"Long before it's in the papers"
May 04, 2009

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Tiny “invisibility cloak” is like a magic carpet

May 4, 2009
Courtesy Berkeley Lab
and World Science staff

Re­search­ers have cre­at­ed a ti­ny “car­pet cloak” that con­ceals ob­jects un­der it from de­tec­tion us­ing light near the hu­man-vis­i­ble part of the spec­trum. While the car­pet it­self is vis­i­ble, the bulge of the ob­ject un­derneath it “dis­ap­pears” from the view of in­stru­ments that use this near-infrared light.

These three im­ages de­pict how light strik­ing an ob­ject cov­ered with the car­pet cloak acts as if there were no ob­ject. (Im­age by Thom­as Zent­graf)

Re­search­ers say they’re hope­ful that with more pre­cise fab­rica­t­ion their strat­e­gy should yield a true “in­vis­i­bil­ity car­pet” that works in the ar­ea of the col­or spec­trum uti­lized by hu­man eyes, and at a larg­er size.

The car­pet works such that shin­ing a beam of light on the bulge shows a re­flec­tion iden­ti­cal to that of a beam re­flected from a flat sur­face.

“We have come up with a new so­lu­tion to the prob­lem of in­vis­i­bil­ity based on the use of di­e­lec­tric [non­con­duc­ting] ma­te­ri­als, said Xi­ang Zhang of the Uni­ver­s­ity of Cal­i­for­nia Berke­ley and Law­rence Berke­ley Na­tional Lab­o­r­a­to­ry in Cal­i­for­nia, lead­er of the re­search team. 

“Our op­ti­cal cloak not only sug­gests that true in­vis­i­bil­ity ma­te­ri­als are with­in reach, it al­so rep­re­sents a ma­jor step to­wards trans­forma­t­ion op­tics, open­ing the door to ma­ni­pu­lat­ing light at will for the crea­t­ion of pow­er­ful new mi­cro­scopes and faster com­put­ers.”

Zhang and his team pub­lished a pa­per on their work in the re­search jour­nal Na­ture Ma­te­ri­als.

Pre­vi­ous work by Zhang and his group in­volved com­plex “metama­te­ri­als”—composites of met­als and elec­tric­ally in­su­lat­ing sub­stances whose ex­tra­or­di­nary op­ti­cal prop­er­ties arise from their un­ique struc­ture rath­er than their com­po­si­tion. With these me­tal­lic metama­te­ri­als, Zhang and his group found that light can be bent back­wards, a prop­er­ty un­prec­e­dent­ed in na­ture.

The new cloak cre­at­ed by Zhang and his team is made ex­clu­sively from di­e­lec­tric ma­te­ri­als, which are of­ten trans­par­ent to vis­i­ble light. The cloak was dem­on­strat­ed in a rec­tan­gu­lar slab of sil­i­con 250 mil­lionths of a mil­li­me­ter thick that serves as an op­ti­cal “waveg­uide” that forc­es light waves along cer­tain di­rec­tions. Light is con­fined in the ver­ti­cal di­men­sion but free to prop­a­gate in the oth­er two di­men­sions. 

A care­fully de­signed pat­tern of holes — each 110 mil­lionths of a mil­li­me­ter wide — per­fo­rates the sil­i­con, trans­forming the slab in­to a meta­ma­te­rial that forc­es light to bend like wa­ter flow­ing around a rock. In the ex­pe­ri­ments re­ported in Na­ture Ma­te­ri­als, the cloak was used to cov­er an ar­ea that meas­ured about 3.8 thou­sanths of a mil­li­me­ter by one-tenth as much.

Right now the cloak op­er­ates for light be­tween 1,400 and 1,800 nanome­ters in wave­length, mean­ing the light waves are slightly long­er than the vis­i­ble type, Zhang said. But he main­tains the cloak is rel­a­tively easy to make and should be up­wardly scal­a­ble. “In this ex­pe­ri­ment, we have dem­on­strat­ed a proof of con­cept for op­ti­cal cloak­ing that works well in two di­men­sions,” said Zhang. “Our next goal is to real­ize a cloak for all three di­men­sions.”

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Researchers have created a tiny “carpet cloak” that conceals objects under it from detection using light near the human-visible part of the spectrum. While the carpet itself is visible, the bulge of the object underneath it “disappears” from the view of instruments that use this near-infrared light. Researchers say they’re hopeful that with more precise fabrication their strategy should yield a true “invisibility carpet” that works in the area of the color spectrum utilized by human eyes, and at a larger size. The carpet works such that shining a beam of light on the bulge shows a reflection identical to that of a beam reflected from a flat surface. This means the object itself has essentially been made invisible. “We have come up with a new solution to the problem of invisibility based on the use of dielectric [nonconducting] materials,” said Xiang Zhang of the University of California Berkeley and Lawrence Berkeley National Laboratory in California, leader of the research team. “Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.” Zhang and his team published a paper on the work in the research journal Nature Materials. Previous work by Zhang and his group involved complex “metamaterials”—composites of metals and electrically insulating substances whose extraordinary optical properties arise from their unique structure rather than their composition. With these metallic metamaterials, Zhang and his group showed that light can be bent backwards, a property unprecedented in nature. The new cloak created by Zhang and his team is made exclusively from dielectric materials, which are often transparent to visible light. The cloak was demonstrated in a rectangular slab of silicon 250 millionths of a millimeter thick that serves as an optical “waveguide” that forces light waves along certain directions. Light is confined in the vertical dimension but free to propagate in the other two dimensions. A carefully designed pattern of holes — each 110 millionths of a millimeter wide — perforates the silicon, transforming the slab into a metamaterial that forces light to bend like water flowing around a rock. In the experiments reported in Nature Materials, the cloak was used to cover an area that measured about 3.8 thousanths of a millimeter by one-tenth as much. Right now the cloak operates for light between 1,400 and 1,800 nanometers in wavelength, meaning the light waves are slightly longer than the visible type, Zhang said. But he maintains the cloak is relatively easy to make and should be upwardly scalable. “In this experiment, we have demonstrated a proof of concept for optical cloaking that works well in two dimensions,” said Zhang. “Our next goal is to realize a cloak for all three dimensions, extending the transformation optics into potential applications.”