Shape of things to comeScientists gain better understanding of physics behind invisibility cloaks

Published 27 August 2007

Perfect invisibility cloaks are difficult to achieve, but for many military, law enforcement, and civilian uses, imperfect cloaks may do; one intriguing use of cloaks: Making cities more beautiful by hiding ugly industrial facilities for aesthetic reasons

We have written in the past about invisibility cloaks — that is, materials which can throw a cloak of invisibility around objects, fulfilling a fantasy that is as old as ancient myths and as young as Star Trek and the Harry Potter novels. If the problems associated with creating invisibility cloaks are resolved, they will have many applications in defense and homeland security as well. Questions remain, though. Is a perfect invisible cloak theoretically possible? Are there certain wavelengths — such as those in the visible spectrum — which cannot be made invisible? How will using imperfect materials affect the performance of a cloak? Ah, question. Scientists from Zhejiang University and MIT have recently analyzed the physics behind invisibility cloaks in an effort to answer some of these questions. They have published their research on invisibility cloaks in a recent issue of Physical Review Letters. The group analytically demonstrated how electromagnetic waves interact with invisibility cloaks made of metamaterials, an interaction which is often different from conventional scattering with regular particles. Their findings will likely be useful for cloak design and applications, an exciting research area that is still in its early stages.

“When an electromagnetic wave is incident onto a conventional sphere, part of the radiation will be scattered in all directions; while for a metamaterial cloak, the incident wave will smoothly pass through the cloak undeflected,” Chen explained to “It is very interesting that a perfect metamaterial cloak shows no reflection or absorption but rather allows the Poynting power to bypass the hidden object. Our research also shows that the Poynting power inside of the cloak is not uniform: when close to the inner boundary of the cloak, the power flow density is close to zero, while near the outer boundary of the cloak, the power flow density becomes large.”

The first invisible cloak made of metamaterials was created last year by Duke University researchers David Shurig and others. Metamaterials, which are composed of a man-made matrix of tiny metal wires and loops that control electromagnetic waves, can create an area in space in which no electromagnetic waves propagate. The light waves thus flow around the cloaked object like water in a creek flows around a rock, appearing on the other side in such a way that an observer can no tell that the waves flowed around an obstacle. In the first