How to Drive Light Round the Wrong Bend
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NEWS NATURE|Vol 445|25 January 2007 How to drive light round the wrong bend Can visible light ever be manipulated so that it bends the wrong way? If it could, a range of futuristic devices would be tantalizingly close NEGATIVE RESULT to reality, such as a lens for imaging features Researchers used surface plasmons to carry View from above light across a gold prism — achieving the first Negatively smaller than the wavelength of light, or a shield refracted negative refraction of visible light. wave to render objects invisible. Surface Several scientists have written off such ‘nega- Slit plasmon Normal wave tive refraction’ in the visible range as practically Visible light impossible but a group is now claiming to have enters through slit Positively achieved it, spurring a debate about what con- refracted stitutes true refraction. wave Light bends in a specific way when it passes Silver Silver from one medium to another — an effect called refraction. Negative refraction describes a situ- Surface plasmons 50 nm When surface plasmons ation in which light bends the opposite way. created which emerge from the prism, It happens only if the direction in which the carry the light they are negatively refracted (see inset) peaks and troughs travel along a light wave can 500 nm Gold be reversed relative to the direction in which Silicon nitride the wave itself is travelling. A material with a negative refractive index Silver would focus light perfectly instead of dispers- ing it. This led John Pendry of Imperial Col- lege London to predict that a ‘perfect’ lens could be made, which would image features smaller But achieving similar effects for visible light unveiled a material that they say has a negative than the wavelength of light. Some asserted that has seemed well out of reach. Radiation in refractive index for visible light. Dionne pre- refraction could only ever have a positive value. the microwave and infrared ranges has wave- sented the results on 11 January at Nanometa But the debate was settled in 2003 when nega- lengths in the order of micrometres or centi- 2007, a conference on nanophotonics and meta- tive refraction was demonstrated for micro- metres, so the components of the material used materials held in Seefeld, Austria, and the group waves1,2 and later for infrared waves. to negatively refract them are also on this scale. has submitted them for publication. Researchers achieved the effect with ‘meta- But building something equivalent for visible Rather than try to create a material with materials’ that had components of roughly light, with a wavelength of some 500 nanome- components as small as the wavelength of vis- the same size as the light’s wavelength. More tres, is a huge challenge. ible light, theoreticians recently suggested tak- recently, Pendry used a metamaterial to bend Now Jennifer Dionne and Henri Lezec, ing advantage of electromagnetic waves called light around an object to create an ‘invisibility working in Harry Atwater’s group at the Cali- surface plasmons, created when light hits free shield’, also for microwaves3. fornia Institute of Technology in Pasadena, have electrons oscillating on the surface of a metal, to guide the light in the desired direction. This is what Dionne and Lezec have now done. Their device, called a waveguide, consists of the insulator silicon nitride sandwiched between E. SCHREMPP/SPL two sheets of silver. Light enters the device through a slit in the upper silver sheet. Once inside, the light wave couples with oscillating electrons in the silver to create a surface plasmon wave that travels along the metal’s surface. But embedded in the silicon nitride is a gold-coated prism, with a gap between it and the upper silver sheet that is just 50 nanometres wide (see graphic). As the surface plasmon wave crosses this gap, it is refracted. Dionne says that she has detected light with wavelengths of 480–530 nm (blue- green) emerging from the device having under- gone negative refraction. The refractive index reached as low as −5 (compared with +1.33, for A straw in a glass of water seems disjointed because of refraction (left). But in this rough mock-up of light travelling from air into water). what would happen if water had a negative refractive index (right), the effect is startling. The underside Passing the surface plasmons through the of the water’s surface can be seen but not the bottom of the glass. For more accurate models, see ref. 4. thin gap above the prism confines their move- 346 NATURE|Vol 445|25 January 2007 NEWS WHEAT FUNGUS SPREADS OUT OF AFRICA Stem rust threatens key crops in Asia. www.nature.com/news ment, so only one mode of surface plasmon CIMMYT MOWBRAY, D. wave can get through. At certain wave- PR’s ‘pit bull’ takes on open access lengths of light, the frequency of the surface plasmon wave