 7215 0054 [email protected] Page 1

Executive Summary From infants we are familiar with the rainbow – the visible, coloured, part of the electromagnetic spectrum. But electromagnetic waves, of which the visible is only a minor part, enter many more parts of our lives, enabling mobile and fixed wire communication, medical imaging, visual displays, security sensors, drug detection and so much more. Personal mobile communications are all pervasive with phones, electronic notebooks, laptops and smart cards becoming able to communicate wirelessly with each other and our environment. Everything, even our clothes, will soon send and receive electromagnetic radiation in some form (other than just colour), to transfer our data, monitor our health, check our security and network with our friends. In getting physically closer to the emitting and receiving surfaces, or antennas, we enter a new regime known as the near field. Here the electromagnetic radiation is dominated, not by the propagating beams, familiar in light from a torch say, but by non-propagating fields, which are strong close to the emitting surface but decay away rapidly in distances comparable to a wavelength of the radiation. When using a mobile phone or lying in a medical imaging machine for example, the near fields are important on a human scale and their management is crucial to the safe and efficient performance of such devices. Near field control and manipulation is also vital at optical frequencies where resolution of very small objects is necessary for making electronic integrated circuits, detecting low levels of chemical or biological agents and the manipulation of molecules. Better insights and radical new discoveries are emerging about near field radiation across a wide frequency spectrum from radio frequency and microwaves to the infrared and visible. The world is poised for greater exploitation of these effects across a broad front. This review considers the new science very close to the emitting and receiving surfaces. We find that new discoveries about metal surfaces are breaking the traditional rules of optics. Electromagnetic radiation can pass efficiently through holes much smaller than the wavelength and produce locally intense sources, which can be steered or captured to make new “electronic” circuits, antennas or visual displays. Artificial materials have been demonstrated which can amplify, steer and image the near fields at RF, microwave and THz frequencies enabling radically new antenna designs, a paradigm shift in medical magnetic imaging and many other applications. These technologies combined with innovation in high dielectric antenna design open the prospect of novel antennas, integrated circuits, sensors, scanners, displays and imagers, which have the potential to outperform conventional technology. The UK proves to have a world lead in many aspects of the new technologies combining a strong University and industrial base with understanding of the fundamental physics through to product applications, especially in antennas and photonics activity. In the face of strong programmes in the US but improving support from Brussels, the review recommends a focused approach to the antenna opportunities, local near field communications and optical sensing in collaboration with the key industrials and an enhanced research and development programme into the potential of RF near field control in medical imaging and elsewhere. This to be combined with strategic support for innovation in all areas where the UK has capability, especially in photonic components, photonic integration and the continued support for nanotechnology infrastructure, essential for the realisation of these technologies. Page 2

Contents CONTENTS ...................................................................................................................................................3 INTRODUCTION ............................................................................................................................................5 1) NEAR FIELD PHYSICS – BASIC PHYSICS BUT NEW IDEAS ...........................................................................6 More than just light beams.....................................................................................................................6 Capturing the near fields .......................................................................................................................7 Summary for Section 1 ...........................................................................................................................8 2) NEW PHYSICS AND NEW DISCOVERIES – THE STATE OF THE SCIENCE OF NEAR FIELDS .............................9 Amplifying the near fields – a perfect lens...........................................................................................10 Tunnelling light through metal plates – giant light transmission ........................................................12 Managing the dielectric constant for compact antennas and near field control..................................14 High magnetic permeability materials for imaging magnetic fields ....................................................15 Summary for Section 2 .........................................................................................................................17 3) A NEAR FIELD MANIPULATORS TOOL BOX, THE WORLD STATE OF THE SCIENCE ....................................17 Effective media at RF (MHz) ...............................................................................................................19 Effective media at microwaves (GHz) to THz ......................................................................................20 Discrete elements in the microwave spectral domain ..........................................................................22 Effective media in the IR to visible spectral domain ............................................................................23 Surface media in the microwave domain .............................................................................................23 Surface media in the IR to visible spectral domain..............................................................................25 Summary for Section 3 .........................................................................................................................28 General philosophy..............................................................................................................................30 Novel antennas.....................................................................................................................................30 Medical imaging ..................................................................................................................................33 Table 2. Applications in medical imaging...........................................................................................34 Photonic interconnects for sub wavelength circuits and active devices ..............................................35 Flat panel displays...............................................................................................................................37 Sensors (microwave, RAMAN, fluorescence).......................................................................................38 Near field imaging at optical frequencies (surfaces, molecular and biological systems)....................38 Optical data storage ............................................................................................................................39 SERS / SNOM.......................................................................................................................................39 Medical therapeutics............................................................................................................................40 Ensuring safe medical procedures and environment ...........................................................................40 Summary for Section 4 .........................................................................................................................41 5) FUNDING: EXISTING AND FUTURE OPPORTUNITIES ................................................................................42

