The Peculiar Properties of Metamaterials

Take a look around; everything you see is made of some kind of material. Your desk might be wood, the carpet is composed of wool or perhaps polymer fibers and the windows are made from glass. These are all materials, and materials have properties such as hardness, strength, mass and ductility. Some materials can of course do more interesting things, like transmit light (glass) or conduct electricity (copper,) but they all have one thing in common: their physical properties are set by their chemical composition. For example, copper conducts electricity because of the way each atom interacts with its neighbors.

Metamaterials are different. A metamaterial is one where the properties result from its structure rather than its composition. That difference can be rather hard to appreciate, so let’s use a simple analogy: the domestic, man-made sponge.

Artificial sponges are made from cellulose. Cellulose doesn’t hold a lot of water, so what enables a sponge to soak up a bucket load? The answer is that it’s the holes that hold the water. In other words, it’s the structure that does the job of soaking up moisture, not the cellulose itself.

So a metamaterial can be thought of as a sponge, but one where the structure has been tweaked at the microscopic level to give it specific properties. This results in materials with properties that don’t exist in nature, such as negative refraction.

Most people are familiar with refraction because instances exist all around us. It’s refraction that makes the bottom of a swimming pool look closer than it really is. It’s refraction that makes a pencil appear to bend when dipped into a beaker of water, and it’s refraction that steers light through the lens of a camera onto the sensor.

Refraction takes place when light passes from one medium, such as air, to another medium, like water or glass. The change in density between the two materials actually bends the light, and it does so in a very predictable way that results from the properties of the material and the wavelength of the light. A great example of this is when a prism is used to split white light into the colors of the spectrum: the different wavelengths bend by slightly different amounts as they pass from glass to air, resulting in the familiar rainbow.

In a metamaterial the structure has been engineered to bend light the other way than what the laws of physics normally dictate. (This doesn’t mean the laws are broken; physicists just understand how to create a structure that behaves differently to what happens in nature.) This is what is termed the property of negative refraction.

Now it’s fair to ask why scientists and engineers would be interested in such materials. The answer is that metamaterials hold the promise of being able to steer electromagnetic waves, not just light but longer wavelengths such as microwaves, in very precise ways that make possible inventions we’ve only dreamed of.

One application, of great interest to the military, is surfaces that can absorb rather than reflect radar. This ultimate “stealth” technology would make ships and aircraft invisible to opposing forces. Another area of research is in designing smaller antennas for portable electronic devices like phones. Here metamaterials hold the promise of smaller, yet more powerful radio devices to make us even more connected.

Metamaterial technology is also being investigated for its potential to improve the efficiency of solar cells, and even as a means of protecting buildings against earthquakes, but some scientists are chasing a bigger prize: invisibility!

The ability to disappear has been a dream for centuries, but scientists working on metamaterials are slowly turning it in to reality. The key is to engineer a structure that provides the peculiar property of negative refraction with visible light. When this is achieved metamaterials will quickly make the jump from research lab to reality, and the world will never be the same again.