Carbon nanotubes as optical antennae

Researchers at Cornell University have found that carbon nanotubes’ light transmission behaves as a scaled-down version of radio-frequency antennae found in walkie-talkies, except that the interaction is with light instead of radio waves. The principles that govern the interactions between light and the carbon nanotubes are the same as those between the radio antenna and the radio signal.

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 – according to the research, which was published in Nature Nanotechnology, Vol. 6, p. 51-56 (2011).

The researchers used Rayleigh scattering of light – the same phenomenon that makes the sky look blue – from carbon nanotubes grown in the lab. They found that, although the propagation of light scattering is mostly classical and macroscoping, the color and intensity of the scattered radiation is determined by intrinsic quantum properties; i.e., the nanotubes’ simple carbon-carbon bonded molecular structure determines how they scatter light, independent of their shape, which differs from the properties of today’s metallic nanoscale optical structures.

A methodology developed in their lab eliminated problematic background signals by coating the surface of a substrate with a refractive index-matching medium to make the substrate “disappear” optically, not physically. The technique, which allowed them to see the various light spectra produced by the nanotubes, is detailed in Nano Letters, Vol. 11, No. 1, pp. 1-7 (2011).

The technique also allows for quick, easy characterization of a large number of nanotubes, which could lead to ways of growing more uniform batches in the future.