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Table 3. Key issues and unresolved problems in near field science....................................................45 Summary for Section 6 .........................................................................................................................48 7) CONCLUSIONS AND RECOMMENDATIONS ..............................................................................................49 8) ACKNOWLEDGEMENTS ..........................................................................................................................51 9) GLOSSARY ............................................................................................................................................52 10) BIBLIOGRAPHY ....................................................................................................................................55

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Introduction The aim of this report is to review the state of the science in the understanding and exploitation of so-called near field effects in electromagnetics. These fields are properly part of the make up of the electromagnetic radiation around an emitting surface such as a radio antenna and have been addressed and understood in antenna design for many decades. Since they decay over distances of the order of a wavelength they are not widely observed and their relevance has been confined to detailed antenna design at long wavelengths and specialist effects in optical sensing and molecular manipulation, for example. However recent discoveries have uncovered the possibility of manipulating and exploiting these fields in a wider range of applications and over a much wider spectral range from MHz right through to visible light.

Such applications could impact many

areas of life including medicine, communications, electronics, chemical and biological detection, security and advanced scientific research. We therefore review the concepts, look at the new discoveries and detail the progress of the research worldwide and especially where the UK has a strong or leading position. In Section 1 we explain what near fields are and how they can be observed and used in conventional optics. Section 2 then reviews the recent discoveries in the area of new materials which are structured to enhance the near fields and produce in some cases totally new physics.

The areas of research are broken into convenient domains in

Section 3 based on the spectral range they apply to and the technology being used to exploit the near fields. The applications and future perspective is outlined in Section 4 giving some concept of the near term exploitation and the 5 to 10 year foresight. How the work is currently being funded and where future funding might arise is addressed in Section 5. In Section 6 we summarise the key issues, again using the technology domains defined in Section 3 and we make some general conclusions and recommendations in Section 7.

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The publication rate in these new areas has grown exponentially over the last three or four years. Section 8 contains a representative publication list, referenced to relevant Sections of the document. It is deliberately limited to the most recent publications except where earlier seminal papers are essential for understanding and background. 1) Near field physics – basic physics but new ideas

More than just light beams Apart from the familiar sun, sources of electromagnetic radiation (emr) can take many forms depending on the wavelength and include aerials at radio, wave-guide horns at microwave, and sources such as light emitting diodes or lasers in the visible and infrared. More fundamentally an electromagnetic source is usually where emr emerges from one medium into another (the metal antenna to the air or the wave guide mouth to free space). We can view such interfaces between two media as a surface with material 1 on one side and material 2 on the other. Each material can be described by general properties such as refractive index (or more fundamentally dielectric permittivity ε and magnetic permeability µ). In general there may be a number of such interfaces in series or adjacent to each other. It is sometimes convenient to imagine the interface to be covered in an array of point sources, reradiating the emr from one side of the interface to the other. A long way away from the interface (typically a few wavelengths) these sources have combined into propagating waves and we have the traditional radio, microwave or optical “beam” which carries our broadcasts, illuminates our homes or tracks ships and aircraft (radar). This so-called “far field” regime is the most familiar to us and treating it on its own is a very powerful approximation which is valid beyond only a wavelength or so from the source interface. It is most conveniently described in terms of ray tracing and we have well-known laws for the behaviour of these rays as they pass through various media and their interfaces (Snell’s Law describing refraction at interfaces between media with two different refractive indices for example – light rays entering a pond). However, a full solution of the emr source (described by Maxwell’s equations) reveals that there are other field components in addition to the propagating electromagnetic field. Page 6

These

contributions are only significant close to the source interface and are called “near fields”. Near fields are solutions of the equations, which do not propagate like the beams in the far field but decay exponentially in distances of order a wavelength. They do not carry any energy away from the source unless they are intercepted before they die away. It is these fields, their practical effects and the ways in which they can be intercepted, manipulated and indeed amplified which are the subject of this rapidly expanding area of science reviewed here. Capturing the near fields The importance of the near fields can be better understood when we realise that although they decay in distances of the order of a wavelength, for many emr applications this region (long or short) is very significant in practical terms. It can be quite long; for example the long-wave radio transmitter sends out waves, which are hundreds of metres long. Of more relevance to this review, the radio frequency waves used in a Magnetic Resonance Imaging (MRI) scanner in medical diagnosis have wavelengths of a few metres and the whole imaging procedure inside the magnet takes place in the near field of the radio frequency (RF) coil. This creates an almost purely magnetic RF near field with very little electric field and is exactly what is needed for safe imaging of patients. In another relevant example, the waves from a mobile phone have wavelengths in the tens of centimetres so when you hold a phone to your ear your head is in the near field regime and in this case the electric fields can be relatively large and enter the head. When you are “in” the near field these fields are as large as, and in some conditions larger than, the fields of the familiar propagating waves far away from the source. An example from classical optics shows how the near field can be observed and exploited. If a beam of light is incident on an interface from the higher refractive index side (say from underwater in a pond) there is an angle of incidence from the normal to the interface plane beyond which the beam is “totally internally reflected” (TIR) and no propagating wave passes into the lower refractive index medium. Think for example of a prism where light enters on one face, is totally reflected at the 2nd face and exits at the 3rd Page 7

