Nanoharp Hits High Note

By BBC News Online Science Editor Dr David Whitehouse

April 4, 1999

First they brought you the world's smallest guitar, now they
bring you world's smallest musical instrument - the

The microscopic guitar made by Cornell University
researchers two years ago was just a whimsical demonstration
of new nanofabrication technology.

But the nanoharp is a true stringed instrument that plays
real music, if only we could hear it. It plays a tune too
high for even a dog to hear.

The nanoharp's strings vibrate at frequencies as high as 380
MHz and are probably the fastest human-made moving object.
The human ear cannot hear frequencies above 22 kHz.

"This is another use for our new ability to make microscopic
mechanical systems," said Harold Craighead, Cornell's
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."

The new device is carved out of a single crystal of silicon.
The strings are actually silicon rods 50 nanometres (nm) in
diameter, ranging from about 1,000 to 8,000nm long. A
nanometre 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.

Cornell University scientists hope to use it to study the
properties of very small vibrating systems - ones that some
day could be used to make extremely sensitive chemical
sensors, for example.

Vibrating Strings

The researchers are studying resonance effects in these
microscopic systems.

They make the silicon rods vibrate by applying a radio
frequency voltage signal through the silicon base.

Then they measure the resulting vibrations by bouncing laser
light off the strings and observing the reflected light with
a sensitive interferometer.

The researchers have measured the highest frequency man-made
vibrating strings, and the smallest vibrating strings,
smaller by a factor of four than anyone else has measured.
Many aspects of the nanoharp's behaviour have yet to be

As with a full-sized harp, the resonant frequency at which
one of these tiny strings vibrates depends on the length and
the mass.

However, scientists say that the 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," they say.

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