Home >Documents >Instant Expert 7 - Metamaterials

Instant Expert 7 - Metamaterials

Date post:03-Jun-2018
Category:
View:220 times
Download:0 times
Share this document with a friend
Transcript:
  • 8/11/2019 Instant Expert 7 - Metamaterials

    1/8

    METAMATERIALS

    INSTANTEXPERT

    7

    John Pendry

  • 8/11/2019 Instant Expert 7 - Metamaterials

    2/8

    The Russian engineer Victor Veselago was one of thedriving forces behind the early development ofmetamaterials. In 1967, he predicted the possibility ofmaterials with a negative refractive index which wouldbend light the wrong way (see diagram, opposite).

    All materials in nature have a positive refractiveindex. The refractive index is a measure of the

    property that makes submerged objects appearcloser to the surface than they really are, andswimming pools shallow when they are not. Thegreater the refractive index of a material, thecloser to the surface an object within it appears.The logical consequence of this law is that a fish ina pond filled with a negatively refracting materialwould appear to have jumped right out of it (left).

    Veselagos argument that negative refraction

    should be possible was complex. Light consistsof both electric and magnetic fields, with energyshared equally between the two. Light propagatesby tossing energy between the fields as if theelectric and magnetic components were ina dance. Atoms in a material with a positiverefractive index water, glass or most other

    transparent materials respond by aligningtheir own electric and magnetic fields to theapplied fields so as to slow down the dance.

    Veselago imagined a material that alignedits electric and magnetic fields in the oppositedirection to those in the light beam. This, hereasoned, would reverse the dance and create theeffect of negative refraction. His dream was onlyrealised with the advent of metamaterials.

    NEW OPTICS, NEW THEORIES

    The idea that a materials internal structure couldinfluence its response to light has been debated sincebefore the time of James Clerk Maxwell, the 19th-century Scottish physicist who demonstrated howlight, magnetism and electricity are all aspects thesame phenomenon.

    In the 1950s, researchers studying long-rangeradio communication built structures made from thinmetallic wires to simulate how Earths ionosphereinteracts with radio transmissions. But it wasnt untilthe 1990s that we finally realised just how radicallya materials structure could influence its properties.

    I helped the GEC-Marconi company to produce aso-called split-ring structure. Manufactured byetching a copper circuit board with rings a fewmillimetres across, it had a particularly strongresponse to radar signals, which produced electriccurrents in the copper that in turn produced aninduced magnetic field (see image, above left).

    This structure had another interesting property.Most magnetic materials align in the same directionas the applied field, like a compass needle pointing

    north. In contrast, the new metamaterial aligned itsmagnetism in the opposite direction. In 2000, DavidSmith, then at the University of California, San Diego,took this split-ring structure and used it to make thefirst material capable of bending radiation in theopposite direction to normal materials such as glass.This was the negative refraction whose existence hadbeen predicted decades earlier by Victor Veselago.

    A decade on, and we now have metamaterials withexquisitely intricate structures, like the onesfashioned from microscopic helices of gold by MartinWegeners group at the Karlsruhe Institute ofTechnology in Germany (see image, bottom right).When light shines on the material, electric currents are

    THE ROAD TOMETAMATERIALS

    LIGHT REFRACTED BY MATERIAL WITHA NEGATIVE REFRACTIVE INDEX MAKES FISH

    APPEAR AS IF ITS OUT OF THE WATER

    Metamaterials havebeen built for

    radar waves (mainimage, and right, top)

    and visible light(right, bottom)

    ii | NewScientist

    J O H N

    P E N D R Y

    induced in the helices. Interestingly,the size of the currents dependsstrongly on whether the light is leftor right-circularly polarised. Theresponse is stronger than even asolution of sugar, one of the mostoptically active natural substances.