face. At the 2nd face, providing the conditions are correct, TIR has occurred and all the light is sent towards the 3rd face. Now just beyond the total reflecting interface, in the lower refractive index material (say free space), although no propagating waves exist there are exponentially decaying near fields which are solutions of Maxwell’s equations at the interface. They carry no energy and so to the distant observer they do not exist and all the energy appears in the reflected beam, which is then incident on the 3rd face of the prism. However, if another material is brought near to the reflecting face 2 from the free space (lower index) side, when it comes within a wavelength of the light (less than a few hundred nanometres in the case of visible light) the near field “tail” enters the new material and the oscillating field crosses the gap (a process described as tunnelling) and then appears again as a propagating wave in the new material. Energy is now transferred across the gap into the new material, depleting that reflected towards the 3rd face of the prism. The total internal reflection is said to be “frustrated”. We have in essence detected, manipulated and exploited the near field of the emr source (in this case the 2nd face of the prism). By looking at the changes in the beam reflected towards the 3rd face we can deduce information about the material approaching the 2nd face and frustrating the TIR. A simple fingerprint sensor works in this way. The print is imposed onto the 2nd face of the prism and the image is “projected in the 3rd face beam” (the pattern frustrates the TIR where it touches the 2nd face and patterns the reflected beam towards the 3rd face).

Frustrated total internal reflection is the essence of a powerful near field

microscopy technique called Scanning Near Field Optical Microscopy (SNOM). Here a sharpened optical fibre is the 3rd medium, which frustrates the TIR in the sample surface allowing the tip to “sniff” the near field of the surface and detect details at a molecular level in a sample, which is effectively the second prism face in our example. Summary for Section 1 •

A source of emr is generally described as an interface between two media of different refractive index from which radiation is emitted.

•

Two types of electromagnetic field are created: o Far fields which propagate, can be described by beams and ray tracing. Page 8

o Near fields, which normally do not propagate, carry no energy and decay in a wavelength or so. •

Near fields can be found at an interface where the beams are totally internally reflected (TIR).

•

Near fields may tunnel into a third high index medium if it is brought within a distance less than the decay length (about a wavelength) and “frustrates” the TIR.

2) New physics and new discoveries – the state of the science of near fields

Near fields exist and can be and are exploited. Many wavelengths away from the interface they are small (exponentially decaying) but very close to the source (interface) (within a wavelength) they are as large as the propagating waves (and in the new materials we are to discuss they are enhanced and can be very large indeed). Moreover they are not beams in the sense that we like to think about propagating (far field) emr and they cannot be manipulated like beams. You cannot, for example focus them using conventional lenses since they just never make it to the other side of the lens. This is actually more fundamental than may at first appear because if you think of an image formed in the far field by a conventional lens it in fact is not a “perfect” image of the source since it has fundamental parts of its emr make up missing – the near fields. The new discoveries and in some cases new physical effects that have emerged in the last few years are all about enhancing and controlling these near fields and enabling them to be used in ways not previously considered. To do this we have to make and exploit materials with properties very different from the “classical” materials that we are used to. For simplicity we will often treat the materials as effective uniform media (although in practice they are often strongly anisotropic and locally structured, for most discussions the structure is small compared to the wavelength of the emr). We can consider such materials as described by their dielectric permittivity ε and magnetic permeability µ and the interesting physics appears when we can separately control these values in specific

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spectral ranges, making them large and even arranging for one or both of µ and ε to be negative. We can encompass the wide sweep of new science and applications by considering four inter-related classes of such materials: •

Materials where the refractive index is negative (achieved by making ε and µ negative simultaneously): these are the so-called Left Handed Materials (LHMs) or more descriptively, Negative Index Materials (NIMs).

•

The involvement of surface plasmons in structured (usually metal) surfaces (at wavelengths where ε is again required to be negative in the metal) and known collectively as plasmonics.

•

Large ε materials for compact and near field controlling antennas (these may also incorporate independent control of µ to achieve impedance matching).

•

Finally, materials with relatively large µ at for example RF frequencies, which have no DC magnetic response (such materials do not occur naturally) for applications in magnetic imaging and near field control in medical and other applications.

