Nanocups Brim with Potential
Superlenses. Ultra-efficient solar cells. Cloaking devices. Once the stuff of science fiction, these may soon be possible, thanks to a metamaterial that collects light and emits it in a single direction.
Created by Naomi Halas, an award-winning pioneer in nanophotonics, and graduate student Nikolay Mirin, the metamaterial uses tiny, cup-shaped particles called nanocups.
Mirin had been trying to make a thin gold film with nano-sized holes when it occurred to him that the knocked-out bits were worth investigating. Previous work on isolated gold nanocups had given researchers a sense of their properties, but Mirin found a way to lock ensembles of nanocups into sheets that orient the nanocups in a unified direction. The resulting metamaterial — a substance that gets its properties from its structure and not its composition — excels in capturing light from any direction and focusing all of it in one 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.
This means that the observer does not see the material, but what is behind it. "The material should not only retransmit the color and brightness of what is behind it," Mirin said, "but also bend the light around, preserving the original phase information of the signal."
Halas — Rice’s Stanley C. Moore Professor in Electrical and Computer Engineering and professor of chemistry, of biomedical engineering and of physics and astronomy — said the embedded nanocups are the first true three-dimensional nanoantennas, and their light-bending properties are made possible by electronic surface excitations known as plasmons.
Electrons inside plasmonic nanoparticles resonate with input from an outside electromagnetic source in the same way that a pool struck by a drop of water ripples. The particles act the same way radio antennas do, with the ability to absorb and emit electromagnetic waves that, in this case, include visible wavelengths.
"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 haven’t thought of yet."
Because nanocup ensembles can focus light in a specific direction, they make good candidates for thermal solar power. "Solar-generated power of all kinds would benefit," said Halas. "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."
In addition, a solar panel that focuses light into a beam that’s always on target without having to track the sun would save a lot of money on machinery and the energy needed to power the machinery.
Using nanocup metamaterial to transmit optical signals between computer chips has potential, and it also might be used in enhanced spectroscopy and to create superlenses."
"We’d like to implement the material into some sort of useful device," said Halas of her team’s next steps. "We also would 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 haven’t thought of yet."