Cornell plucks its latest microscopic stringed instrument to study vibrating materials at record high frequencies

nanofabricated device
Dustin Carr/Cornell Nanofabrication Facility
Electron microscope image of a nanofabricated device to study microscopic resonances. The "Strings" are rods 50 nanometers thick, ranging in length from 1000 to 8000 nanometers. This view is from the top, although the angles make it appear to be a perspective view.

ATLANTA -- From the folks who brought you the world's smallest guitar, now meet the nanoharp.

But while the microsopic guitar made by Cornell University researchers two years ago was just a whimsical demonstration of new nanofabrication technology, this new "stringed instrument" plays the real music of science, serving as a platform to study the physics of very small vibrating systems.

"This is another use for our new ability to make microscopic mechanical systems," said Harold Craighead, Cornell professor of applied and engineering physics, who supervised the research. "By making things very small you bring out properties that aren't evident in larger materials. We can combine this information with other types of measurements made by researchers in materials science to help understand how materials behave. Right now we're working with silicon, but the methods can eventually be applied to other materials."

The new device, carved out of a single crystal of silicon with advanced versions of the methods used to build tiny electronic circuits, 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 nanometers (nm) in diameter, ranging from about 1000 to 8000 nm long. A nanometer is one billionth of a meter, making each string about 150 atoms thick. The entire device is about the size of a red blood cell.

Dustin Carr, a research support specialist at the Cornell Nanofabrication Facility and a graduate researcher in the Cornell physics department, described the tiny device in a talk, "Nano-Mechanical Resonant Systems in Single-Crystal Silicon," today (March 23) at the 1999 annual meeting of the American Physical Society in the Georgia World Congress Center, Atlanta.

Carr and Craighead work with postdoctoral associate Stephane Evoy, graduate student Lidija Sekaric and Jeevak Parpia, Cornell professor of physics. They built the device using electron-beam lithography and what's called "released silicon" technology, which refers to nanostructures that have been undercut to be freely suspended in space.

nanoharp from an angle
Dustin Carr/Cornell Nanofabrication Facility
A view of the nanoharp from an angle, showing that the strings are suspended above the silicon substrate.

The researchers are studying resonance effects in these microscopic systems. In the macroscopic world, plucking a string tuned to middle C, for example, will cause a nearby string tuned an octave higher to vibrate, responding to energy transmitted through the air. Nanodevices operate in a vacuum, but their vibrations can be transmitted through the silicon base.

The researchers make the silicon rods 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 sensitive interferometer.

"We've measured the highest frequency man-made vibrating strings, and the smallest vibrating strings, smaller by a factor of four than anyone else has measured," Carr said. "There is lots of interesting behavior that we're still working on trying to understand."

The researchers have measured vibrations at frequencies from 15 Mhz up to 380 Mhz, Carr said." The system can detect a motion of as little as one nanometer, or possibly less."

As with a full-size harp, the resonant frequency at which one of these tiny strings vibrates depends on the length and the mass. However, Carr said, these microscopic strings are not under tension like those in a musical instrument, and the resonant frequency of the nanoharp's strings follows a different rule, varying as the square of the length, like a metal bar struck by a hammer. "It's really more like a xylophone than a harp," he said.

Eventually, Parpia said, the group plans to examine the behavior of these oscillators at very low temperatures. "When you drive a mechanical oscillator, the oscillation increases in amplitude with the amplitude of the drive, but at very low temperatures the relationship becomes non-linear. The intent is to take these very small oscillators and see if they behave differently than the larger devices we've worked with in the past."

The Cornell group is noticing unusual effects in their measuring system, Carr said. "The light intereacts with the system in a very special way. The wavelength of the light we use to measure the vibrations is just a little bit larger than the size of the device. There may be some interesting optical effects," Carr said.

Parpia noted that the measuring system could be turned around, using small oscillators to modulate light. "It could be a very inexpensive way of modulating light with a very narrow frequency range," he said. "Or it could be used as a very good filter to select a particular band of frequencies."

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