It is not the purpose of this review to explain these vast and expanding new technologies in detail. Indeed each is developing a myriad of sub-facets even as we watch. The wavelengths at which these concepts can be demonstrated are also expanding with various aspects now investigated from the radio frequency bands (MHz) via the microwave (GHz) and THz bands and into the infrared and visible. We take representative examples of these different classes of materials in action to illustrate the concepts and indicate the applications. Amplifying the near fields – a perfect lens If we return to our TIR interface and consider a material approaching into the near field to frustrate it we can formalise the arrangement by imagining three slabs of material with respectively high, low, high refractive index. If TIR is set up at the first high to low interface the evanescent near field will appear in the 2nd low index material. The amount Page 10

of light we can capture into the third high index material will depend on the thickness of the low index “gap” material through which the near field light has to “tunnel”. Since the near field modes are exponentially decaying it will be easily seen that the amount of light transmitted into the third medium will fall off exponentially with the width of the second medium. To make this more useful it would be valuable to be able to amplify the evanescent fields and remove this dependence on the gap width. This trick is exactly what it turns out you can achieve with a material which we refer to as a Negative Index Material (NIM) or a Left Handed Material (LHM) for reasons that will become clear later.

These materials are also known in the literature as metamaterials (“beyond”

materials) or Veselago materials (after the researcher who first proposed them in 19681). In a NIM the properties are manipulated so that the refractive index becomes negative. It turns out that this may be achieved in more than one way although the easiest to explain (though not to achieve) is to consider a material where both the dielectric permittivity and the magnetic permeability are negative simultaneously in a particular spectral range. Negative dielectric permittivity is easier to arrange than negative magnetic permeability. Indeed the former occurs naturally, in metals below the bulk plasma frequency for example (note by the way the link with surface plasmons which only occur when the dielectric permittivity is negative – below the bulk plasma frequency.)

To obtain

negative permeability we had to wait for the work of Pendry et al.12 in the UK and putting them both together to make real life NIMs for the first time was down to Smith et al.2 in the USA. In an NIM everything appears to be opposite. The index of refraction is negative so that light, for example, bends the ‘wrong way’ when it enters the high (negative) index medium from a low positive index medium. A light source located behind a plate of NIM would appear to be in front of it. Because ε and µ are both negative, EM waves appear to transmit energy one way while “travelling” in the other. Technically, the group and phase velocities of the waves are of opposite sign. Consequently the familiar right-hand rule involving the relationship between the electric and magnetic field components and Page 11

the wave-vector of the wave becomes a left-hand rule; hence “left handed material” as sometimes described. Among other strange consequences, the Doppler effect is reversed: as is the Cerenkov effect3, with charged particles passing though a medium emitting light in a cone behind the particle rather than in front. Finally evanescent (near field) modes will actually grow in the NIM. If we introduce a layer of NIM between say the gap material (2) and the “receiving” material (3) in our structure, the NIM and gap layer no longer cause a decay of the evanescent waves tunnelling through material 2. If the conditions are exactly right (and no losses are assumed) the transmitted energy into the third layer will be 100 %, i.e. all the light energy is transmitted through the near field “tunnel” barrier and no light is reflected via TIR4. What is more this result is independent of the thickness of the gap material (2). It must be stressed that this is a theoretical prediction and losses in real materials and other effects such as surface waves will reduce this value. Nevertheless this amplification effect has been demonstrated experimentally in recent work on silver films5. This amplification of the near fields was used by Pendry to predict that a slab of NIM material would in fact behave as a “perfect lens” which actually transfers the evanescent as well as the propagating modes and produces a genuine image of the source.6 Even after additional effects are taken into account the NIM slab is expected to show significant advantage over conventional methods for manipulating near fields. Tunnelling light through metal plates – giant light transmission An important result in classical optics is that light cannot “resolve” structure that is much less than a wavelength in size in an interface. A further consequence of this is that not much light will pass through a hole in a surface if it is much less than a wavelength in diameter. This imposes limits on optical lithography in semiconductors for example and Page 12

forces engineers to use shorter wavelength light or even electrons to pattern semiconductor chips with fine geometries. If a hole is punched in a metal foil with diameter much smaller than the wavelength of light incident then Bethe predicted that the transmission should be proportional to the fourth power of the radius divided by the wavelength. Most of the light should be reflected from the foil as if the hole was not there. However, in 1998 Ebbesen7 reported light being transmitted through a thin film of gold with a large array of holes each less than one-tenth the wavelength of light. Subsequent study has confirmed that what is happening is that the light very close to the metal surface is coupling to collective excitations of electrons in the metal and forming surface waves (full name “surface plasmon – polaritons” to reflect the combination of the light with the surface plasmon excitations, referred to as surface plasmons or SPs for short). These surface waves literally gather up the light falling on the metal and funnel it through the holes. The result has been repeated using silver, chromium and aluminium plates, and has shown that up to 50 % of the light hitting a perforated metallic film passes through it, even when only 20 % of the surface is pierced. The effect is colour sensitive, with some wavelengths of light boosted by 1,000 times and others not at all, and it works just as well with a few holes as it does with many thousands.

