Science fiction and fantasy tales are full of the ability to “cloak” characters with invisibility. Whether it is a spaceship with a cloaking device, or a young wizard with an invisibility cloak, the interest in rendering someone or something invisible captures our fancy. Scientists have succeeded in creating the illusion of invisibility by bending light around a region for concealment. These types of devices have limitations, however; one of these limitation that the device normally has to be touching the object to be rendered invisible – or in very close proximity.
Instead of bending light, though, what if transformation optics was used to create invisibility – or even give an object the appearance of a completely different object? A team of theorists at the Hong Kong University of Science and Technology propose a technique that might be able to accomplish just that, with a remote device. The groupâ€™s proposal appears in Physical Review Letters: â€œIllusion Optics: The Optical Transformation of an Object into Another Object.â€
â€œRight now, invisibility devices bend light, steering it around an object to make it appear transparent, as if it werenâ€™t there,â€ Che Ting Chan tells PhysOrg.com. Chan is one of the scientists who proposed the idea of a device that would use illusion optics to transform objects within a confined space. â€œOur proposed device would have special properties. It would employ a type of illusion that makes an object look exactly like another object.â€
Metamaterials, which are manmade with special properties, would be used. A â€œcomplementary mediumâ€ would be employed to optically cancel a specified area. Then, a â€œrestoring mediumâ€ would be used to make the cancelled space â€œreappearâ€ as something else. For invisibility, the restored area would look like air. â€œIf it looks like air,â€ Chan explains, â€œthen it is transparent. You can see through it. It is like making something invisible.â€ This method could also, theoretically, be used to â€œtransformâ€ objects. â€œYou could make a ball look like a rock,â€ Chan says. â€œYou could hide something in plain sight, as something else.â€
In addition to transforming the way an object looks to others, this device would have another advantage over current invisibility devices. â€œYou wouldnâ€™t have to need the device to touch the object being hidden,â€ Chan insists. â€œYou could remotely control a particular area to entirely exclude a specific wave without having to be as close.â€
While the device sounds promising, Chan admits that there are difficulties involved in building such a cloak. â€œIn addition to positive refractive material, which is not a big problem, we would require a negative refractive index.â€ Positive refractive materials are easily made, but something with a negative refractive index would require special design. â€œSome sort of structure to create a phase delay would be needed,â€ Chan says.