Spectral cloaking is a fundamentally new approach to an age-old challenge: It’s a way to change the color of light waves as they pass through an object, resulting in invisibility. A new study from the National Institute of Scientific Research (INRS) in Montreal has offered the first demonstration of a device built on this principle.

Objects are visible because they modify the energy of the light waves that interact with them. Most cloaking devices today work by altering the paths followed by light — making waves propagate around, rather than through, an object. These work to conceal an object, but only when it is illuminated with a single color of light — a significant limitation, considering that sunlight and most other light sources are broadband, containing many colors (or frequencies). There are also temporal cloaking devices, which tamper with light’s propagation speed in order to temporarily conceal an object for a prescribed length of time.

With both approaches, the altered paths that light follows can make it apparent to observers that something isn’t quite right. In other words, there is a detectable distortion that gives away the presence of the cloak.

The new device, by contrast, operates by selectively transferring energy from certain colors of the light wave to other colors. It first shifts 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. After the wave has passed through the object, the device reverses the color shift, restoring the light wave to its original state.

The INRS researchers built their cloaking device from two pairs of two commercially available electro-optical components: a dispersive optical fiber, to force the different colors of a broadband wave to travel at different speeds; and a temporal phase modulator, to modify the optical frequency of light depending upon when the wave passes through the device. The team demonstrated its approach by placing an optical filter in between the two pairs of components, then illuminating it with a short pulse of laser light. This resulted in what appeared to be a laser pulse propagating through a non-absorbing medium.

The device could be used to secure data transmitted over fiber optic lines, and to improve technologies used for sensing, telecommunications and information processing. For example, selectively removing and subsequently reinstating colors in broadband waves used as data signals could allow more data to be transmitted over a given link, helping to alleviate logjams as data demands continue to grow. The technique could also be used to reorganize the signal energy spectrum in order to minimize problems such as dispersion, nonlinear phenomena and other undesired effects that impair data signals.

For their study, the researchers demonstrated spectral cloaking when the object was illuminated from only one spatial direction. Theoretically, however, the concept could be extended to make three-dimensional objects invisible from all directions; the team plans to continue its research toward this goal, while also working to advance practical applications for single-direction spectral cloaking in one-dimensional wave systems.

The research appears in the Optical Society journal Optica.