Nanoscale vibrating strings called nanoresonators are of tremendous interest to researchers because of their potential as signal generators in electronic circuits or as sensors in a variety of instruments.

Now Cornell researchers have made nanoresonators that can be more sharply tuned than ever. In technical terms, the devices at room temperature perform several times higher than any previously achieved.

The work is described in the Jan. 7 issue of the journal Applied Physics Letters.

A vibrating device has a resonant frequency at which it will naturally vibrate when activated. It's never just a single frequency, but a small range around a center point. Quality factor, or Q, refers to how narrow that range is. A radio with a high-Q tuner would be very selective, rejecting signals from stations very near the frequency to which it is tuned.

In mechanical nanoresonators, high Q makes it easier to detect small changes in mass, which can be used to detect the presence of bacteria, viruses or other biological molecules, and to detect small changes in the environment around the device. Q is measured by watching the "ringdown time" of a resonator after it has been set to vibrating. The narrower its range of vibration, the less energy it will lose with each swing, and the longer it will continue to vibrate. Adding just a few molecules of air or other gas to the space around a high-Q device damps its vibration and noticeably shortens the ringdown time, making the device useful as a pressure sensor in very rarified gases.

High-Q resonators have been demonstrated at millimeter scales, but Q declines as devices get smaller. The Cornell research team found they could increase the quality factor by stressing the material as it was formed. In previous work they used stressed silicon nitride to make strings 200 by 102 nanometers wide by 60 microns long (a nanometer is a billionth of a meter; a micron is a millionth of a meter) with a Q of 207,000. In the process they found that Q increased with length (and increased with lower frequency). In the latest work, a string 275 microns long exceeded the milestone "million" value for the first time, with a Q of 1.3 million.

To test the usefulness of the new devices as pressure sensors, the researchers operated a string resonator at pressures ranging from high vacuum to room air pressure, and found that the quality factor decreased from 1.1 million to about 1,000 as pressure increased from vacuum to 1 Torr (A Torr is 1 millimeter of mercury; 760 Torr = 1 atmosphere). Previously demonstrated devices, they said in their report, have been limited to sensing pressure down to about 0.1 Torr, but the high-Q devices, they said, would extend the sensing range to below 0.001 Torr, while remaining effective at high pressures.

The research was conducted by Cornell graduate student Scott Verbridge; Harold Craighead, the C.W. Lake Jr. Professor of Engineering; and Jeevak Parpia, Cornell professor of physics. It was supported by the Defense Advanced Research Projects Agency, and the devices were made at the Cornell NanoScale Facility, supported by the National Science Foundation.