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The Quest for the Superlens

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The Quest for the Superlens THE QUEST FOR THE CUBE OF METAMATERIAL consists of a three- dimensional matrix of copper wires and split rings. Microwaves with frequencies near 10 gigahertz behave in an extraordinary way in the cube, because to them the cube has a negative refractive index. The lattice spacing is 2.68 millimeters, or about one tenth of an inch. 60 SCIENTIFIC AMERICAN COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. Superlens Built from “metamaterials” with bizarre, controversial optical properties, a superlens could produce images that include details fi ner than the wavelength of light that is used By John B. Pendry and David R. Smith lmost 40 years ago Russian scientist Victor Veselago had Aan idea for a material that could turn the world of optics on its head. It could make light waves appear to fl ow backward and behave in many other counterintuitive ways. A totally new kind of lens made of the material would have almost magical attributes that would let it outperform any previously known. The catch: the material had to have a negative index of refraction (“refraction” describes how much a wave will change direction as it enters or leaves the material). All known materials had a positive value. After years of searching, Veselago failed to fi nd anything having the electromagnetic properties he sought, and his conjecture faded into obscurity. A startling advance recently resurrected Veselago’s notion. In most materials, the electromagnetic properties arise directly from the characteristics of constituent atoms and molecules. Because these constituents have a limited range of characteristics, the mil- lions of materials that we know of display only a limited palette of electromagnetic properties. But in the mid-1990s one of us (Pendry), in collaboration with scientists at Marconi Materials SCIENTIFIC AMERICAN 61 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. Technology in England, realized that a the electrons within the material’s at- Another important indicator of the “material” does not have to be a slab of oms or molecules feel a force and move optical response of a material is its re- one substance. Rather it could gain its in response. This motion uses up some fractive index, n. The refractive index is electromagnetic properties from tiny of the wave’s energy, affecting the prop- simply related to ␧ and ␮: n = ± ͌ʲ␧ ʲ . In structures, which collectively create ef- erties of the wave and how it travels. By every known material, the positive val- fects that are otherwise impossible. adjusting the chemical composition of a ue must be chosen for the square root; The Marconi team began making material, scientists can fine-tune its hence, the refractive index is positive. In these so-called metamaterials and dem- wave-propagation characteristics for a 1968 Veselago showed, however, that if onstrated several that scattered electro- specifi c application. ␧ and ␮ are both negative, then n must magnetic waves unlike any known ma- But as metamaterials show, chemis- also take the negative sign. Thus, a ma- terials. In 2000 one of us (Smith), along try is not the only path to developing terial with both ␧ and ␮ negative is a with colleagues at the University of Cal- materials with an interesting electro- negative-index material. ifornia, San Diego, found a combina- magnetic response. We can also engineer A negative ␧ or ␮ implies that the tion of metamaterials that provided the electromagnetic response by creating electrons within the material move in elusive property of negative refraction. tiny but macroscopic structures. This the opposite direction to the force ap- Light in negative-index materials be- possibility arises because the wavelength plied by the electric and magnetic fi elds. haves in such strange ways that theorists of a typical electromagnetic wave—the Although this behavior might seem par- have essentially rewritten the book on characteristic distance over which it var- adoxical, it is actually quite a simple electromagnetics—a process that has ies—is orders of magnitude larger than matter to make electrons oppose the included some heated debate question- the atoms or molecules that make up a “push” of the applied electric and mag- ing the very existence of such materials. material. The wave does not “see” an netic fi elds. Experimenters, meanwhile, are work- individual molecule but rather the collec- Think of a swing: apply a slow, stea- ing on developing technologies that use tive response of millions of molecules. In dy push, and the swing obediently moves the weird properties of metamaterials: a a metamaterial, the patterned elements in the direction of the push—although superlens, for example, that allows im- are considerably smaller than the wave- it does not swing very high. Once set in aging of details finer than the wave- length and are thus not seen individu- motion, the