Tiny tubes tune in colors

For more than a century, electric devices have been transmitting and receiving radio signals via antennas that range from skyscraping radio towers to telescoping cell phone aerials. Now, scientists have shown that far smaller antennas can snatch visible light waves from the air.

COLOR CODED. A carpet of vertically oriented carbon nanotubes shimmers with rainbow reflections because different-length tubes within the carpet respond to different colors. Y. Wang, et al./Applied Physics Letters

To physicists, visible light differs from radio waves in that light’s wavelengths are only a millionth to a billionth as long as those of their radio cousins. Antennas must typically extend about one wavelength to do a good job of receiving an electromagnetic signal of that same wavelength. Only recently have technologists made structures tiny enough—on the scale of viruses—to have a chance at picking up light.

Building on those advances, Krzysztof Kempa of Boston College and his colleagues have created arrays of standing carbon nanotubes (SN: 9/18/04, p. 180: Available to subscribers at Nanotech Goes to New Lengths: Scientists create ultralong carbon nanotubes), each about a thousandth the thickness of a human hair and roughly the length of a wavelength of light.

In the Sept. 27 Applied Physics Letters, the team reports two signs that such tubes respond to light in the same way as a cell phone’s antenna responds to an incoming call. First, the impinging light’s oscillating electric field excites the tubes only when the oscillations are aligned with the tubes. Second, each color energizes only tubes close in length to that color’s wavelength.

Because “antennas are theoretically the best converters of radiated energy into electricity,” a new type of highly efficient solar cell may be in the offing, says Kempa. Other possibilities, he says, are future optical devices that will receive data in businesses and homes at much higher rates than are affordable today.

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