The concept of an invisibility cloak sounds like pure science fiction, but hiding something from view is theoretically possible, and in some very-controlled cases it's experimentally possible too. Now, researchers have developed a new device that works in a completely different way to existing cloaking technology, neatly sidestepping some past issues and potentially helping to hide everyday objects under everyday conditions.
We see objects because light bounces off them in a particular way before landing on your retinas, and they get their colors by reflecting more light of that particular color. The basic concept of cloaking objects involves finding ways to disrupt that process and build them into devices or metamaterials.
Over the years, most techniques we've seen (or not seen, as the case may be) involve bending light waves around the object. Others work on a principle called "temporal cloaking." In this situation, light at the leading edge of a wave is sped up, while that at the trailing half is slowed down, creating a gap in the middle. If something were to move through that gap, no light would hit it, letting it pass through like parting the Red Sea. At the other end, the gap is closed by slowing down and speeding up the light waves, and an observer would be unaware that the object was there at all.
Well, almost unaware anyway. Therein lies the problem with both types of cloaking – messing with the travel time of the light waves can create a visible "shimmer" effect like that in the Predator movies, alerting viewers that something is amiss.
"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," says Luis Romero Cortés, lead author on the study. "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."
For the new device, which the team calls a "spectral cloak," light isn't redirected around the object but is made to pass right through instead. The device selectively shifts light from one part of the spectrum to another, before shifting it back to normal on the other side. If you're cloaking a green object, for example, the device shifts green light to the blue part of the spectrum so it can pass through, then reverses the process. With no green light to bounce off the object, it becomes effectively invisible.
The device itself does all this using two commercially available components, placed in pairs on opposite sides of the object. First a dispersive optical filter forces different colors of light to travel at different speeds, then that light hits a temporal phase modulator, which changes the frequency of the light based on when it passes through. That lets the team manipulate the light to avoid reflecting the color of the object, and once the wave has passed through, those components work in reverse to restore the original state of the light.
In tests, the team beamed short laser pulses at an optical filter, which can be tuned to absorb certain colors of light but not others. Using their device, the researchers were able to effectively conceal the optical filter, without letting it absorb any light.
Of course, for any practical cloaking use the device would need to function in three dimensions, but the team is working on that. Along with the long-term goal of cloaking objects, the technology could be put to use in fiber optics communications systems, making them more secure and efficient.
The research was published in the journal Optica.
Source: The Optical Society of America