    This array of coppersplit rings was the firstmetamaterial to havenegative magnetism meaning that its internalmagnetic field aligns inthe opposite direction toan applied magneticfield. It wasmanufactured at

    GEC-Marconi in 1998 byMike Wiltshire and wasdesigned to respond tomicrowaves with awavelength of about3 centimetres. Thecopper rings areapproximately0.5 centimetres across

  • 8/11/2019 Instant Expert 7 - Metamaterials

    3/8

    David Smith was workingat the University ofCalifornia, San Diego,when he adapted thesplit-ring structure tocreate the firstmetamaterial with anegative refractive index(below, top). The copperloops change the magneticresponse, while thincopper wires on thesurface behind providethe required electricalresponse. Smiths grouphas made extensivecontributions to thedevelopment ofmetamaterials, includingthe first working invisibilitycloak in 2006. It, too,operated at radarfrequencies

    R A D I O

    M I C R O W A V E

    LIGHT REFRACTED BYA MATERIAL WITH ANEGATIVE REFRACTIVE INDEX

    LIGHT REFRACTED BYA CONVENTIONAL MATERIAL

    AN ENGINEERING CHALLENGE

    The structures that make up metamaterials have to be smaller thanthe wavelength of the radiation to be manipulated

    AMOUNT OF BENDING IS DETERMINEDBY A MATERIALS REFRACTIVE INDEX

    S U B - M I L L I M E T R E

    F A R I N F R A R E D

    N E A R I N F R A R E D

    V I S I B L E

    U L T R A V I O L E T X - R A Y

    G A M M A R A Y

    10 2(100m)

    10 0(1m)

    10 -2(1cm)

    10 -4(100m)

    10 -6(1m)

    10 -8(10nm)

    10 -10(100pm)

    10 -12(1pm)

    Wavelength (metres)

    8 January 2011 | NewScientist | iii

    How a material affects light falling upon it is dictated in part by itschemical composition, but its internal structure can have an evenstronger inuence. Silvered mirrors are highly reecting, but black-and-white photographs also owe their blackness to silver billions of

    nanometre-scale spheres of the metal embedded in the lm. Thisdramatic difference arises because the silver spheres are muchsmaller than the wavelength of light.

    Metamaterials extend this concept with articial structures thatmight be nanometres across for visible light, or as large as a fewmillimetres for microwave radiation. Their properties are engineeredby manipulating their structure rather than their chemical composition.

    The possibilities these materials open up are limited only by ourimagination, and not by the number of elements in the periodic table.As a result, metamaterials research has exploded during the past

    decade. It has given us optical properties we once thought wereimpossible, including negative refraction never found in nature, andnovel devices such as invisibility cloaks.

    HISTORY OF METAMATERIALS

    J O N P E N D R Y P R O F E S S O R M A R T I N W E G E N E R U N I V E R S I T Y O F K A R L S R U H E

    D U K E U N I V E R S I T Y

  • 8/11/2019 Instant Expert 7 - Metamaterials

    4/8

    A SLAB OF NEGATIVELY REFRACTINGMATERIAL CAN FOCUS DETAILS SMALLER

    THAN THE WAVELENGTH OF LIGHT

    iv | NewScientist

    General relativity predicts that extremely massiveobjects will cause severe distortions to thesurrounding space. Perhaps the best known of these

    objects are black holes singularities in space-timefrom which even light cannot escape. Some peoplehave asked if metamaterials can be used to mimicblack holes, but this is not possible as metamaterialslack the built-in energy source required to producethe Hawking radiation that real black holes emit.

    But there is something metamaterials can do thatis even more spectacular: they can create negativeoptical space. Think of space as a sheet of rubber thatcan be compressed. By pushing hard enough on thesheet, it should in principle be possible to fold spaceback on itself so that light moving in the folded spacepasses the same point three times. A fish swimmingin this space would come into focus three times, themiddle focus being inverted.

    Transformation optics tells us that a portion of thefolded space must have a negative refractive index.This negative optical space has a remarkable property.Light leaving an object can be thought of as defocusingas it travels away in all directions. Negative opticalspace cancels out this effect. As a result, a lens madeof a metamaterial that creates negative optical spaceis optically perfect. It effectively eliminates the spaceseparating the object and image, so that the objectand image coincide.

    Another limitation of ordinary lenses is that theycan never resolve details smaller than the wavelengthof the light being used. Thats because light diffractsor bends around objects of a similar size to its ownwavelength, so it can never be focused to a sharppoint. The lens is said to be diffraction limited.

    When I calculated in 2000 that a negativelyrefracting lens breaks the diffraction limit, the resultcaused a furore, so entrenched was the notion thatlenses cannot be used to see anything smaller thanthe wavelength of light. Several experiments havesince shown my prediction to be correct. In particular,Nicholas Fang, working at the time with Xiang Zhangat the University of California, Berkeley, demonstrateda lens that could resolve details a

Click here to load reader

Embed Size (px)
Recommended