UC Berkeley researchers manipulate light to create ‘invisibility cloak’

Jeffrey Joh/Staff
Majid Gharghi and Chris Gladden study the ‘carpet cloak’ they helped develop, which makes objects invisible to the human eye by controlling how light bends.

Using optics to manipulate the way light bends when it hits an object, a team of UC Berkeley researchers has developed a cloak to create the illusion of invisibility — a technology that may one day also be used to manipulate solar light energy.

In a paper published on July 13 in the American Chemical Society Nano Letters journal, campus mechanical engineers described the development of a new cloaking method to make objects invisible to the human eye by controlling the way light bends when it moves through the cloak.

The “carpet cloak,” a name given to this new technology by the researchers in campus mechanical engineering professor Xiang Zhang’s nanoscience research lab, was developed in nine months and is a covering made of thin layers of silicon oxide and silicon nitride that sits on a silver mirror. The cloak not only covers an object hidden below, but also bends light away from the bump the object creates to flatten the image viewed by the human eye.

“The cloak has a variable refractive index,” said Chris Gladden, a graduate student researcher and a co-author of the paper. “So by controlling the index of a material, we can control the way light moves through it. We can hide something under the cloak, and using the cloak, we flatten the bump.”

Although the cloak that Gladden and post-doctoral research fellow Majid Gharghi developed is about 5 inches wide, it only works to create the illusion of invisibility on microscopic objects roughly 300 nanometers in diameter — about the size of a red blood cell.

Because this cloak works for objects less than 100 times its size, Gladden said scaling up this technology for a human-sized “invisibility cloak” would be difficult. If the current cloak were scaled up, it would need to be the size of mid-sized room in order to work for one person, he said.

Gladden and Gharghi have been studying the properties of nanomaterials like silicon oxide and silicon nitride to identify which material combination can provide the a higher index of refraction because, according to Gharghi, the higher the index, the slower the light travels through the material and the easier it is to flatten the bump.

“Right now we are limited by the maximum refractive index of the materials we use,” Gladden said. “But that does not limit the potential of a new material. If we are able to find a material with a much higher (refractive) index, we can create a much more compact cloaking.”

This is not the first time Zhang’s group has used optics to create invisibility “cloaks.” In 2009, the group used artificial materials to hide objects on a flat surface, and in February, Zhang’s group developed GRIN plasmonics — the use of specialized lenses to reroute and reflect light on the nanoscale.

In contrast to the GRIN plasmonics technology, which according to Gladden, was limited to creating invisibility on the red visible light and infrared light spectra only, the “carpet cloak” creates invisibility across the entire visible light spectrum.

Because of this property, Gharghi said the technology could one day be layered on top of solar cells to manipulate solar energy and light, though he added that nanomaterials exhibit certain properties that disappear when they are scaled up to a human size.

“The unique properties of nanomaterials make scaling up very difficult,” Gharghi said. “Right now there are no nanofabrication techniques that can be used to size up for the human body.”