For thousands of years, people have been captivated by the prospect of invisibility. In a report published in Science two months ago, researchers at the University of California, Berkley describe how they were able to effectively render an object invisible to red light, using a method developed by researchers at the University of Toronto.
Physicists have known for some time that an object can be made invisible by preventing light waves from carrying information about that object. For the past decade, researchers have focused on bending light away from objects in order to prevent information from reflecting off of their surfaces. This approach has been made possible by the development of meta-materials — engineered surfaces with unusual electromagnetic properties.
The meta-material approach has proven effective in masking objects from light waves in radio and microwave frequencies, however, real-world applications remain unlikely. This is because meta-material cloaks must be substantially bigger than the objects that they mask in order to be effective.
In 2013, researchers at U of T changed the game by proving that an alternative cloaking technique was possible. Dr. Eleftheriades, an accomplished researcher in the field of meta-materials, and his PhD student, Michael Selvanayagam, conducted the experiment. Along with the meta-material approach, these researchers ditched the technique of bending light. In its place, the researchers introduced light interference.
You may recall from your high school physics classes that waves cancel each other out when they are out of phase — that is, when two different waves are pushing and pulling in opposite directions. The same principle applies to light waves. Making use of this phenomenon, Eleftheriades and Selvanayagam built an array of antennas that can detect the frequency of incoming light waves and output the corresponding inverted wave, in order to cancel the outgoing information. Using this device, Eleftheriades’s team was able to make a large metal cylinder invisible to radio waves.
By replacing meta-materials with a series of antennas that could be theoretically printed flat, Eleftheriades and Selvanayagam demonstrated that practical cloaking could be accomplished.
The researchers at Berkley, lead by Dr. Xiang Zhang, extended Eleftheriades’s findings by actually assembling a thin layer of microscopic gold antennas. These antennas were tuned to selectively interfere with visible light in the red frequency, effectively rendering the cloaked object invisible in the red light spectrum. If you were standing in a room filled with red light, you would not see the cloaked object, from any angle.
Although the researchers are hopeful that this technique can be scaled up to cloak everyday objects from light in the full visible spectrum, there are many technical hurdles that need to be overcome before we find invisible cars parked on St. George street.
For instance, the current technology is limited to static objects with few contours, because the current antennas cannot compensate for motion or shadows. Moreover, scientists still need to devise a way to dynamically interfere with wavelengths of all colours in order to make objects invisible outside of lab conditions.
Despite these limitations, we are one step closer to developing the ultimate product for university students: a blanket that can hide exam-season clutter.