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Nanoscale antenna concentrates light

When radio waves interact with a conventional antenna, the electric field from the radio waves drives the electrons in the antenna back and forth, producing an oscillating voltage on the antenna. That voltage is what is amplified in your stereo and converted into sound by your speakers. Researchers led by Rice graduate student Dan Ward and Rice professor Douglas Natelson have used metal nanostructures in an analogous way to act as antennas for light.

It has been known for years that light can excite "ripples" (called plasmons) in the electronic fluid of nanoscale metal structures, like ripples on the surface of water. As in a conventional antenna, the electrons sloshing around lead to voltages and correspondingly big electric fields near the surface of the metal. These enhanced fields are very useful for spectroscopy, nonlinear optics, and other "photonic" technologies. However, historically it has been very difficult to measure those voltages and fields experimentally, and there have been arguments about the precise size of these effects.

In nanoscale metal gaps like that shown below, the plasmons in the metal lead to an oscillating voltage across the gap, and intense electromagnetic fields in the gap. These gaps are so small (sub-nanometer, at their closest point) that the optically driven voltage can cause electrons to tunnel quantum mechanically from one side of the gap to the other. By measuring that tunneling at low frequencies using a known voltage, and then comparing with the light-driven currents, Ward and Natelson were able to determine the optical voltages and fields. Impressively, the researchers found that the intensity in the gap region can be a million times greater than that of the incident light!

D. R. Ward, F. Heuser, F. Pauly, J. C. Cuevas, and D. NatelsonOptical rectification and field enhancement in a plasmonic nanogap. Nature Nanotechnology, in press (Reprint)

Press Release:
(Rice News & Media Relations press release Nano antenna concentrates light)

(a) False-color scanning electron micrograph showing a nanoscale gap between two gold electrodes. (b) A comparison of electrically driven tunneling (black) and optically driven tunneling (red), allowing the researchers to infer the optically produced voltage and local electric field. (c) Artist's conception of the nanogap tip region, where the displaced electrons (blue) lead to a dramatically enhanced optical intensity (red) at the atomic scale.