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December 21, 2010

Carbon nanotubes could be ideal optical antennae

Cornell researchers have discovered that carbon nanotubes can transmit and receive light at the nanoscale. Carbon nanotubes, cylindrical rolled-up sheets of carbon atoms, might one day make ideal optical scattering wires -- tiny, mostly invisible antennae with the ability to control, absorb and emit certain colors of light at the nanoscale.

The researchers used the Rayleigh scattering of light -- the same phenomenon that creates the blue sky -- from carbon nanotubes grown in the lab. They found that while the propagation of light scattering is mostly classical and macroscopic, the color and intensity of the scattered radiation is determined by intrinsic quantum properties. In other words, the nanotubes' simple carbon-carbon bonded molecular structure determined how they scattered light, independent of their shape, which differs from the properties of today's metallic nanoscale optical structures.

Nature Nanotechnology - Single-walled carbon nanotubes as excitonic optical wires


Although metallic nanostructures are useful for nanoscale optics, all of their key optical properties are determined by their geometry. This makes it difficult to adjust these properties independently, and can restrict applications. Here we use the absolute intensity of Rayleigh scattering to show that single-walled carbon nanotubes can form ideal optical wires. The spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics, quantum-dot-like optical resonances and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity of ~8 e2/h, which we derive using an exciton model, suggesting universal behaviour similar to that observed in nanotube conductance. We further demonstrate a radiative coupling between two distant nanotubes, with potential applications in metamaterials and optical antennas.

"Even if you chop it down to a small scale, nothing will change, because the scattering is fundamentally molecular," Park explained.

They found that the nanotubes' light transmission behaved as a scaled-down version of radio-frequency antennae found in walkie-talkies, except that they interact with light instead of radio waves. The principles that govern the interactions between light and the carbon nanotube are the same as between the radio antenna and the radio signal, they found.

To perform their experiments, the researchers used a methodology developed in their lab that completely eliminates the problematic background signal, by coating the surface of a substrate with a refractive index-matching medium to make the substrate "disappear" optically, not physically. This technique, which allowed them to see the different light spectra produced by the nanotubes, is detailed in another study published in Nano Letters.

18 pages of supplemental information at Nature Nanotechnology

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