Not Seeing is Believing

Wednesday, May 25, 2011 @ 03:05 PM gHale

“Seeing something invisible with your own eyes is an exciting experience,” said Physicist Joachim Fischer.

For about one year, Fischer and fellow Physicist Tolga Ergin have worked on refining the structure of the Karlsruhe Institute of Technology (KIT) invisibility cloak to such an extent that it is also effective in the visible spectral range.

In invisibility cloaks, light waves receive guidance by the material so they leave the invisibility cloak again as if they had never been in contact with the disguised object. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are the result of using complex mathematical tools similar to Einstein’s theory of relativity.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the deflected light. For example, the relatively large radio or radar waves require a material “that can be produced using nail scissors,” said Professor Martin Wegener at KIT’s Center for Functional Nanostructures (CFN). At wavelengths visible to the human eye, materials have to be in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These “logs” have precisely defined thicknesses in the range of 100 nm. These logs influence and guide the light waves normally deflected by the curvature so the reflected light corresponds to that of a flat mirror.

“If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum,” Fischer said.

Last year, the Wegener team presented the first 3D invisibility cloak. Until then, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could occur at wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough came from the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3D structures with optical properties that do not exist in nature, metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

“The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before,” Ergin said. “We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication.”

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