This phenomenon

introduces a whole class of effects in which the emr near fields around a metal surface interact with the surface plasmons and some natural or artificial surface structure (e.g. other holes or surface corrugations) and enhance the near field effects8. Strong resonant behaviour of near fields around surface structure can enhance the local electric field substantially and is the origin of another important microscopic technique known as surface enhanced Raman spectroscopy (SERS), where these enhanced fields are used to probe the properties of molecules in the surface of a sample. The beaming effect through sub-wavelength holes and slits can also be demonstrated at microwave frequencies9. By capturing and manipulating the near fields in this way we are effectively breaking the rules about resolution of small objects being limited to a wavelength or so. (The rules assume that only far fields are involved). By properly harnessing the near fields we are able to resolve structure and penetrate holes and slits much smaller than the wavelength and this has enormous implications for many applications as we shall see later. Page 13

Managing the dielectric constant for compact antennas and near field control In designing antennas for mobile communications (mobile phones) and for airborne systems (civil and military) size and weight are critical, as is efficiency. In the mobile phone environment we have also to add safety since the antenna is used close to human tissue. The latter point is also an efficiency issue since the hand and the head are media having high dielectric constants which can influence the antenna, spoiling its tuning and efficiency and affecting reception and battery life. The fields around a simple dipole antenna can be divided into near field terms, which are both magnetic (the so-called induction field term) and electric, and far field terms (magnetic and electric). The near field terms fall off rapidly with distance from the dipole whereas the far fields dominate at distances of order a wavelength and above. With wavelengths of 10s of centimetres the human tissue is in the near field. The near field terms do not contribute to powerflow from the antenna (unless lossy materials are present – see below); they are stored energy in the magnetic and electric fields. The antenna is effectively surrounded by rather dense electromagnetic fields associated with the resonance of the antenna.

High

dielectric constant human tissue will “attract” and distort this energy into the high dielectric region since emr energy concentrates into the high dielectric path (just like electric current finds its way along high conductivity paths in, for example, a lightning conductor). Recent developments of mobile antennas10 are looking to manipulate these high energy near fields around the antenna by using materials with high relative dielectric constants (approaching that of human tissue - anything from 35 to 100 compared to air which is 1). These materials “capture” the near field by loading the antenna and reduce the amount of field entering the nearby human tissue. These materials can be used as cores to metal antennas or as the antennas themselves. We thus have antennas which are more efficient, safer, and very much smaller. The increase in efficiency also improves battery life reducing cost and poisonous battery waste in the environment. High dielectric materials are however quite expensive, heavy and can be difficult to manufacture and process. For some applications it is possible to use versions of the Page 14

NIMs discussed above. Airborne system manufacturers (Boeing) are utilizing negative refractive index materials to make lenses, which have the same refracting power as a high dielectric lens. This arises because in a conventional lens the performance is symmetrical about the value of refractive index n=1 (not n=0). So a concave lens made from material with n=-1 has the same performance as a conventional convex lens with n=+3. This latter can only be achieved with, say, a heavy glass or ceramic material whereas the NIM can be made of lighter polymer materials with metallic patterning for example. This is a major advantage where weight is critical. It is also possible to use the independent control of µ and ε in a metamaterial to have a high value of ε whilst ensuring that the impedance (given by the ratio of ε and µ) is still matched to free space. There is also a way to do this using the plasmonics demonstrated at Exeter University. The use of metamaterials with negative µ has also been utilized as a screen to reduce near field radiation from a mobile phone entering the head11. High magnetic permeability materials for imaging magnetic fields Relatively high values of dielectric permittivity ε are available over quite a wide spectral range. However materials with relative magnetic permeability much different from 1 are only available at frequencies up to the microwave region. Magnetic materials in the visible are nonexistent. With the advent of the NIMs the ability to design the magnetic permeability µ, independently of the dielectric permittivity ε and to relative values greater than or less than 1, allows the possibility of magnetic materials in a new variety of spectral ranges. Larger values of µ in the visible and IR regions would open the possibility of magnetic functions such as isolators, common at microwaves, to be used in optics. However a very important example is down in the MHz (RF) region where medical imaging uses alternating magnetic fields. With wavelengths of several metres, all the control is in the near field. Indeed we find here examples of extreme near field where the ratio of wavelength to length scale can be as great as 1000 to 1. By using the resonant structures Page 15

designed for NIM it has been shown to be possible to design flux guides and lenses which can control the RF magnetic fields in, for example, an MRI machine14 but NOT be sensitive to the very large DC magnetic fields inside the machine. There are three distinct magnetic fluxes present in MRI machines, as shown in the table below: Field

Strength

Frequency

Static polarising field: B0

0.25-3 T

DC

Gradient fields: G

10 –200 mT/m

10-1000 Hz

RF fields: B1

∼25 µT

1-125 MHz

Here, the main field B0 is required to be homogeneous to typically 25 % of market share), BAeSystems (a major military and civil aerospace comms Page 30

supplier and innovator) and start-ups in novel near field antennas such as Sarantel (growing far east market) and Antenova. The near market: Antennas for mobile and looking towards third generation mobile and wireless local area networks.

