The experiment confirmed that the device was able to transform the light waves in the range of frequencies that would have been absorbed by the optical filter, then completely reverse the process as the light wave exited the filter on the other side, making it look as though the laser pulse had propagated through a non-absorbing medium. One pair of these components was placed in front of the optical filter while the other pair was placed behind it. The second is a temporal phase modulator, which modifies the optical frequency of light depending on when the wave passes through the device. The first component is a dispersive optical fiber, which forces the different colors of a broadband wave to travel at different speeds. The cloaking device was constructed from two pairs of two commercially available electro-optical components. The team demonstrated their approach by concealing an optical filter, which is a device that absorbs light in a prescribed set of colors while allowing other colors of light to pass through, that they illuminated with a short pulse of laser light. Then, once the wave has cleared the object, the cloaking device reverses the shift, reconstructing the wave in its original state. 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 for it to reflect. To do this, the cloaking device first shifts the colors toward regions of the spectrum that will not be affected by propagation through the object.
"Our proposed solution avoids this problem by allowing the wave to propagate through the target object, rather than around it, while still avoiding any interaction between the wave and the object."Īzaña and his team accomplished this by developing a method to rearrange different colors of broadband light so that the light wave propagates through the object without actually "seeing" it. "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, National Institute of Scientific Research (INRS). This alteration of the wave's temporal profile can make it apparent to observers that something is not as it should be. In either approach, different colors of an incoming light wave must follow different paths as they travel through the cloaking device, thus taking different amounts of time to reach their destination. Other approaches, called "temporal cloaking," tamper with the propagation speed of the light such that the object is temporarily concealed as it passes through the light beam during a prescribed length of time. Most solutions for invisibility cloaking involve altering the paths that light follows so that waves propagate around, rather than through, an object. 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. "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." "Our work represents a breakthrough in the quest for invisibility cloaking," said José Azaña, National Institute of Scientific Research (INRS), Montréal, Canada. Researchers demonstrate the new approach in Optica, The Optical Society's journal for high impact research. After the wave has passed through the object, the device restores the light to its original state. The spectral cloak operates by selectively transferring energy from certain colors of the light wave to other colors.
The new device, called a spectral invisibility cloak, is designed to completely hide arbitrary objects under broadband illumination. However, sunlight and most other light sources are broadband, meaning that they contain many colors. Most current cloaking devices can fully conceal the object of interest only when the object is illuminated with just one color of light.
The concept, theoretically, could be extended to make 3-D objects invisible from all directions a significant step in the development of practical invisibility cloaking technologies. The approach could be applicable to securing data transmitted over fiber optic lines and also help improve technologies for sensing, telecommunications and information processing, researchers say.
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