Sound From Light

ENG Researchers Lead the Way to Nanoscale Detection Devices

By Michael Seele

Two College of Engineering faculty researchers are combining nanomechanics with photonics to produce tiny devices that may one day detect minute traces of cancer, or the tiniest traces of biological or chemical hazards in the environment. AME assistant professors Kamil Ekinci and Todd Murray are studying the vibrations of nanoscale wires that could be used to detect the presence of particles measured in tens of atoms.

Other researchers have plucked such wires and measured the vibrations before, but mostly in a cold vacuum, where the vibrations are exaggerated, and using electrical stimulation and measurement techniques. In their paper, “Photothermal Operation of High Frequency Nanoelectromechanical Systems,” published in Applied Physics Letters, Murray, Ekinci and graduate student Ashwin Sampathkumar demonstrate how to stimulate and measure such wires at room temperature outside of a vacuum using only light. Their approach dramatically decreases the size of the apparatus required and more closely mimics real-world conditions.

The research is identifying the resonance frequencies of the wires, which, like harp or guitar strings, resonate at frequencies corresponding to their lengths. Establishing this baseline data is crucial to the development of the technology since the presence of even a single molecule on a wire will alter its resonance frequency and indicate the presence of an unwanted substance.

“These wires are very sensitive to added mass,” said Murray. “They can detect trace amounts.”

“Putting mass on them changes resonance frequency,” Ekinci added. “Using this method, it would be possible to detect how much mass is present.”

The researchers carved the wires out of wafers of chromium-coated silicon 250 nanometers thick. The wires are, in effect, strung between two wafers at different lengths, similar to a harp, only about a million times smaller. The nanowires range in length from 2.5 microns to 10 microns. Using a new light microscope built specifically for this experiment, the researchers focused a laser to produce the smallest spot of light physically possible — less than one micron, or one-millionth of a meter, across — and used it to “pluck” each wire at various points along its length. The laser produced vibration in the wires not by physical contact like a guitar pick, but by heating specific spots. A second tightly-focused laser was used to measure resultant vibrations.

Ekinci and Murray recently won a National Science Foundation grant to fund the next step of the project, which involves multiplying these devices into arrays that would work more efficiently.

“Obviously, the probability of a single molecule finding a single wire is pretty small,” Murray explained. “Adding hundreds or thousands of these wires increases the probability that a molecule is going to land on one of them.”

Ekinci is also exploring the possibility of immersing these arrays in liquid and measuring how the medium affects vibration. This would be critical to developing medical applications for the technology, he said.

These nanoscale sensors register changes in mass when light “plucks” each wire at various points along its length.

Kamil Ekinci & Todd Murray with their device
(Click to view larger image)