Filtronic already supply very efficient, innovatively

designed antennas aimed at a market that expects to pay only 50 cents each. New technology and expensive material antennas have to offer a lot more AND cost in. Sarantel10 are addressing this with a highly automated production line for high dielectric ceramic-loaded BiHelix antennas. The feed system and high dielectric core control the near field to give high stability to clutter and human tissues which cause gain impairment and down tuning as well as unpopular radiation into the head (SAR). Improvements in emergency location applications for mobile devices are also attractive. Antenova are developing multiple resonant dielectric antennas to control near fields and achieve high fractional bandwidth. They are designing for applications in spectral ranges from 8.5 MHz to 2.5 GHz. Two-antenna systems for frequency division duplex architectures with good isolation are now required. There is work ongoing to reduce to one but it must cost in. Filtronic are developing a more complicated antenna but it includes microwave chips and does more than just an antenna to get value back. High isolation antennas are needed for Multiple Input Multiple Output (MIMO) systems since you need 40 dB isolation between antennas. In wireless communications the cross talk is much reduced in the near field. Near Field Communications (NFC) promoted by Philips and Sony42 offers an alternative for short range “inter-device” communications. This is good for cluttered wireless environments, MIMO applications etc. Future look: To control near fields around human tissue we need to synthesize materials which have ε comparable to human tissues, are cheap and lightweight and easy to manufacture – this opens the market up to wearable antennas. Inside the dielectric the wavelength is 1/10th that in air making possible miniscule antennas, which could even be Page 31

used for chip-to-chip communications. There is also a need for more than a pure resonant dielectric antenna that could be coupled together with plasmonics and NIM-style materials to, for example, enhance magnetic properties and grade the dielectric constant to design the emr properties. In considering enhanced magnetic properties we note that the current generation of antennas primarily uses electric dipole radiators and therefore the near field is almost purely electric. The body absorbs radiation through the electric component of the fields, so a magnetic dipole with essentially magnetic near fields has the potential for much reduced SAR. Magnetic metamaterials, now demonstrated, give the flexibility to design with the magnetic rather than electric field in mind and might therefore offer lower SAR antenna designs.

Other possible growth areas are in

automotive applications where 77 GHz station-keeping (cruise control) radar has antennas with a lens.

They are looking to reduce lens material cost and may use

metamaterials. One could also use metamaterials to manipulate the microwave phase and intensity over direction to achieve a scanning radar without moving parts. Steerable antennas have been investigated at Filtronic for 3G mobile. However 3G will have 6 zones not 3 and it is interference limited not noise limited. One might consider a simple beam steer antenna in a handset but it turns out that it is interference limited not power budget limited and steering only helps the latter. The stealth property makes plasma antennas (Plasma Antennas Ltd, Cambridge) attractive in military applications, but domestic applications could be base station antennas to provide reduced interference and improved performance. As the property of the plasma changes with frequency, electronic microwave beam steering is a possibility. Looking further out to the exploitation of the new technologies a few ideas are: NIMs in ultra high sensitive phase shifters. Recent experiments (S. Sridhar) show phase shifts as high as 1000 radians for a small 1 GHz change in frequency. Also Sridhar shows that in electronically controlled NIM filters the pass-band can be tuned and losses can be reduced by better design. This would enable electronically controlled filters made Page 32

of NIM opening applications such as superlens, beam steering, open cavities and directional couplers. A limitation here is that isotropic NIMs are essential and not yet demonstrated in effective media NIMs. An answer might be Photonic crystals (PhCs), which offer a controlled means to achieve negative refraction and flat lens imaging at microwave frequencies. The principal advantage of PhCs is that they can be easily scaled to a wide range of frequencies up to optical. By virtue of the design these materials are isotropic in 2D and in some cases 3D (work on this at Kent University). Other potential applications are leaky wave antennas based on backward wave propagation. Military applications looked at by Boeing include high refractive index/ low reflection lenses, both high ε and matched impedance and relative ε=-1 which is equivalent to ε=3 for a lens. In the UK BAeSystems are evaluating NIM materials looking at radical alternatives to conventional antennas. Other possibilities are near field ground penetrating imaging for beneath soil (using NIM lens for sub-wavelength near field imaging to distinguish objects under the ground) and ultra low frequency communications (submarine communications etc., where low frequency magnetic fields are required to penetrate the water), which is linked to the penetration of aqueous bodies for medical imaging etc. Medical imaging The application of metamaterials in medical imaging has a strong (almost unique) position in the UK at Imperial College. Current demonstrations are in MRI (where the ability to control and shape RF fields has wide ramifications, particularly when combined with current developments of scanners with larger numbers of parallel receiver channels) but exploitation is also expected to be in the wider area of imaging and near field control in medical diagnosis. All applications are future look at present. Some broad potential application areas for metamaterials in MRI are summarised in the table below.

