New Concept for Spectral Cloaking

Researchers and engineers have long sought ways to conceal objects by mani­pulating how light interacts with them. A new study offers the first demon­stration of invisi­bility cloaking based on the mani­pulation of the frequency of light waves as they pass through an object, a funda­mentally new approach that overcomes critical short­comings of existing 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. The concept, theoreti­cally, could be extended to make 3D objects invisible from all directions; a signi­ficant step in the develop­ment of practical invisi­bility cloaking techno­logies.

A broadband wave illuminates an object, which reflects green light, making the object detectable by an observer monitoring the wave. A spectral invisibility cloak transforms the blocked green color into other colors of the wave’s spectrum. The wave propagates unaltered through the object, without seeing its color and the cloak subsequently reverses the previous transformation, making the object invisible to the observer. (Source: L. R. Cortés & J. Azaña, INRS)

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. The new spectral invisi­bility cloak is designed to completely hide arbitrary objects under broadband illu­mination. The spectral cloak operates by selectively trans­ferring energy from certain colors of the light wave to other colors. After the wave has passed through the object, the device restores the light to its original state.

“Our work represents a break­through in the quest for invisi­bility cloaking,” said José Azaña, National Institute of Scientific Research (INRS), Montréal, Canada. “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.” 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. Most solutions for invisi­bility cloaking involve altering the paths that light follows so that waves propa­gate around, rather than through, an object. Other approaches like 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.

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. This altera­tion of the wave’s temporal profile can make it apparent to observers that something is not as it should be. “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. “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.”

Azaña 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. 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. 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. Then, once the wave has cleared the object, the cloaking device reverses the shift, recon­structing the wave in its original state.

The team demon­strated their approach by concealing an optical filter, that they illu­minated with a short pulse of laser light. The cloaking device was constructed from two pairs of two commercially available electro-optical components. The first component is a dispersive optical fiber, which forces the different colors of a broadband wave to travel at different speeds. The second is a temporal phase modulator, which modifies the optical frequency of light depending on when the wave passes through the device. One pair of these compo­nents was placed in front of the optical filter while the other pair was placed behind it.

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. While the new design would need further development, the demon­strated spectral cloaking device could be useful for a range of security goals. For example, current telecommu­nication systems use broadband waves as data signals to transfer and process infor­mation. Spectral cloaking could be used to selec­tively determine which opera­tions are applied to a light wave and which are made invisible to it over certain periods of time. This could prevent an eaves­dropper from gathering information by probing a fiber optic network with broadband light.

The overall concept of reversible, user-defined spectral energy redistri­bution could also find appli­cations beyond invisi­bility cloaking. For example, selec­tively removing and subsequently reinstating colors in the broadband waves that are used as telecommu­nication data signals could allow more data to be transmitted over a given link, helping to alleviate logjams as data demands continue to grow. Or, the technique could be used to minimize some key problems in today’s broadband telecommu­nication links, for example by reor­ganizing the signal energy spectrum to make it less vulnerable to dispersion, nonlinear phenomena and other undesired effects that impair data signals.

While the researchers demon­strated spectral cloaking when the object was illu­minated from only one spatial 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. In the meantime, the team is also working to advance practical appli­cations for single-direction spectral cloaking in one-dimen­sional wave systems, such as for fiber optics based appli­cations. (Source: OSA)

Reference: L. R. Cortés et al.: Full-field broadband invisibility through reversible wave frequency-spectrum control, Optica 5, 779 (2018); DOI: 10.1364/OPTICA.5.000779

Link: Institut National de la Recherche Scientifique – Énergie, Matériaux et Télécommunications INRS-EMT, Montréal, Canada

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