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Making light bend backwards

Technology could allow ultra-potent microscopes, new electronic devices 

Oct. 17, 2007
Special to World Science  

While de­vel­op­ing new types of lens­es, re­search­ers have crafted a lay­ered ma­te­ri­al that makes light bend in a way na­ture nev­er in­tend­ed. 

Light nat­u­rally bends, or re­fracts, in a spe­cif­ic way when it trav­els from one ma­te­ri­al to an­oth­er. This cre­ates, for ex­am­ple, the il­lu­sion of a drink­ing straw look­ing bent when placed in wa­ter. 

Neg­a­tive re­frac­tion com­pared to nat­u­ral re­frac­tion. In nat­u­ral re­frac­tion, light go­ing from one ma­te­ri­al to an­oth­er bends in some di­rec­tion to the op­po­site side of the "nor­mal," an im­ag­i­nary line per­pen­dic­u­lar to the sur­faces. In neg­a­tive re­frac­tion, light bends back from the nor­mal.

But the new ma­te­ri­al, crafted from al­ter­nat­ing lay­ers of semi­con­duc­tors, re­fracts light back­wards—a phe­nom­e­non called neg­a­tive re­frac­tion, re­searchers say.

Neg­a­tively re­fract­ing ma­te­ri­als have been made be­fore. But this is the first that’s fully three-di­men­sion­ and made to­tally of semi­con­duc­tors, the in­vest­i­ga­tors said. Semi­con­duc­tors are sub­stances that can switch be­tween elec­tric­ally con­duct­ing and non-con­duct­ing states, which makes them key com­po­nents of elec­tron­ic de­vices.

The negative-refraction se­mi­con­duc­tor struc­ture could be use­ful in in­stru­ments such as chem­i­cal threat sen­sors, com­mu­nica­t­ions equip­ment and di­ag­nos­tics tools, the sci­en­tists said. Semi­con­duc­tors “are ex­tremely func­tion­al ma­te­ri­als. These are the things from which true ap­plica­t­ions are made,” said en­gi­neer Claire Gmachl of Prince­ton Un­ivers­ity in New Jer­sey, one of the re­search­ers.

Nat­u­ral re­frac­tion is why lens­es have to be curved, a trait that lim­its im­age res­o­lu­tion. The new ma­te­ri­al makes flat lens­es pos­si­ble, Gmachl and col­leagues said—theoretic­ally al­low­ing for the crea­t­ion of mi­cro­scopes that can fo­cus on ob­jects as small as DNA strands.

A lim­ita­t­ion of the new ma­te­ri­al, though, is that it works only with in­fra­red light, a type of light with slightly low­er en­er­gy than the vis­i­ble. But the re­search­ers said they hope the tech­nol­o­gy will ex­pand to oth­er wave­lengths in the fu­ture. 

The sub­stance is in a class of ma­te­ri­als called meta­ma­te­ri­als, made of tra­di­tion­al sub­stances, such as met­als or semi­con­duc­tors, ar­ranged in very small al­ter­nat­ing pat­terns that mod­i­fy their col­lec­tive prop­er­ties. This en­ables me­ta­ma­te­ri­als to ma­ni­pu­late light in ways that nor­mal ma­te­ri­als can­not. Sci­en­tists are al­so in­ves­ti­gat­ing the pos­si­bil­ity that cer­tain me­ta­ma­te­ri­als could form in­vis­i­bil­ity cloaks.

The re­frac­tion find­ings, by a team led by Prince­ton en­gi­neering grad­u­ate stu­dent An­tho­ny Hoff­man, ap­peared in the Oct. 14 on­line is­sue of the re­search jour­nal Na­ture Ma­te­ri­als.

* * *

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Homepage image: Artist's illustration of negative refraction. (Credit: Keith Drake)

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While developing new types of lenses, researchers have crafted a layered material that makes light bend in a way nature never intended. Light naturally bends, or refracts, in a specific way when it travels from one material to another. This creates, for example, the illusion of a drinking straw looking bent when placed in water. But the new material, crafted from alternating layers of semiconductors, refracts light backwards—a phenomenon called negative refraction, physicists say. Negatively refracting materials have been devised before. But this is the first that’s fully three-dimensional and made totally of semiconductors, the researchers said. Semiconductors are substances that can switch between electrically conducting and non-conducting states, which makes them key components of electronic devices. These traits can make the technology useful in instruments such as chemical threat sensors, communications equipment and diagnostics tools, the scientists said. Semiconductors “are extremely functional materials. These are the things from which true applications are made,” said engineer Claire Gmachl of Princeton University in New Jersey, one of the researchers. Natural refraction is why lenses have to be curved, a trait that limits image resolution. The new material makes flat lenses possible, the scientists said—theoretically allowing for the creation of microscopes that can focus on objects as small as DNA strands. A limitation of the new material, though, is that it works only with infrared light, a type of light with slightly lower energy than the visible. But the researchers said they hope the technology will expand to other wavelengths in the future. The substance is in a class of materials called “metamaterials,” made of traditional substances, such as metals or semiconductors, arranged in very small alternating patterns that modify their collective properties. This approach enables metamaterials to manipulate light in ways that normal materials cannot. Scientists are also investigating the possibility that certain metamaterials could make invisibility cloaks. The refraction findings, by a team led by Princeton engineering graduate student Anthony Hoffman, appeared in the Oct. 14 online issue of the research journal Nature Materials.