Cloaking For Stronger Security

Thursday, May 5, 2011 @ 08:05 AM gHale

For Joachim Fischer watching things disappear “is an amazing experience.”

But making items vanish is not the reason he creates invisibility cloaks. Rather, the disappearing act is a quality demonstrations of the capabilities new optical theories and nanotechnology construction methods now enable.

This new area, called “transformation optics” has turned modern optical design on its ear by showing how to manipulate light in ways long thought to be impossible. They promise to improve dramatically such light-based technologies as microscopes, lenses, chip manufacturing and data communications.

Fischer described the first-ever demonstration of a three-dimensional invisibility cloak that works for visible light—red light at a wavelength of 700 nm—independent of its polarization (orientation) at this week’s Conference on Lasers and Electro Optics in Baltimore. Previous cloaks required longer wavelength light, such as microwaves or infrared, or required the light to have a single, specific polarization.

Fischer, of Karlsruhe Institute of Technology in Germany, makes the tiny cloak — less than half the cross-section of a human-hair — by direct laser writing (i.e. lithography) into a polymer material to create an intricate structure that resembles a miniature woodpile. The precisely varying thickness of the “logs” enables the cloak to bend light in new ways. The key to this was incorporating several aspects of a diffraction-unlimited microscopy technique into the team’s 3-D direct writing process for building the cloak. The dramatically increased resolution of the improved process enabled the team to create log spacings narrow enough to work in red light.

“If, in the future, we can halve again the log spacing of this red cloak, we could make one that would cover the entire visible spectrum,” Fischer added.

Practical applications of combining transformation optics with advanced 3-D lithography (a customized version of the fabrication steps used to make microcircuits) include flat, aberration-free lenses easily miniaturized for use in integrated optical chips, and optical “black holes” for concentrating and absorbing light. If the latter can also work for visible light, they will be useful in solar cells, since 90 percent of the Sun’s energy reaches Earth as visible and near-infrared light.

Meanwhile, in a move that could advance the use of Terahertz radiation in security, medicine and communications, a research team from Northwestern and Oklahoma State universities cloaked a three-dimensional object from view in a broad range of Terahertz frequency light, which lies between infrared and microwaves. The team also discussed this development at the Conference on Lasers and Electro Optics.

Cheng Sun of Northwestern describes how they were able to build a rigid sponge-like cloaking structure less than 10 millimeters long on a side in 220 layers, each precisely defined to vary the index of refraction and bend light to render invisible anything located beneath a shallow concave bump on the cloak’s bottom surface. The group showed that the physical geometry and the spectrographic signature of a chemical strip about the width of 10 human hairs disappeared when cloaked.

Concealing tiny objects from view is not the team’s ultimate goal, Sun said. Rather, this latest demonstration shows the new “transformation optics” principles and 3-D lithography techniques they used to make the cloak can also enable optical components for guiding, collimating, and focusing terahertz light in a variety of ways—in new medical and scientific diagnostic tools, airport security scanners, and data communication devices.

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