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Table 2. Applications in medical imaging Relative Permeability µ >>1

Isotropy

Anisotropic

Isotropic µ=0 Anisotropic Isotropic µ< 0 Anisotropic Switchable

Applications Flux ducting, RF yokes Surface noise reduction Endoscope RF faceplate Magnetic “wires” RF flux delivery RF screens Coil screening RF illumination Focussing for imaging and RF delivery Shielding Flux mirrors Magnetically conducting planes Shielding Enhanced parallel imaging

Flux guide work points the way to novel flux transfer devices that may allow new receiver coil configurations. There is not enough space in MRI to make conventional lenses but reflection optics may be possible opening the way for true "optical-style" imaging of RF field systems - the classic MRI method is quite slow and cumbersome. The idea is to use the metamaterial optics to image a plane of spins onto an image plane of detectors, removing the need to scan so there would be a reduced time period for a patient in the machine, and reduced RF exposure. Imperial College have made an application to Health Technologies Development – (Dept of Health and EPSRC Initiative) to make a metamaterial yoke with flux compression to go around the head to keep the RF flux out of the patients brain (so as to be able to meet the need to apply an energy density 100s or 1000s of times greater than we do currently). Other key applications may be:

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(1) Screening (a system which is not absorbing, rather than just dumping power into the absorber and wasting it as at present) – this is also a means of reducing unwanted coupling between different sub-systems; and (2) Tracking radio pills swallowed by the patient for internal imaging without putting the patient in an MRI machine or using endoscopes or markers on the patient’s body – this removes the space constraint. Such a pill could transmit a video signal, so metamaterials could be used to set up an imager. Photonic interconnects for sub wavelength circuits and active devices Using surface plasmons (SP) to control light in metal surfaces has opened applications in interconnects to manipulate and direct light between components of a sub-wavelength circuit. The future will most probably include going from passive devices such as plasmon wave-guides to dynamic and active devices, such as modulators or light emitters8. These will start as improvements to existing devices and then spawn totally new device concepts through sub-wavelength integration and new physics. Enhanced near field intensity also opens the way to non-linear devices. The near market: Micromanaged Photons (MMP) use thin metal layers to confine the light in the guiding plane. The light travels as a SP in the metal surface and the PhC is created by modulating the thickness of the metallic film.

This leads to cheaper

processing and lower wave-guide loss due to out-of-plane scattering. They are also looking to use electro-optic polymers in the cladding layers for active control. Matching the wave-guide modes to the optical fibre is also possible, thus reducing on- and off- chip coupling losses. In long-range SPs the loss / mm can be high even in the best metals but because the components are small and close together the loss per function can be small. There is no UK start up in this area but a very strong science base which would enable progress if a suitable application can be found.

A word of caution however.

There might be

opportunities in the telecom sector for plasmonics but competition and progress in Page 35

ordinary electronics is very severe and the current conservatism in the systems market suggests looking elsewhere for early wins. Future look: Structured metal foils (such as the Ebbesen holey foils) can be described as miniature phased array antennas in the optical regime and can transmit and receive light along a particular direction. These are arrays of sub-wavelength holes locally structured (corrugated) to produce intense and low dispersive beams of light whose wavelength can be controlled. Applications include spatial and spectral multiplexers (re-routing light according to wavelength), coupling in and out of fibres and optimising near field devices for microscopy and data storage. Diffraction-reducing properties reduce the intrinsically high beam divergence of common optical devices, LEDs and lasers. This offers a unique path to photonic miniaturization without scaling limits in the sub-wavelength regime, such as low transmittance and severe diffraction. Nanoscopic light sources (or receivers) used for light delivery to (or collection from) localized areas in photonic and opto-electronic devices as well as for studies of optical dynamics of individual and interacting quantum objects. Single photon sources. One generic area that needs to be addressed is the development of a near field to far field converter in order to use the locally manipulated near fields to generate a propagating far field that can then be transmitted. Sources for THz radiation (non-linear generation):

a possible route is to use field

enhancement of microwaves to put several watts of power into a nanosize area and use third harmonic generation in the strongly enhanced fields to convert from, say, 200 GHz to 600 GHz (or do that in reverse to do imaging). It is possible to use SP enhanced nonlinear effects to make optical switches, for example, and Exeter are looking at new nonlinear elements (prime area for future work). Applications of plasmonic systems in the microwave area are being developed by Qinetiq, UK, supported by Exeter University. They include very thin microwave absorbers used for screening in, for example, buildings on airfields. This reduces interference with low angle secondary surveillance radar. Page 36

(Plasmonic cladding is much thinner and lighter than “conventional” ferrite materials. The latter have to be at least a quarter of the wavelength thick - equivalent to several centimetres for radar wavelengths). Thin, lightweight absorbers have other obvious applications for screening microwaves both for civil and military purposes.

