Nanotubes work like radio antennas to convert light into electricity.
Date: Thursday, September 23, 2004 @ 21:36:55 MST
Topic: Science

"Miniature aerials pick up light" by Mark Peplow

Radio aerials have been around for over a century, and routinely receive information carried by radio waves into our homes. Now, finally, scientists have built an aerial that can do the same for light waves. The tiny antennas could be used in solar cells, or 'optical computers' that would move data round as light beams.

Radio waves, like light waves, consist of an oscillating electric field. When radio waves hit a receiving aerial, which is generally made of metal wire, they move electrons back and forth inside it. This current can then be amplified and the signal converted into sound.

But the aerial needs to be a roughly similar size to the wavelength of the incoming wave. This is easy enough for radio waves, which can have wavelengths measured in metres, but light-wave cycles are just a few hundred nanometres long, about 10,000 times smaller than the head of a pin.

So physicist Yang Wang and his colleagues at Boston College, Massachusetts, have made an array of carbon nanotubes of just that length. The 50-nanometre-wide tubes make ideal miniature aerials because they conduct electricity well, so electrons can move freely up and down the tubes.

When the researchers shone light waves at the tubes, they detected a current, resulting from electrons bouncing up and down in the tubes at around 1015 times every second.

They have also created an array of nanotubes with a steady gradient of relatively short tubes at one end, through to long tubes at the other. This means that the whole array can detect visible light of any colour, says Zhifeng Ren, who worked on the project.

The team found that when the light waves were oriented so that their electric field was perpendicular to the nanotubes, the electrical response disappeared. This confirms that the light wave's electric field is responsible for the current, says Wang.

The work is tantalizing, says Mark Welland, a nanotechnology expert at the University of Cambridge, UK. He hopes the development could benefit optical computing. An array of carbon nanotubes could convert the light beam of data inside such a computer into an electrical signal, he says, providing an interface with conventional electronics.

The nanotubes might also enable a radically new design of solar cell, he says. Arrays of the long, thin wires could be spread over large areas to catch as much light as possible, convert it to electricity and deliver it along a circuit. A single material that can do all these things is "exactly what the ideal solar cell would consist of," he points out.

1. Wang Y., et al. Appl. Phys. Lett., in the press (2004).

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