Movies have often managed to mess up invisibility even when they have the ability to go beyond the restrictions of reality. For example, in the movie “Predator”, an alien uses a cloak-like-object to hide from others but is easily thwarted by distortions of light bending around said cloak. Interestingly, researchers--in our world-- have managed to get across this light distortion problem. They have built an ultrathin “invisibility cloak” that can turn objects into perfect, flat mirrors.
In theory, the goal of an invisibility cloak is to bend light around an object. Unfortunately, materials that have the ability to do this are hard to shape and work only from narrow angles. In this condition, the cloaked object is visible if you were to walk around it. Luckily, a new cloak avoids this problem as it is thin and flexible enough to wrap around any shape. And of course, it can be adjusted to complement any background that’s behind it. What’s more, it can create illusions.
A group, led by Xiang Zhang, director of materials science at Lawrence Berkeley National Laboratory, constructed a thin film consisting of a 50-nanometer-thick layer of magnesium fluoride topped by a varying pattern of tiny, brick-shaped gold antennas, each 30 nanometers thick. The gold antennas (bricks) were built in six different sizes, ranging from about 30 to 220 nanometers long and 90 to 175 nanometers wide.
To test their creation, the scientists covered a tiny, weirdly shaped object (measuring a bit more than one-thousandth of an inch) with the thin film. They, then, shined an infrared light at the wrapped object and saw that the light reflected back almost perfectly without revealing where the object was. It was as though there was just a flat mirror in its place.
The reason for the invisibility was due the gold antennas scattering light that reflects off of it. In a normal case, light reflecting off an object (even a mirror) will scatter a bit, more so if the shape is irregular. Sometimes, the waves of light may create interference patterns. An interference pattern changes the phase and frequency of a certain light wave. (Phase is an angle measurement that tells you how far along a light wave you are; two waves 180 degrees out of phase cancel out.) Hence, any reflected light appears as colors or a reflection depending on the object.
The new cloak of invisibility changes that. The gold bricks on the film reflect the light in a way that the phase and frequency of the light are maintained. In other words, the cloak makes it seem like the light were colliding with a perfect mirror and the cloaked object wasn’t even there. Even the edges are nowhere to be seen.
With the right tuning of the gold antenna bricks, it’s not too difficult to make the light being reflected look like anything you want. Usually, it’s just the background or the floor, but it could easily be something else. A cloak that was large enough could be draped over a helicopter and make it look like a bike. And the light doesn’t necessarily need to be infrared either. It could just as easily be of another wavelength.
The reflection trick also works from any angle, and the cloak doesn't have to be a peculiar shape — it can be wrapped around anything, and still functions properly. It's also thin and light, according to the researchers.
But there is a disadvantage, a pretty big one: If someone were wearing this cloak, he or she would have to stay still for it to work, since the tuning has to be matched to the background. With this in mind, Andrea Alù, an associate professor of electrical engineering at the University of Texas at Austin, has done extensive research on cloaking systems. He is doubtful that scientists can create the kind of illusion Zhang’s group is looking for.
"They had a small object, a little bump," Alù states. "With a larger object, I can't take advantage of that … when I illuminate it, a portion is not illuminated; it's in shadow. As such, the illusion of the perfect reflector would be broken.” That being said, the new findings show you can influence how light reflects using nanometer-scale structures on a thin film. "The beauty of the paper is that you can control the reflection surface at the subwavelength scale," Alù said.
Zhang said the cloaking technology's reflectivity offers another application: displays. Right now, any big projection has to use a relatively flat surface. But if the phase and frequency of the light reflected from it could be finely controlled, that problem could go away. A projection surface could be any shape, and the resulting picture would not be distorted. Zhang added that this kind of material has been fabricated before, and that a next step would be to make a lot of it at industrial scales, tuning the antennas to different wavelengths of light.
The cloak of invisibility is a mark of our advances in technology. It is also a mark of the things we would do to make our imagination a reality.