Making light bend backwards

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.

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, 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