November 12, 2013
Quest For ‘Invisibility Cloak’ Remains Elusive, Or So It Appears
redOrbit Staff & Wire Reports - Your Universe Online
Limitations in current cloaking technology allow objects to be temporarily concealed at only narrow wavelengths of light or microwaves, while many newer designs actually make things more visible, experts at the University of Texas acknowledged on Monday.
Current cloaking devices, which are made of a variety of materials such as planes of glass, work by bending light around objects to make them temporarily invisible. Scientists have also developed ways to deflect and reposition other kinds of waves, such as sounds waves used in sonar equipment.
Although these breakthroughs are inspiring and will likely lead to further advancements, they come with serious drawbacks, the researchers noted in their study published in the journal Physical Review X.
Andrea Alu, an electrical engineer at the University of Texas at Austin and Francesco Monticone looked at three popular types of "passive" cloaks – a plasmonic cloak, a mantle cloak, and a transformation-optics cloak. Passive cloaks are those that don’t require electricity.
The researchers calculated that, when tested over the entire range of the electromagnetic spectrum, all three techniques scattered more waves than the object they were trying to conceal.
This is perhaps the most significant problem to overcome with current devices, the researchers said.
The limited range of wavelengths and types of waves means that a cloak designed to deflect microwaves, for instance, would not deflect visible light. Furthermore, making an object invisible to red light can make it appear bright blue, increasing its overall visibility.
Assuming one looked at all wavelengths of light, one would actually see the cloaked object “more than the uncloaked object it is trying to hide,” the researchers said.
"If you suppress scattering in one range, you need to pay the price, with interest, in some other range," Alu told James Morgan of BBC News.
In recent years, scientists have developed invisibility cloaks from metamaterials, which can be artificial structures with special light scattering properties. Other cloaks have been made from static materials to warp light. Someday, these devices could be made from dynamic components such as a network of electronic amplifiers that connect metal patches to create an active and changeable surface.
The use of such an “active” design would make cloaks invisible over a broader range of wavelengths and lighting conditions, the researchers said.
In the current study, Alu and his team proposed a new design that uses amplifiers to coat the surface of the object in an electric current, creating an ultrathin cloak that could hide objects at a frequency range "orders of magnitude broader" than any available passive cloaking technology.
"Our active cloak is a completely new concept and design, aimed at beating the limits of [current cloaks] and we show that it indeed does," Alu told the British news agency.
"If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution,”Alu added. "We are looking into realizing this technology at the moment, but we are still at the early stages."
For now, a cloak that allows complete invisibility is "impossible" with current passive designs, according to the study team.
"When you add material around an object to cloak it, you can't avoid the fact that you are adding matter, and that this matter still responds to electromagnetic waves," Alu said.
A much more promising avenue is active cloaking technology that relies on electrical power to make objects vanish.