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Nanophotonics: no shortage of big ideas

29 Sep 2014

Invisibility cloaking? Color-changing wallpaper? Medical sensors? It's all possible with nanophotonics, discovers Andrew Williams.

Controlling the way that light interacts with matter on the scale of the wavelength of that light is the realm of nanophotonics, and the emerging field promises to spawn any number of commercial applications.

While tales of invisibility cloaks attract plenty of media attention, more prosaic uses in consumer electronics and medicine appear closer to realization.

And at the forefront of their development is a team of researchers within the renowned Cavendish Laboratory at the UK’s University of Cambridge.

According to the head of its dedicated Nanophotonics Centre, Jeremy Baumberg, there is huge application potential for the nascent technology.

One of the key themes of his group’s current work is in the area of structural color - where the color of a material can be controlled by changing its internal structure, rather than with pigments or dyes. As a result, one outcome could be textiles that clothes whose colors do not fade with time.

Oranges in a crate
The nanophotonics team has already used this approach to create an entirely new material called polymer opal: a pliable, rubber-like substance whose physical structure can be “tuned” to reflect specific colors. As Baumberg explains, the material is comprised of thousands of tiny polymer spheres, each about 200 nm across, that are stacked in an ordered fashion. “Like oranges in a crate,” is how he describes it.

By altering the spacing of the spheres, something that can be done by simply stretching the opal, its color can be tailored to any part of the visible spectrum. Baumberg shows a sample being stretched from blue to green and back again - but others may stretch from red to green and then blue, before “reversibly relaxing” back to the original color. These changes can even be localized so that a pattern – such as a logo – is revealed when the opal is stretched.

So where might such a phenomenon find commercial application? Baumberg is considering a number of possibilities. They range from the deadly serious, like banknote security, to the playful: one idea is to develop wallpaper that can change color on demand.

“Nokia has shown interest in using the material to coat mobile phones, because when you bend [polymer opal] around the corner of a device it gets stretched, resulting in colour changes at the corner, so it highlights all of your design features," he adds.

And it is already possible to make polymer opal in an impressive bulk: “We can produce it in a kilometer length,” Baumberg reveals. “Instead of very expensively digging it out of the ground in Australia we can do it cheaply. The cost of the components is pretty negligible. We're still trying to get to the final commercialization - but we're [getting] closer.”

Medical diagnostics
The team is also working on ways to improve medical diagnostics. With Oren Scherman, director of the Melville Laboratory for Polymer Synthesis in Cambridge’s chemistry department, they have created a novel chemical sensor based on a barrel-shaped molecular container known as cucurbituril (CB).

CB acts like a minuscule test tube. It allows individual molecules to enter its barrel shape, effectively isolating them from a chemical mix. And now the team has worked out how to identify the contents of each barrel, using a combination of light and gold nanoparticles.

“We want to use the fact that we're confining light to small volumes to actually see things in those volumes, and the things we want to see are molecules,” Baumberg says. “What we can do is trap the light - so if I take two gold particles and bring them close together it turns out that the way the light shakes the electrons in them traps the light in the gap in the middle.”

He also points out that whereas most sensing equipment requires the kind of precision that can only really be achieved in a laboratory, this new technology has the potential to become a low-cost, reliable and rapid sensor for mass markets. The amount of gold required for the test is tiny, and the gold particles self-assemble with CB at room temperature.

“If you make it very efficiently, essentially it's not much different from a DVD player,” says Baumberg. “It has very high-quality optics, it has a good laser, it has filters in it and it has detectors in it.

“Those are very cheap these days and everyone has them around. It's not the case that everyone will have [these medical diagnostic devices] in the same way, but you can conceive of them being very cheap.”

Funding challenges
It is, of course, one thing to talk about mass production and quite another to actually achieve it. Funding for both the fundamental science and industrial transfer are critical.

In terms of the former, Baumberg says that he has had no problems securing research funding within the UK – as indicated by the group’s recent receipt of a £6 million grant from the Engineering and Physical Sciences Research Council (EPSRC) to continue the fundamental work.

However, securing the kind of development funding needed for subsequent scale-up and commercialization presents a much tougher challenge.

“The funding councils in the UK say, ‘well, it's not really research anymore, so it's not really for us to fund’. But the companies say, ‘well, how do you de-risk this technology? It looks really good, but can you scale it up to the pre-pilot stage?'

“They want us to reduce the risk first but we need money to do that. So getting across that ‘valley of death’ is always tricky,” he says.

“If I want to do this next stage of demonstrating that something's going to be really useful, that funding is always difficult to find - because it's a lot of money and a big risk and it's always unclear who is going to take that risk.”

Seeding wild ideas
At the European level, where Baumberg is involved in a number of application bids for funding under the European Commission’s Horizon 2020 innovation program, it is too early to say how well the new funding schemes are functioning, or whether claims of reduced bureaucracy are really true.

“It looks similar to the previous ones; I'm not holding my breath for less administration or so on,” says Baumberg. “Horizon 2020 is a gargantuan amount of money and it's the only thing in the EU budget that's increased, because everybody believes in the science and technology.”

However, one part of the European funding scene that he very much approves of is the European Research Council (ERC) element, describing it as “spectacular”. That is largely because it funds individual scientists on the basis of excellence, without imposing requirements in terms of industrial connections, impact or country representation.

"Britain has done incredibly well out of that,” Baumberg says. “It's one thing in Europe that's got the US scared, because they see that Europe is now funding people to do wild stuff, future stuff that will seed new industries.

“It's not explicitly industry-driven, but at the same time that's where new ideas come from. Unfettering people is really good. So that is a spectacular success in my view.”

About the Author

Andrew Williams is a freelance writer based in Cardiff, UK.

Scitec Instruments LtdBristol Instruments, Inc.Synopsys, Optical Solutions GroupSensofar Metrologyart Photonics GmbHEdmund OpticsDiffraction International
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