Doug Natelson and graduate student Dan Ward, from Rice University, have discovered how to make a light-harvesting antenna from two gold tips separated by a gap only a few nanometers wide. As light source they used a laser, whose rays, once trapped in the gold tips, get concentrated into a tiny space, increasing the light intensity in the gap by a thousand times.
Aided by colleagues from Germany and Spain, the two had issues with the size of the antennas: “You can ignore the fact that your car antenna is built out of atoms; it just works,” said Natelson, a Rice professor of physics and astronomy, and also electrical and computer engineering. “But when you have tiny pieces of metal very close to each other, you have to worry about all the details. The fields are going to be big, the situation’s going to be complicated and you’re really constrained. We’ve been able to use some physics that only come into play when things are very close together to help figure out what’s going on.”
They assessed the light’s amplification by measuring the electrical current flowing between the gold tips. Because they were so close to each other, the charged flowed in between them via quantum tunneling, which had the electrons get pushed from one tip to the other, excited by the laser.
The team concluded that the amplification is a plasmonic effect (due to surface plasmons – oscillating electrons in metallic structures). Natelson explains that if “you’ve got a metal structure, you shine light on it, the light makes the electrons in this metal structure slosh around. You can think of the electrons in the metal as an incompressible fluid, like water in a bathtub. And when you get them sloshing back and forth, you get electric fields.
“At the surfaces of the metal, these fields can be very big – much bigger than those from the original radiation,” he said. “What was hard to measure was just how big. We didn’t know how much the two sides were sloshing up and down – and that’s exactly the thing we care about.”
The golden tips, however, had been cooled down to 80 degrees Kelvin, and had been electrically insulated from their silicon bases.
Although their research is targeting high-sensitivity sensors and non-linear optics, this kind of light harvesting could prove itself useful for solar panels, for example. There have also been other researchers who used surface plasmons in the past to enhance the power of solar cells.
Their entire study has been published in the Nature Nanotechnology magazine.