A unit of soldiers is going on a mission deep behind enemy lines. As they criss-cross through city streets, no one notices them. At the same time, a stealth bomber has recently taken off from its base. And just outside the city, a tank battalion is getting in formation for the inevitable assault.
Fortunately, the enemy has absolutely no idea what is happening. Not only is the bomber deflecting radar waves, the cloaking technology in the tanks' armor and soldiers' uniforms is making visible light literally bend around them so they can evade detection.Thanks to advances in metamaterials, this hypothetical situation is attracting attention and may soon be seen on the battlefield.
Electromagnetic radiation, which includes radio waves, radar (microwaves) and visible light, arises from oscillations in electric and magnetic fields. The classification of the wave is based on its associated wavelength and frequency, which are related by the speed of light.
The ability to see an object is the result of electromagnetic radiation in the form of visible light bouncing off the object and reflecting into a lens (such as in the human eye or in a camera).
The goal of metamaterials is to bend these electromagnetic waves around the object so that the visible light reaching the observer or infrared light reaching an infrared detector is not the light reflected by the object - instead it is the light coming from behind it.
All matter exhibits properties that are the result of their molecular makeup. The properties of metamaterials are determined by both the molecular composition of the materials and the specific orientation of those molecules.
Chiral molecules have the same structure and chemical formula, but have opposite orientation. This is comparable to a left and right-handedness, or a mirror image. The orientation of these molecules with respect to each other therefore affects the properties of the material overall.
In order for the metamaterials to be effective, its structural features must be smaller than the wavelength of the radiation it's manipulating. The technology already exists for radio and microwave radiation, which have relatively large wavelengths, and eventually allow for radar cloaking of stealth aircraft. These structures only need to be on the order of a few centimeters and millimeters.
However, for cloaking of visible light, they need to be on the order of micrometers and nanometers (millionths and billionths of a meter).
Self-assembly methods for metamaterials become relevant at this small scale. One of these methods, known as convective assembly, involves a substrate in a colloidal suspension. As the solvent evaporates away, dissolved particles build up and are ordered into a "nucleus" structure, upon which more particles collect to form a crystal.
While researchers have been able to make tightly packed three-dimensional structures using this method, specifics about the conditions and associated mechanisms that allow for this are not fully understood.
Another method involves engineering the surfaces of particles to separate them by size, therefore creating an osmotic pressure gradient that forces the particles together.
Moniraj Ghosh, a graduate student in the Department of Chemical and Biomolecular Engineering at Hopkins, along with researchers at the University of Pennsylvania, has used this method to assemble closely-packed structures from rough-faced particles, making it possible to synthesize metamaterials on a visible light-bending scale.
Militaries from many major countries, including Britain and the United States, are investing in metamaterials research. For example, DARPA (the U.S. Defense Advanced Research Projects Agency) has provided $15 million for development of "urban obfuscants" for use in combat.
These would act as shields for soldiers and double as invisibility cloaks. What's more, the material they would be fabricated from would be capable of repairing itself. With technology like this on the horizon, the scenario above might soon be a reality.
