Meta is better
Creating “impossible” optical materials
Lenses in one form or another have been used by humans since antiquity for magnifying small objects, lighting fires and even correcting defects in human vision. Since the day a Dutch spectacle maker discovered that two suitable lenses could form a telescope; the science of optics has gone from strength to strength. Today for the same price as a bag of groceries, one can purchase a sophisticated multi element camera lens that is truly a masterpiece of optical engineering.
In spite of all the progress, there are limitations too. All of the different types of glass available to optical engineers today have, for the most part, broadly similar optical properties. So there’s only so much you can do with them.
To a large extent this is dictated by what nature gives us to work with. A glass is composed of atoms with charged nuclei and negative electrons sharing orbitals with their neighbours. Light is an oscillating electromagnetic wave. So when it enters this charged environment complex interactions occur between its own electric and magnetic fields and that of the lattice atoms. Seen from a distance this interaction has the effect of bending the beam.
Given this mechanism of operation, it’s fairly clear that glasses with different atoms and different electron configurations will bend the beam in slightly different ways – a property known as refractive index. But in general all atoms and all glasses tend to behave in fairly similar ways. And because we only have a hundred or so elements to chose from there are limits to what can be achieved.
However in recent years, scientists have been developing ways to cheat. Rather than simply making optical materials out of homogeneous blends of atoms, it’s possible to create little clumps of atoms and alternate them to make an artificial lattice. These so called metamaterials can be engineered to have properties that would be totally impossible to achieve with conventional compounds.
Fabrication of such materials for visible wavelengths is technically extremely difficult but with microwaves, which of course have the exact same physics, it’s becoming a hot area of research.
Dr Ilya Shadrivov leads a group at the Australian National University developing metamaterial optics for microwave applications. “In the past we’ve created materials with negative refraction and other exotic properties,” He says, “But these have all been fixed architectures.”
The microwave metamaterials are fabricated in a similar way to a printed circuit board with a lattice of carefully engineered copper tracks that interact with the passing microwaves in just the same way the atoms of a glass interact with light. But to date, even though these new microwave metamaterials have exotic properties, those properties have been fixed. This means that a microwave lens or mirror made in this way can only behave in one way.
“It occurred to us that if instead of building the lattice using simple tracks, we used electronic circuitry including light-sensitive photo-diodes, then we could control the refractive properties at will.” Dr Shadrivov says, “On the prototype I was changing the direction of the microwave beam simply by lighting the metamaterial with the torch on my phone which was really cool!”
The group and their collaborators are now producing metamaterials in which each “meta-atom” element has its own LED illumination. This enables them to not only vary the properties of the material but to do so differently in different regions.
The technology can potentially have applications in radar and communications. “Using materials like this we can create a satellite dish that isn’t a dish at all, it’s a flat disc. And we can change properties like its focal length and reflectivity without changing its physical shape at all.”
Radar and communication dishes that reflect, refract or simply disappear under optical command offer the possibility of exciting new devices for both military and civilian applications.