Lasers may power nano-motors

By MIKE MARTIN, UPI Science News  |  April 23, 2002 at 10:22 AM
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CAMBRIDGE, Mass., April 23 (UPI) -- Laser light may be the fuel of the future for nanotech robots and motorized tools so small they can manipulate individual cells and molecules, researchers at Harvard University and the Weizmann Science Institute report.

"Ultrafast frequencies -- tens of gigahertz -- are typical rotational frequencies in molecules and could be used to spin other molecules," paper co-author and Harvard physicist Hossein Sadeghpour told United Press International from Cambridge.

"We are able to show that light can spin nanotubes with ultra-high frequencies," Weizmann Institute chemical physicist Petr Kral said in a recent paper from Rehovot, Israel.

Kral and Sadeghpour used lasers to transfer the angular or sideways momentum of infrared light-bearing photons to carbon nanotubes. The tubes, thousands of times thinner than a human hair and with walls only one atom thick, then rotated like whirring turbines.

Scientists at the Department of Energy's Oak Ridge National Laboratory designed the first laser-driven nanomotor -- a cylindrical carbon tube surrounded by a carbon sleeve. In theory, applying an oscillating laser field would make the tube rotate enough to power a tiny motor, chemists Donald Noid, Bobby Sumpter and computer scientist Robert Tuzun explained in a groundbreaking 1995 paper.

"When we started to consider nanotechnology, we thought that a method would be required to use some form of energy that could create mechanical motion," Oak Ridge National Laboratory senior scientist Donald Noid told UPI. "The most simple approach we could envision was that of rotation of nanotubes with lasers."

The Oak Ridge team only simulated a laser-powered nanomotor, however. The Harvard-Weizmann team said they have the blueprint for a working prototype.

"The approach of this newest paper by Kral and Sadeghpour strikes me as immensely clever and innovative," said Robert Tuzun, a professor in the Department of Computational Science at SUNY in Brockport, New York. "They have captured the essential physics of the problem in a simple model -- an important step in the experimental realization of the rotational excitation of nanotubes."

Providing power at the nanoscale, where devices can be consist of only a few molecules, poses unusual problems.

"It is hard to deliver energy, momentum or angular momentum to nanosystems, since these systems cannot be well attached to external electrical or mechanical contacts," Kral told UPI.

"It is much 'cleaner' to use optical methods to manipulate, levitate and impart angular momentum to nanotubes," Sadeghpour said.

Kral and Sadeghpour envision an "optical method" that transfers rotational energy from laser-excited photons -- individual packets of light energy -- to "phonons," individual packets of vibration energy nestled in the carbon nanotubes.

"Some vibrations not only carry energy, but also angular momentum -- the ability to rotate things," Kral told UPI.

The laser-excited, vibrating phonons transfer their angular momentum to the bodies of the nanotubes, which respond by spinning like the propellers of the miniaturized submarine "Proteus" envisioned by Isaac Asimov in his classic novel of nanotechnology, "Fantastic Voyage."

Carbon nanotubes can be 100 times stronger than steel, Kral explained, making them perfectly suited for demanding tasks that require extreme precision -- even some future device resembling Asimov's molecule-sized submarine, which searched for a brain tumor by traveling through blood vessels.

"One could also use this kind of motor -- a spinning nanotube -- to build gyroscopes with unique properties," Kral said. "Such systems are very demanding when we want to stabilize objects in space, like the Hubble telescope, where worn gyroscopes were recently replaced."

Tuzun told UPI that "much of the basic physics required for mechanical devices at the nanometer size scale is still not well understood. Papers such as the one by Kral and Sadeghpour are important steps in this understanding."

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