Nanoharp hits high note

Each of the strings is only 150 atoms wide

By BBC News Online Science Editor Dr David Whitehouse

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

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 strings viewed from the side

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. 
 
 

The nanoguitar

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 explained.

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.