Other

applications include using the beaming effect from slits in metal foils for beam steering in automotive radar with no moving parts. Applications are also envisaged in the infrared spectrum. Jeremy Baumberg at Southampton is developing spherical reflector optical cavities with dimensions below 10 microns to give localised tunable regions of greatly-enhanced light intensity suitable for applications in nano-scale optics, microlasers, quantum information processing and surface-enhanced single molecule detection43. They can be coupled to quantum dots for enhanced luminescence emission and microlasing. Other photonic applications: Organic photodiodes actually lose light due to SPs in the cathode. It is possible to use surface structuring to recover this. In the reverse process solar cells could use SPs to enhance the absorption of light.

Finally, there are

applications in molecular photonics, where metallic nanostructures could be used as a basis for manipulating molecular properties and addressing single molecules (optically). Flat panel displays Ebbesen’s group have filed seven different patents for plasmonic applications (NEC Labs related) one of which is likely to be related to flat panel displays. These are operated by tuning the transmitted light through dimpling the surface and varying the incidence angle (beaming light approach) to turn colours on and off. For example Tineke Thio (head researcher on these devices at NEC Labs and colleague of Ebbesen) has a prototype device that can change two laser beams transmitted through an optical sieve from red to yellow to green without using the filters or polarisers required by current liquid crystal displays (LCDs). In a few years' time, this could lead to brighter, clearer display screens Page 37

for mobile information devices that are six times more efficient than today's monitors. The UK has complementary technology in displays and there may be spin off here. Sensors (microwave, RAMAN, fluorescence) Sensor applications include those based on Raman, or fluorescence detection tailored for specific molecules using colloidal nanoparticles. Microwaves in the near field can interrogate individual molecules (which are microwave resonant), and could be used to meet the requirement for DNA sampling. The potential for this area could be huge as outlined by Levine et al.44. There is a strong UK position in microwave sensing based in Exeter (Sambles) and UMIST (Wu). Near field imaging at optical frequencies (surfaces, molecular and biological systems) The real opportunity here is to provide spatial resolution for optical spectroscopy in the length range of 10 nm. This is the size of biological proteins and the size of quantum confinement in semiconductors. Near field technology might then allow direct imaging of membrane proteins (although notoriously problematic since they tend to want to exist and function properly only across certain membranes, which may not be amenable to near field sensing), the wave functions in artificial quantum structures and bio-imaging on the nanoscale, down to single molecules (modifying radiative properties of single molecules by modifying their environment). The main obstacle is to produce robust and cheap sensors (many groups employ very advanced techniques, like e-beam lithography, which is less likely to be used for commercial products). There is also a growing field of highresolution ultra-fast laser processing which might offer a possible production method. This can readily make features of 200 nm size, and in certain materials can make gratings over large areas of as small as 30 nm pitch (this fine structure depends on inherent materials properties, however). Finally there are possibilities to use metal nanoparticles as contrast agents for biomedical microscopy (similar to fluorophores or quantum dots), but fluorescent proteins will be dominant in this sector for the foreseeable future. Page 38

In medical imaging, optics has a potential to produce cheap techniques with the key feature of being able to discriminate different cells. Early tumour detection is an obvious challenge. THz imaging is covered in the EEMS project state of the science review ‘Picturing people: non intrusive imaging’. Optical data storage Data storage is one of the focusing and promising objectives for near field applications. The rapid technological progress is because it is commercially driven. Technology in the US drives these applications with massive activity in Japan (Tominaga) and Taiwan (Tsai). A key challenge is to increase the scan speed. The UK can only watch and learn we would suggest. SERS / SNOM There is clear application potential in aperture-less near field microscopy (leading group is Hillenbrand, MPI Munich), mainly if combined with spectroscopy like infrared, fluorescence or Raman. This would offer below 10 nm spatial resolution and it is nondestructive and sensitive to chemical composition (see the imaging and sensor applications above). Not yet available commercially, but it could be a new product useful for scientific research at the beginning. Since there is still a lot of instrumental development and research on contrast mechanisms to be done we still have to wait some years for real industrial applications. However if near field microscopy is only a research tool, this will always be a minor field in science (a few companies and maybe 10 to 50 microscopes sold annually). The technical development of the microscopy systems is not an area where the UK has a strong position and so it is only mentioned here as an important example which will drive the science worldwide. An extension of this area is the use of the near field to physically manipulate individual molecules. This subject is covered in the EEMS project state of the science review ‘Manufacturing with light: photonics at the molecular level’.

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Medical therapeutics There is possible interest in pulsed electromagnetic fields for bone stimulation (osteoporosis) and magnetic nerve stimulation (trans-cranial magnetic stimulation). These are near field in the sense of being r