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February 2000 (Volume 9, Number 2)
Electron microscope image of a nanofabricated device to study microscopic resonances. The "Strings" are rods 50 nanometers thick. Photo courtesy: Dustin Carr, Cornell Nanofabrication Facility, Cornell Univ.
The same group of researchers fabricated a microscopic guitar two years ago as a whimsical demonstration of their fledgling nanofabrication technology. (The original nanoguitar is included on the APS Timeline of Physics wall chart.) Cornell professor Harold Craighead, who supervised the research, says the nanoharp is just another use for their newfound ability to make microscopic mechanical systems. "By making things very small, you bring out properties that aren't evident in larger materials," he says. "We can combine this information with other types of measurements made by researchers in materials science to help understand how materials behave." Although the current device is made of silicon, Craighead says that the same methods can eventually be applied to other materials as well.
The nanoharp is carved out of a single crystal of silicon using advanced versions of the same methods used to build tiny electronic circuits: electron beam lithography and "released silicon" technology, which refers to nanostructures that have been undercut to be freely suspended in space. It consists of two endpieces, one square and one triangular, with several "strings" of varying lengths stretching between them. The strings are actually silicon rods 50 nm (150 atoms) in diameter, ranging from 1000 to 8000 nm in length, and the entire device is about the size of a single red blood cell.
As with a full-size harp, the resonance frequency at which a string vibrates depends on the length and the mass. However, the microscopic strings are not under tension like those in a musical instrument, and hence the resonant frequency of the nanoharp's strings follow a different rule, varying as the square of its length, similar to a metal bar being struck by a hammer. "It's really more like a xylophone than a harp," says Dustin Carr, a research support specialist in Craighead's lab and graduate researcher in Cornell's physics department, who was one of the featured speakers on the topic at the APS meeting.
The nanoharp's purpose goes beyond mere scientific whimsy. Craighead and his fellow researchers are studying resonance effects in these microscopic systems. They cause the silicon rods to vibrate by applying a radio frequency voltage signal through the silicon base. They then measure the resulting vibrations by bouncing laser light off the strings and observing the reflected light with a highly sensitive interferometer. The effect is similar to the way in which plucking a string tuned to middle C, for example, causes a nearby string tuned an octave higher to vibrate in response to energy transmitted through the air. The team has measured vibrations at frequencies from 15 MHz up to 380 MHz, and the system can detect a motion as little as one nanometer. Eventually they hope to examine the behavior of these oscillators at very low temperatures.
For more information about Harold Craighead's nanofabrication research at Cornell University, see http://www.hgc.cornell.edu.
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