Metamaterials offer a real possibility that our most far-fetched fancies could one day become real as rocks. From invisibility cloaks and perfect lenses to powerful batteries, their super-power applications seem to be never-ending. So far these applications haven’t been developed, even though scientists have been studying these metamaterials for more than 15 years.
"Not many real metamaterial devices have been developed," says Elena Semouchkina, an associate professor of electrical engineering at Michigan Technological University.
Invisibility cloaks don’t exist and there isn’t a perfect lens app that lets you see viruses with your smartphone. This is partly because, traditionally, researchers oversimplify how metamaterials actually work. Semouchkina says their complications often have been ignored.
So her team set out to investigate those complications and discovered that the magic of metamaterials is driven by more than just one mechanism of physics.
Metamaterials may seem complex and futuristic but according to Semouchkina, they are fairly simple. Metamaterials are engineered materials that have properties not found in nature. Typically, they are built of multiple identical elements fashioned from conventional materials, like metals or conductive materials.
These designer materials work by bending the paths of electromagnetic radiation — from radio waves to visible light to high-energy gamma rays — in new and different ways. How metamaterials bend those paths (refraction) drives their peculiar waves around a cloaked object, accelerating them on their way and reuniting them on the other side. An onlooker could see what was behind the object, while the object itself is invisible.
The conventional approach among metamaterial researchers has been to relate a metamaterial’s refractive properties to resonance. Each building block of the metamaterial vibrates like a tuning fork as the electromagnetic radiation passes through, causing the desired type of refraction.
Semouchkina wondered if there might be additional factors involved in bending the paths of the waves.
"Metamaterials seem simple, but their physics is more complicated," she says, explaining that she and her team focused on dielectric metamaterials, which are built of elements that don't conduct electricity.
The team ran a few computer simulations and made a surprising discovery. It was the shape and repetitive organization of the building blocks within the metamaterial — their periodicity — that affected the refraction. Resonance seemed to have little or nothing to do with it.
The metamaterials they studied have characteristics of another type of artificial material, photonic crystals. Like metamaterials, photonic crystals are made of many identical cells. They also behave like the semiconductors used in electronics, except they transmit photons instead of electrons.
"We found that the properties that go along with being a photonic crystal can mask the resonance of metamaterials, to the point they can cause unusual refraction— including negative refraction, which is necessary for the development of a perfect lens," Semouchkina says, "Basically, we need to recognize that some of these structures can exhibit properties of photonic crystals, and we need to take their physics into account. It's an evolving field, and it's a lot more complicated than we've given it credit for."
Semouchkina’s team is working on developing invisibility cloaks using photonic crystals, but she stresses that metamaterials research can have other real-world applications. One of her projects focuses on using metamaterial concepts to improve the sensitivity of magnetic resonance imaging (MRI), which could lead to better medical diagnostics and advances in biological research.
Understanding the underlying physics of metamaterials will speed up the development of many devices.
A paper on this research was published in Journal of Physics D: Applied Physics.