Ghost Illusions From Metamaterials

Researchers at the National University of Singapore have released details from an upcoming paper (to be published in the journal Advanced Functional Materials this month) about using metamaterials to generate controlled “ghost” images of an object. While not yet done in optical light, this technique continues to illustrate the odd behaviors one can create with metamaterials. I first learned of this through an article in phys.org, though the live science article provides additional information and perspective.

Metamaterials are funny things, I think it is fair to say. If you’ve not yet run across the concept, take a look here. In essence, they are materials which, through their construction and structure, induce new or unexpected behavior in light or sound (waves). This is done through exploiting the wavelengths with shapes and sizes which are multiples or fractions of the wavelengths of interest. One of the more interesting properties being heavily explored is the concept of a negative refractive index. One of the most popularized notions of metamaterials and negative refractive index work is that of the “invisibility cloak” where light could be bent around an object rendering it invisible. In practice, metamaterials can do many of these things, but they have key limitations. They are typically rigid structures that must be carefully placed around the object and only work on narrow bands of light or sound, specific wavelengths (think radar or microwave or one particular tone, sonar).

The team at NUS, lead by Dr. Qiu Cheng-Wei, have taken this idea of bending light around an object a step further. They have created a metamaterial setup which can not onlyhide the subject object, but can also create “ghost” images of that object. The researchers can set the system up such that false images appear around or displaced from the real object. From the live science article:

In principle, the researchers can manipulate what illusions are seen by tinkering with the device in any number of different ways �" for instance, by changing the shape and thickness of the rings, their number or how far apart they are, or the patterns of copper on the rings. They can make the original item vanish and make any number of ghost images appear in its stead, as well as engineer the appearance of those illusions.

"We can control how the ghost objects look in many ways," Qiu said.

The phys.org article observes:

Their research has opened up a completely new avenue for cognitive deception through light-matter behaviour control. This would have wide applications in defence and security. Their findings will also pave the way for the design of new optical and microwave devices such as those for detection and communication. The team will further develop this technique to make larger microwave devices to achieve radar "ghosts" and aircraft camouflage suitable for defence purpose.

Right now, their system consists of concentric rings of flexible printed circuit board with copper traces and only works on radar light. Given that the system is rings around the target object, the illusion can only work in the same plane as the rings, viewed through them, but Dr. Qui notes that it can be made to work three dimensionally via the use of concentric shells. Optical light is a matter of changing the wavelengths handled by the material, which may be easier said than done given the very small wavelengths in play (approximately 390-700 nm).

Neat stuff, in my opinion. It is still a long way away from “invisibility cloaks” or practical stealth technology, however. One thing that was special about this particular approach was the management and use of scattering instead of just refraction. I’d like to see if it is possible to have a single metamaterial surface (perhaps multi-layered?) which can not only support optical wavelengths, but all optical wavelengths. I’m thinking there may be the potential for a new kind of display technology lurking in here. Looking through a shell of optical metamaterials, it may be possible to tune and adjust it such that any image can be displayed anywhere in the sphere, creating a new kind of holographic display. Just speculating, I can’t back this up. It would be cool, though.