is close to the frequency of the oscillating electrons within the bulk of The author of Nail ’Em! to focus on simple messages, “We’re like any firm under the metal. In this case the surface plasmon Confronting High-Profile such as “Public access equals siege,” says Barbara Meredith, wave and the oscillating electrons interact Attacks on Celebrities and government censorship”. He a vice-president at the in such a way that the direction of travel of Businesses is not the kind of hinted that the publishers organization. “It’s common the wave’s peaks and troughs is reversed, figure normally associated with should attempt to equate to hire a PR firm when you’re giving negative refraction. the relatively sedate world of traditional publishing models under siege.” She says the AAP For Dionne, the goal of “peeking round scientific publishing. Besides with peer review, and “paint a needs to counter messages the corner” has been achieved. “It’s like writing the odd novel, Eric picture of what the world would from groups such as the Public alchemy,” she says. “But it works.” Dezenhall has made a name look like without peer-reviewed Library of Science (PLoS), an Others in the field are more cautious. for himself helping companies articles”. open-access publisher and Mark Stockman, a theoretician at Georgia and celebrities protect their Dezenhall also recommended prominent advocate of free State University in Atlanta, is concerned reputations, working for joining forces with groups that access to information. PLoS’s about the system’s inefficiency, pointing out example with Jeffrey Skilling, may be ideologically opposed to publicity budget stretches that only about 1% of the light gets through. the former Enron chief now government-mandated projects to television advertisements Dionne emphasizes that enough light gets serving a 24-year jail term such as PubMed Central, produced by North Woods through to be detected directly and says she for fraud. including organizations that Advertising of Minneapolis, thinks improvements can be made. Although Dezenhall declines have angered scientists. One a firm best known for its role And some are unconvinced that it offers to comment on Skilling and suggestion was the Competitive in the unexpected election true negative refraction. Allan Boardman, a his other clients, his firm, of former professional theoretician from the University of Salford, Dezenhall Resources, was also “Media massaging wrestler Jesse Ventura to the UK, and Vladimir Shalaev from Purdue reported by Business Week is not the same governorship of Minnesota. University in West Lafayette, Indiana, who to have used money from oil The publishers’ link with are also trying to negatively refract visible giant ExxonMobil to criticize as intellectual Dezenhall reflects how light, argue that the experiment simply the environmental group debate.” seriously they are taking recent shows negative refraction of plasmons, Greenpeace. “He’s the pit bull developments on access to rather than of light itself. “It’s not negative of public relations,” says Kevin Enterprise Institute, a information. Minutes of a 2006 refraction per se,” says Boardman. “They’ve McCauley, an editor at the conservative think-tank based in AAP meeting sent to Nature got to qualify it a lot more.” magazine O’Dwyer’s PR Report. Washington DC, which has used show that particular attention But others such as Nikolay Zheludev of Now, Nature has learned, oil-industry money to promote is being paid to PubMed the University of Southampton, UK, say this a group of big scientific sceptical views on climate Central. Since 2005, the NIH doesn’t really matter, because the end result publishers has hired the pit bull change. Dezenhall estimated has asked all researchers is the same. “If everything is correct, this is to take on the free-information his fee for the campaign at that it funds to send copies of a grand claim,” says Zheludev. “Yes, they had movement, which campaigns $300,000–500,000. accepted papers to the archive, negative refraction,” agrees metamaterials for scientific results to be In an enthusiastic e-mail sent but only a small percentage and plasmonics expert Eli Yablonovitch made freely available. Some to colleagues after the meeting, actually do. Congress is from the University of California, Los Ange- traditional journals, which Susan Spilka, Wiley’s director expected to consider a bill les. “I don’t see much controversy there.” depend on subscription of corporate communications, later this year that would make Pendry is also convinced, although charges, say that open-access said Dezenhall explained submission compulsory. he says he didn’t expect to see the effect journals and public databases that publishers had acted Brian Crawford, a senior demonstrated so soon. “It is very impres- of scientific papers such as the too defensively on the free- vice-president at the American sive,” he says.