Small Chirps Could Provide Big Insights on Tiny Structures

Chirps, short bursts of (often annoying) high-pitched sounds, are generally a way of conveying information. Birds chirp to warn their feathered friends of impending danger. Male crickets chirp to announce their intentions to females. Smoke alarms chirp to keep you awake all night until you finally get up and change that low battery.

Now, new research reveals that chirps can convey important information about tiny objects. As reported this week in the American Physical Society’s journal Physical Review Letters, a group of scientists from the University of Nottingham in the United Kingdom and the National Academy of Sciences in the Ukraine demonstrate that bursts of chirped sound lasting less than a billionth of a second could help uncover the properties of things like single cells and nanoscale structures in microelectronics.

This image shows how the frequency, or pitch, of the transverse sound—the chirp—changes over time. The color map shows the strength of the signal: from black (the minimum) to blue – green– yellow–orange and red (the maximum). In the time from zero to 300 milion-millionths of a second (picoseconds or ps), the pitch increases to about 0.2 Terahertz (THz).
Image Credit: Poyser et. al, PRL.

At first it might seem strange to think that sound can tell us about the structure of an object, but consider ultrasound imaging. Just as x-ray images reveal information about a subject by contrasting how x-rays (a form of light) travel through different materials, ultrasound images reveal information about a subject by contrasting how sound waves travel through different materials. Although the physics of light/matter and sound/matter interactions are different, the technique is fundamentally the same—a signal traveling through a subject is changed by the materials it encounters. By studying these changes, you can learn about the subject.

What does this have to do with chirping? Before we get into that, let’s go back a few years and set the stage. In 2006, the Nottingham-Ukraine collaboration developed something called the saser. A saser is similar to a laser, except that instead of generating a coherent beam of light (the “l” in “laser”), a saser generates a coherent beam of sound waves (the “s” in “saser”). In this case, the team developed a beam of ultrasound waves in the range of 0.3 to 3 terahertz (THz). Sasers in this range have excellent potential for imaging nanometer-sized objects, but scientists have struggled to develop a tool that is sensitive enough to be reliable.

About three years ago the team started exploring whether this sensitivity could be reached by incorporating terahertz sound waves oriented differently than those in the original saser. Their work so-far had focused on using sound waves that were longitudinally polarized, a term that describes when the atoms vibrate in the same direction as the sound wave travels. However, you can get a lot of complementary information from probing matter with sound waves that are transversely polarized, where the atoms vibrate perpendicular to the direction that the sound wave travels.

This investigation required the team to generate transversely polarized waves using a crystal, which turned out to be no easy task. When they finally did get a signal, the signal contained a frustratingly large chirp—a brief burst of sound whose frequency increased over time. The original signal the team was putting into the crystal wasn’t just coming out the other side, it was reflecting off of the surface multiple times, spreading out in time, and generating the chirp in the process.

“Owing to the long distance the sound had travelled, the chirping effect was very prominent in the results, but we couldn’t get the theory to fit,” said University of Nottingham THz acoustics group leader Tony Kent. “This, almost accidental observation, motivated a more systematic study of the chirping effect,” he said.

The systematic study was based on an experiment. First, the team installed a transducer device that converts light into sound onto the surface of a slab of gallium arsenide (GaAs), a material often used in electronics. When excited by a short laser pulse, the transducer produced very short pulses of sound on one side of the 0.44mm slab. The sound traveled through the crystal, but because of the crystal’s structure it didn’t do so uniformly. Sound waves with three different orientations were produced, one nearly-longitudinally polarized and two nearly-transverse polarized, each traveling at different speeds.

Because of their different orientations and speeds, the very short initial pulse spread out in time and space as it traveled across the slab. The researchers took acoustic measurements at the far side of the slab by probing with blue laser pulses and mapped out the resulting chirps created by each type of wave, in addition to how they changed over time and with different variables.

From this information, the researchers show that it is possible to generate, control, and characterize these chirps. In other words, after detecting such chirps you can work backward to determine the properties of the material through which the initial pulse traveled. Furthermore, by limiting the data collection to a specific time window, you can remove some of the noise that usually accompanies terahertz acoustic measurements. Together these characteristics suggest that chirped pulses could form the basis for next generation acoustic measurement tools.

These chirps bring to light possibilities that might keep some enthusiastic researchers up at night, but at least they won’t have to head to the nearest 24-hour gas station to buy new smoke detector batteries.

Kendra Redmond

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