Darkness Ray Beams Invisibility From A Distance
“Anti-resolution” beam bathes objects in darkness, say optical engineers who have built a working prototype
Beams of light play a special role in our cultural heritage: World War II search lights picking out Nazi bombers, Gotham City’s Bat signal summoning help in times of distress and lighthouses warning unsuspecting shipping to stay away. Beams of light are beacons of safety.
Now film-makers and optical engineers have something much more sinister to play with. Chao Wan at the National University of Singapore and a few pals have built a “darkness” beam that bathes objects in the absence of light.
The new device hides macroscopic objects by beaming invisibility from a distance, an entirely different technique to the one used in conventional invisibility cloaks that have received much media coverage in recent years.
The new device turns the conventional approach to optics on its head. Conventionally, optical engineers devise imaging system with the best resolving power possible.
The basic idea is that an imaging system focuses light into a pattern known as a point spreading function. This consists of a central region of high intensity surrounded by a concentric region of lower intensity light and a higher intensity lobe beyond this.
Engineers get the best resolution by narrowing and intensifying the central region while suppressing the outer lobe. (Indeed, one of the more exciting recent developments in imaging is in using this technique to resolve objects that are significantly smaller than the wavelength of the light being using to create the image, a technique known as super-resolution. )
Now Chao and co have taken exactly the opposite approach. Instead of narrowing and intensifying the central region at the expense of the lobes, these guys intensify the lobes while suppressing the central region.
The result is a central region where the field intensity of light is essentially zero. This is a region where objects cannot be resolved, hence the group’s name for this effect: anti-resolution.
The central region is surrounded by a region of high intensity light which acts like a kind of light capsule containing a 3D region of darkness. “A three-dimensional object placed in the optical capsule does not cause scattering and one can therefore see the scene behind the object,” they say.
In effect, it is an invisibility capsule. Chao and co say that the region of darkness can be as much as 8 orders of magnitude bigger than the wavelength of light used in the imaging process. That’s huge!
And the imaging system itself is simple. Chao and co demonstrate it using a laser beam passing through a “lens” consisting of concentric dielectric grooves that are straightforward to manufacture. In their test, they hide an object—a letter ‘N’—that is 40 micrometres in size. That’s significantly larger than conventional invisibility cloaks could do when they first hit the headlines.
Perhaps that’s not surprising given that the new device works in an entirely different way from conventional invisibility cloaks. These are built using bespoke metamaterials that steer light around an object placed inside them. By contrast, Chao and co can effectively beam invisibility from a distance.
There are some limitations, however. The current device works at a single frequency of light so an interesting challenge will be to make broadband lenses that work at a wide range of frequencies.
Beyond that, Chao and co will have to find a killer app for their new device. They say it has many potential applications such as in cloaking and surveillance but give little detail.
Perhaps imaginative readers of the Physics arXiv Blog can help out with suggestions of their in the comments section here.
Ref: arxiv.org/abs/1312.0057: Experimental Demonstration of Light Capsule Embracing Super-Sized Darkness Inside Via Anti-Resolution