Research done at McGill University could result in a simpler way to create negative-index materials, which are substances that interact with light in numerous unexpected and extraordinary ways.
What a “negative-index material” does is not obvious for laypeople, but the name refers to a phenomenon called negative refraction. Refraction happens when light bends as it passes through an object. For example, when a straw is placed in water, it appears bent, which is caused by the light from the straw bending as it goes through the water. This is due to the electric fields of the atoms interfering with the path of the light, causing it to bend. In negative refraction, the light bends in the material, but in a different direction than in normal materials, which is caused by light travelling backwards in the material. Materials which can cause light to do this are extremely rare: in fact, none of them occur in nature. To build them, scientists must assemble hundreds of thousands of small, fine-tuned components called unit cells in repeating patterns; a difficult task, the unit cells are often thousands of times smaller than the period in this sentence.
The proposed method for the construction of negative-index materials would use several innovative techniques to make the process easier. In this case, the unit cells are extremely small gold rings, only a few nanometers in diameter. Gold is ideal for the construction of this material, because at small sizes, it exhibits a phenomenon known as surface plasmonic resonance. This occurs when light that hits the gold’s surface causes its electrons to vibrate at the same frequency as the light, which produces electromagnetic fields which influence how the light travels.
Usually, constructing these rings would be difficult due to their size, but a creative way around this problem has been found. First, a chemical reaction is conducted which causes the atoms of gold to clump together into nanometer-sized spheres. Next, these spheres are arranged into rings with the assistance of a strange partner: a virus.
The tobacco mosaic virus, like most viruses, has a shell to protect its DNA. The shell of this virus is cylindrical, and made up entirely of one type of self-assembling molecule. If many of these molecules are placed in a liquid, they will bond with each other, forming the cylindrical shell on their own. Thus, to make the gold nanoparticles into rings, the researchers attach them to copies of this molecule and let it assemble, causing the gold to form rings on its own. From there, all it takes is a few more chemical reactions, then arranging the gold unit cells in the proper shape.
Negative-index materials are not yet practical to use, as they are hard to make, only work for specific wavelengths of light, and usually don’t work for visible light. In the future, though, they may be used to create lenses which magnify objects more than normal materials can, or even for “cloaking devices” which bend light around an object to make it invisible.
Originally Published in Vol. 47 Issue 9 on February 21st, 2018