Researchers led by condensed matter physicist Doug Natelson and graduate student Dan Ward from Rice University in the US have developed an optical antenna from two gold tips separated by a nanoscale gap - about a hundred-thousandth the width of a human hair - that gathers light from a laser. The tips 'grab the light and concentrate it down into a tiny space,' explained Professor Natelson, leading to a 1,000-fold increase in light intensity in the gap. He said that he expected the discovery to be useful in developing tools for optics and for chemical and biological sensing, even at the single-molecule scale, with implications for industrial safety, defence and homeland security.
'You can ignore the fact that your car antenna is built out of atoms; it just works,' said Professor Natelson. '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.' He pointed out that his team discovered that the key to measuring light amplification was measuring the electrical current flowing between the gold tips.
'Putting the nanotips so close together allows charge to flow via quantum tunnelling as the electrons are pushed from one side to the other,' according to the researchers. They could then get electrons moving by pushing them at low frequencies with a voltage, in a highly controllable, measurable way, and get them flowing by shining the laser, which pushes the charge at the very high frequency of the light.
'Being able to compare the two processes set a standard by which the light amplification could be determined,' Professor Natelson said. He noted that the amplification is a 'plasmonic effect' - plasmons, which may be excited by light, are oscillating electrons in metallic structures that act like ripples in a pool.
'You've got a metal structure, you shine light on it, the light makes the electrons in this metal structure slosh around,' he explained. '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.'
He explained that 'at the surfaces of the metal, these fields can be very big - much bigger than those from the original radiation'. But he said that it was hard to measure just how big they were.
'We didn't know how much the two sides were sloshing up and down - and that's exactly the thing we care about,' he said, adding that by simultaneously measuring the low-frequency electrically driven and the high-frequency optically driven currents between the tips, 'we can figure out the voltage zinging back and forth at the really high frequencies that are characteristic of light'.
The scientist commented that the team studied these enhanced fields because a lot can be done with sensors and non-liner optics. 'Anything that gives you a handle on what's happening at these tiny scales is very useful,' he said.
Contributions to the study were also made by researchers from the Karlsruhe Institute of Technology in Germany and the Autonomous University of Madrid in Spain.
Sources and contacts:
Cordis news release
Nature Nanotechnology: http://www.nature.com/nnano/index.html
Rice University: http://www.rice.edu/
Citation:: Ward, D. R., et al. (2010) Optical rectification and field enhancement in a plasmonic nanogap. Nature Nanotechnology, published online 19 September. DOI: 10.1038/nnano.2010.176.