swing tends to oscillate back length of light used, which might enable ally by the electromagnetic wave. and forth at a particular rate, known optical lithography of microcircuitry As their name suggests, electromag- technically as its resonant frequency. well into the nanoscale and the storage netic waves contain both an electric Push the swing periodically, in time of vastly more data on optical disks. fi eld and a magnetic fi eld. Each compo- with this swinging, and it starts arcing Much remains to be done to turn such nent induces a characteristic motion of higher. Now try to push at a faster rate, visions into reality, but now that Vese- the electrons in a material—back and and the push goes out of phase with re- lago’s dream has been conclusively real- forth in response to the electric fi eld and spect to the motion of the swing—at ized, progress is rapid. around in circles in response to the mag- some point, your arms might be out- netic fi eld. Two parameters quantify the stretched with the swing rushing back. Negative Refraction extent of these responses in a material: If you have been pushing for a while, the to understand how negative re- electrical permittivity, ␧, or how much its swing might have enough momentum to fraction can arise, one must know how electrons respond to an electric fi eld, and knock you over—it is then pushing back materials affect electromagnetic waves. magnetic permeability, ␮, the electrons’ on you. In the same way, electrons in a When an electromagnetic wave (such as degree of response to a magnetic fi eld. material with a negative index of refrac- a ray of light) travels through a material, Most materials have positive ␧ and ␮. tion go out of phase and resist the “push” of the electromagnetic fi eld. Metamaterials ) Overview/ Metamaterials page 60 page Q££Materials made out of carefully fashioned microscopic structures can have resonance, the tendency to oscillate ( electromagnetic properties unlike any naturally occurring substance. In at a particular frequency, is the key to particular, these metamaterials can have a negative index of refraction, achieving this kind of negative response which means they refract light in a totally new way. and is introduced artifi cially in a meta- Q££A slab of negative-index material could act as a superlens, able to outperform material by building small circuits de- today’s lenses, which have a positive index. Such a superlens could create signed to mimic the magnetic or electri- Boeing Phantom Works Phantom Boeing images that include detail fi ner than that allowed by the diffraction limit, cal response of a material. In a split-ring which constrains the performance of all positive-index optical elements. resonator (SRR), for example, a mag- Q££Although most experiments with metamaterials are performed with micro- netic fl ux penetrating the metal rings waves, they might use shorter infrared and optical wavelengths in the future. induces rotating currents in the rings, analogous to magnetism in materials MINAS TANIELIAN H. 62 SCIENTIFIC AMERICAN JULY 2006 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. NEGATIVE-INDEX WEIRDNESS In a medium with a negative index of refraction, light (and all other electromagnetic radiation) behaves differently than in conventional positive-index materials in a number of counterintuitive ways. POSITIVE-INDEX NEGATIVE-INDEX MEDIUM MEDIUM A pencil embedded in a A pencil in water appears negative-index medium would bent because of the water’s appear to bend all the way out higher refractive index. of the medium. n = 1.0 n = 1.0 When light travels from a medium with low refractive When light travels from a index (n) to one with higher n = 1.3 n = –1.3 positive-index medium to one refractive index, it bends toward with a negative index, it bends the normal (dashed line at right all the way back to the same angles to surface). side of the normal. A receding object appears redder because of the Doppler effect. A receding object appears bluer. A charged object (red) traveling faster than the speed of light generates a cone of Cherenkov radiation (yellow) in the forward direction. The cone points backward. In a positive-index medium, the individual ripples of an electromagnetic pulse (purple) travel in the same direction as The individual ripples travel in the overall pulse shape (green) the opposite direction to the and the energy (blue). pulse shape and the energy. MELISSA THOMAS www.sciam.com SCIENTIFIC AMERICAN 63 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. [see box below]. In a lattice of straight swing pushed back when pushed faster The fi rst experimental evidence that metal wires, in contrast, an electric fi eld than its frequency. Wires can thus pro- a negative-index material could be induces back-and-forth currents. vide an electric response with negative ␧ achieved came from the experiments by Left to themselves, the electrons in over some range of frequencies, whereas the U.C.S.D.
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