Nanoscopic 'coaxial cable' transmits light

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PostPosted: Wed May 16, 2007 11:53 am    Post subject: Nanoscopic 'coaxial cable' transmits light Reply with quote

Nanoscopic 'coaxial cable' transmits light
by Jeff Hecht

A way to make nanoscopic metal cables transmit light could lead to innovations in solar cells, artificial retinas and quantum computing components, say researchers.

The trick is to shrink a coaxial cable by a factor of 10,000 so the diameter is smaller than the wavelength of visible light.

Ordinary coaxial cable, or coax, consists of a central wire, surrounded by a layer of a nonconductive "dielectric" material typically a plastic all wrapped in a metal sheath. The structure guides radio waves along the surface of the central wire, so the waves pass through the dielectric material.

Cables a few millimetres in diameter carry video signals between DVD players, video recorders, cable boxes, satellite antennas, and television sets. The wavelengths involved are several times the cable diameter.

"Our coax works just like the one in your house, except now for visible light," says Jakub Rybczynski from Boston College, US, who led the research. The big difference is the cable is nanoscopic, measuring only 300 nanometres in diameter. It is shorter than the shortest visible wavelength and also invisible to the human eye.

Solar efficiency -

A carbon nanotube replaces the inner wire, a film of aluminium oxide replaces the plastic layer and a coating of chromium or aluminium replaces the outer sheath.

Normally light waves cannot penetrate structures smaller than their length. But a length of nanotube protrudes from the end of the cable and acts as an optical antenna to guide the light into the structure . Light waves can then travel through the aluminium oxide layer, guided in the same way that a normal coaxial cable guides radio waves.

Limited wavelengths -

The nanocables are not candidates to replace optical fibres. So far the longest ones stretch only 20 micrometers, and longer cables will only carry light a maximum of about 50 micrometers roughly 100 wavelengths.

However, being able to transmit light on the nanoscale scale could have a range of potential applications, says Michael Naughton, another member of the Boston College team. As the nano-guides are smaller than a wavelength, their behaviour is governed by quantum mechanics. "It's possible we could use this for quantum computers," Naughton said.

However, Naughton's first target is to increase the efficiency of energy conversion in solar cells by tightly packing together arrays of nano-coax filled with photovoltaic material rather than aluminium oxide.

Another possibility is to assemble arrays with optical antennas on one end and electrical output at the other to serve as artificial retinas for people with impaired vision, he says.

Source: New Scientist Tech

Story first posted: 9th January 2007.
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