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3/11/2009

Nanocups brimming with potential
Light-bending particles could lead to optics breakthroughs

BY MIKE WILLIAMS
Rice News staff

Nikolay Mirin had a 'Hey, wait a minute!' moment when he realized the nanocups he was throwing away could be useful. It led the Rice graduate student and his mentor, Professor Naomi Halas, to pursue research that could light the way toward high-powered optics, ultra-efficient solar cells and even cloaking devices.

	TOMMY LAVERGNE
	Graduate student Nikolay Mirin (left) and Professor Naomi Halas have invented the first 3-D nano-antenna, a substance with the ability to bend light.

Nanocups are just what they sound like: very tiny, cup-shaped particles. What makes them special is their ability to bend light. Halas and Mirin have found a way to make material incorporating nanocups that can bend light in a specific direction.

Halas and Mirin are the authors of a paper published last month in the journal Nano Letters that details how they isolated nanocups to create light-bending nanoparticles.

"The truth is a lot of exciting science actually does fall in your lap by accident," said Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering and professor of chemistry and biomedical engineering. “The big breakthrough here was being able to lift the nanocups off of a structure and preserve their orientation. Then we could look specifically at the properties of these oriented nanostructures.”

In earlier research, Mirin had been trying to make a thin gold film with nano-sized holes when it occurred to him the knocked-out bits were worth investigating. Previous work on gold nanocups gave researchers a sense of their properties, but until Mirin's revelation, nobody had found a way to lock ensembles of isolated nanocups while preserving their matching orientation.

COURTESY IMAGE
To make light-bending material, the Rice researchers spread polystyrene or latex colloidal particles on a glass slide, evaporate a layer of gold at various angles on top of the particles, deposit a layer of elastomer on top and then, after curing, lift the slab from the substrate with the oriented nanocups embedded.

His solution involved thin layers of gold deposited from various angles onto polystyrene or latex nanoparticles that had been distributed randomly on a glass substrate. The cups that formed around the particles -- and the dielectric particles themselves -- were locked into an elastomer and lifted off the substrate. "You end up with this transparent thing with structures all oriented the same way," he said.

In other words, he had a metamaterial, a substance that gets its properties from its structure and not its composition. Halas and Mirin found their new material particularly adept at capturing light from any direction and focusing it in a single direction.

Redirecting scattered light means none of it bounces off the metamaterial back into the eye of an observer. That essentially makes the material invisible. "Ideally, one should see exactly what is behind an object," said Mirin. A native of Russia, he came to Houston seven years ago after receiving his bachelor's degree from Lomonosov Moscow State University and anticipates earning his doctorate from Rice this year.

"The material should not only retransmit the color and brightness of what is behind, like squid or chameleons do, but also bend the light around, preserving the original phase information of the signal."

Halas said the embedded nanocups are the first true three-dimensional nano-antennas, and their light-bending properties are made possible by plasmons. Electrons inside plasmonic nanoparticles resonate with input from an outside electromagnetic source in the same way a drop of water will make ripples in a pool. The particles act the same way radio antennas do, with the ability to absorb and emit electromagnetic waves that, in this case, includes visible wavelengths.

Because nanocup ensembles can focus light in a specific direction no matter where the incident light is coming from, they make pretty good candidates for, say, thermal solar power. A solar panel that doesn't have to track the sun yet focuses light into a beam that's always on target would save a lot of money on machinery.

Solar-generated power of all kinds would benefit, Halas said. “In solar cells, about 80 percent of the light passes right through the device. And there's a huge amount of interest in making cells as thin as possible for many reasons.”

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Find out how the Centennial Campaign is creating solutions for tomorrow Halas said the thinner a cell gets, the more transparent it becomes. “So ways in which you can divert light into the active region of the device can be very useful. That's a direction that needs to be pursued,” she said.

Using nanocup metamaterial to transmit optical signals between computer chips has potential, she said, and enhanced spectroscopy and superlenses are also viable possibilities.

“We'd like to implement these into some sort of useful device,” said Halas of her team's next steps. “We would also like to make several variations. We're looking at the fundamental aspects of the geometry, how we can manipulate it, and how we can control it better.

“Probably the most interesting application is something we not only haven't thought of yet but might not be able to conceive for quite some time.”