Science & Environment

Creator of photonic crystals wins Newton medal

photonic crystal Image copyright Eli Yablonovich
Image caption Photonic crystals can manipulate light in the same way that a semiconductor like silicon can steer electrical current

The US physicist who first discovered "photonic crystals" which can repel, trap and steer light is to receive the Newton Medal, the BBC can reveal.

This is the highest honour given by the Institute of Physics in London.

Prof Eli Yablonovitch of the University of California, Berkeley, proposed and created the crystals in the 1980s.

They are now used in data processing and in wave guides for laser surgery; they have also been discovered in bird feathers and the skin of chameleons.

"I'm very, very honoured," Prof Yablonovitch told the BBC.

His seminal description of photonic crystals was published in 1987 when these materials had never been manufactured, and were not known by that name.

It is the second most-cited paper ever published in the prestigious journal Physical Review Letters.

Surprisingly enough, the same concept was described in a second paper three weeks later by Sajeev John, now a professor at the University of Toronto.

Image copyright Institute of Physics
Image caption Prof Yablonovitch made the first photonic crystal while working at Bell Communications Research in New Jersey

Shoulders of giants

Both researchers built on an idea put forward 100 years earlier in 1887, by British physicist Lord Raleigh. He suggested that a material with a repeating, regular structure - such as a crystal - could block light of particular wavelengths.

This happens, Raleigh calculated, because if the light has a wavelength that is similar to the size of the repeated units in the structure, then the waves reflected off its internal surfaces will interfere and cancel each other out. That produces a "stop band" (later called a "photonic bandgap") - a range of light wavelengths that will be repelled by the crystal.

"[Lord Raleigh] developed the idea of the one-dimensional photonic crystal," Prof Yablonovitch explained.

"That's something that we've lived with for over 100 years. But over that entire period, no-one really thought of extending Raleigh's idea into two and three dimensions."

That was where he and Prof John stepped in, almost simultaneously - at a time when controlling light was a subject of great interest, for applications such as optical fibres and solar cells.

"It's just one of those ideas - maybe after 100 years the time had come, and two people thought of it within a few months of one another," Prof Yablonovitch said.

But, he added, expanding the idea into more dimensions, to block light in all directions, was very difficult because "it wasn't clear what the crystal structure should be".

Holes in ceramic

A one-dimensional photonic crystal is a simple stack of layers, but nobody knew what shape a 2D or 3D example would take.

It took Yablonovitch's team four years, and a lot of failed experiments, to produce "Yablonovite" - the first 3D photonic crystal. It consisted of a ceramic material, drilled with three intersecting series of cylindrical holes, 6mm across.

Its power was obvious, Prof Yablonovitch said.

"The multiple scatterings work out such that no matter which way the light tries to go, it's blocked - in every imaginable direction. And even in directions that we cannot imagine, it's still blocked."

Today, 2D photonic crystals are used in "silicon photonics" - integrated circuits that use both light and current to transfer information. These are becoming common in large data centres.

Other researchers have adapted the discovery to guide types of light that are useful in the surgery, including lasers.

"I myself am totally amazed at the applications of these things," Prof Yablonovitch said.

Image copyright Eli Yablonovitch
Image caption The engineered crystals had an array of intersecting cylindrical holes

He particularly appreciates the photonic crystals that have evolved in the natural world. Butterfly wings and the colourful plumage of peacocks and some parrots all contain examples, which were only understood after Yablonovitch and his fellow physicists fully described photonic crystals in the 1980s.

"Other people then started looking in a new way at nature," Prof Yablonovitch said.

"When they started... using electron microscopes and so forth, they discovered that the microstructure corresponded to that of a photonic crystal."

Even the chameleon was recently shown to produce - and control - its colour using the shape of photonic crystals.

Prof Yablonovitch will receive the Isaac Newton Medal at an awards dinner on 5 November, where the Institute of Physics will also present a range of other awards.

Prof Chris Lintott, an astrophysicist at the University of Oxford and BBC Sky at Night presenter, will be awarded the Kelvin Medal for his education and outreach work.

Prof Lintott runs the successful citizen science platform Zooniverse.

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