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Researchers have developed a new and effective approach to invisibility cloaking by manipulating the color of light waves as they pass through an object, which could be used to secure data transmissions, improve technology sensing and advance telecommunications.

For years, scientists and engineers have been looking for ways to conceal objects by manipulating how light interacts with them. When viewing an object, what you are really seeing is the way in which the object modifies the energy of the light waves that interact with it. Past solutions for invisibility cloaking involved changing the paths that the light follows so that waves travel around, rather than through, an object.

Most current cloaking devices can fully conceal the object of interest only when the object is illuminated with just one color of light. However, sunlight and most other light sources are broadband, meaning that they consist of many colors.

"Conventional cloaking solutions rely on altering the propagation path of the illumination around the object to be concealed; this way, different colors take different amounts of time to traverse the cloak, resulting in easily detectable distortion that gives away the presence of the cloak," said Luis Romero Cortés from the National Institute of Scientific Research (INRS).

The research team, led by José Azaña from the INRS in Montréal, Canada, created a new device called a spectral invisibility cloak that can completely hide objects under multi-colored illumination.

Their method for invisibility cloaking rearranges different colors of light so that the light wave propagates the object without actually “seeing” it. To do this, the cloaking device shifts the colors towards regions of the spectrum that would originally not be affected by propagation through the object. For example, if the object reflects green light, then light in the green portion of the spectrum might be shifted to blue so that there would be no green light left for it to reflect. Then, once the wave has cleared the object, the cloaking device reverses the shift, reconstructing the wave in its original state.

"Our proposed solution allows the wave to propagate through the target object, rather than around it, while still avoiding any interaction between the wave and the object,” said Cortés.

The team constructed the new device from two pairs of two commercially available electro-optical components: a dispersive optical fiber, which forces the different colors of a broadband wave to travel at different speeds, and a temporal phase modulator, which modifies the optical frequency of light.

To demonstrate their new approach, the research team concealed an optical filter—a device that absorbs light in a prescribed set of colors while allowing other colors of light to pass through. They illuminated the optical filter with a short pulse of laser light. One pair of the electro-optical components was placed in front of the optical filter and the other pair was placed behind it.

The new spectral cloaking device made it look as though the laser pulse had traveled through the optical filter, making the object entirely invisible to the eye. The experiment confirmed that their device was able to transform light waves in the range of color frequencies that would have been absorbed by the optical filter, then completely reverse the process as the light wave exited the filter.

"Our work represents a breakthrough in the quest for invisibility cloaking," said Azaña. "We have made a target object fully invisible to observation under realistic broadband illumination by propagating the illumination wave through the object with no detectable distortion, exactly as if the object and cloak were not present."

The concept, theoretically, could be extended to make 3D objects invisible from all directions. While the new design needs further development before it could be translated into a Harry Potter-style, wearable invisibility cloak, the approach could be applied to securing data transmitted over fiber optic lines and could also help improve technologies for sensing, telecommunications and information processing, researchers say.

Telecommunications systems use broadband waves as data signals to transfer and process important information. Spectral cloaking could be used to determine which operations are “made invisible” to it, preventing eavesdroppers from gathering information by probing a fiber optic network with broadband light.

Using this method with broadband waves that are used as telecommunication data signals could allow more data to be transmitted over a given link. Or, the technique could be used to minimize some key problems in today’s broadband telecommunication links, including dispersion and other undesired effects that impair data signals.

While the researchers demonstrated spectral cloaking when the object was illuminated from only one direction, Azaña said it should be possible to extend the concept to make an object invisible under illumination from every direction. The team plans to continue their research toward this goal.

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