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Cloaking moves into the visible

24 Jul 2008

Optics.org speaks to researchers who have demonstrated partial invisibility in the visible region of the spectrum for the first time.

Researchers in the US have engineered the first optical material that demonstrates cloaking behaviour in the visible. The team believes that its design validates the basic theory of electromagnetic cloaking and could be a step towards achieving perfect invisibility (Optics Letters 33 1342).

"We are the first group to demonstrate partial invisibility in the visible region of the spectrum," Christopher Davis, a professor at the University of Maryland, told optics.org. "Although ideal cloaking has not been achieved, we have developed a 2D demonstration of cloaking behaviour and we believe that the same design principles could be used to achieve cloaking in 3D."

In order to achieve invisibility, the group needed to manipulate magnetic and electrical field lines. This requires a suitable material with correctly engineered values of permittivity and permeability. Recent theoretical designs for an electromagnetic cloak have been suggested in the microwave frequency range, however extending the operating wavelength into the visible has so far proved difficult.

"In order to achieve electromagnetic cloaking in the visible frequency range, negative magnetic permeability is required," explained Davis. "Such magnetic behaviour is difficult to achieve at very high optical frequencies."

The group turned to plasmonic metamaterials in which plasmons (2D surface light waves) exhibit negative index behaviour naturally. "Conventionally, magnetic permeability must be engineered for the transverse electric polarization of light, while dielectric permittivity must be engineered for the transverse magnetic polarization," commented Davis. "However, since surface plasmons only have one polarization state, engineering the dielectric permittivity is sufficient, resulting in a simpler cloaking design."

In the design, the plasmonic metamaterial is made up of PMMA (polymethylmethacrylate) rings formed on top of a gold film surface. Plasmons propagate consecutively through the regions of gold–air and gold–PMMA interfaces, which have opposite group refractive indices.

"The successive stripes of effective positive and negative refractive index, redirects the plasmon ray propagation along a curvilinear path," explained Davis. "The ability to bend a path of light around a given area is necessary for successful cloaking."

The next step for the group is to increase the size of the cloaking device to ~100 wavelength scale. "Currently our experimental demonstrations are performed on a single wavelength scale," concluded Davis. "A demonstration of cloaking on a larger scale would require considerably smaller losses in the metamaterial."

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