It is relatively easy to measure small movements of large objects, but is much more difficult when the moving parts are on the scale of nanometers, or billionths of a meter. The ability to accurately measure tiny displacements of microscopic bodies has applications in sensing trace amounts of hazardous biological or chemical agents, perfecting the movement of miniature robots, accurately deploying airbags, and detecting extremely weak sound waves traveling through thin films.
Scientists developed a device that measures the motion of particles traversing distances almost unimaginably small — shorter than the diameter of a hydrogen atom, or less than one-millionth the width of a human hair. The handheld device can sense the atomic-scale motion of its tiny parts with unprecedented precision.
The researchers measured subatomic-scale motion in a gold nanoparticle by engineering a small air gap, about 15 nanometers wide, between the gold nanoparticle and a gold sheet. This gap is so small that laser light cannot penetrate it; however, the light did energize surface plasmons — the collective, wave-like motion of groups of electrons confined to travel along the boundary between the gold surface and the air.
The researchers exploited the light's wavelength — the distance between successive peaks of the light wave. With the right choice of wavelength — or equiva-lently, its frequency — the laser light causes plasmons of a particular frequency to oscillate back and forth along the gap, like the reverberations of a plucked guitar string. Meanwhile, as the nano-particle moves, it changes the width of the gap and, like tuning a guitar string, changes the frequency at which the plas-mons resonate.
The interaction between the laser light and the plasmons is critical for sensing tiny displacements from nanoscale particles. Light cannot easily detect the location or motion of an object smaller than the wavelength of the laser, but converting the light to plasmons overcomes this limitation. Because the plasmons are confined to the tiny gap, they are more sensitive than light is for sensing the motion of small objects like the gold nanoparticle. The amount of laser light reflected back from the plasmon device reveals the width of the gap and the motion of the nanopar-ticle; for example, the gap changes due to the motion of the nanoparticle in such a way that the natural frequency, or resonance, of the plasmons more closely matches the frequency of the laser light. In that case, the plasmons are able to absorb more energy from the laser light, and less light is reflected (Figure 1).
To use the motion-sensing technique in a practical device, the gold nanoparticle was embedded in a microscopic-scale mechanical structure — a vibrating cantilever — a few micrometers long, made of silicon nitride. Even when they're not set in motion, such devices never sit perfectly still, but vibrate at high frequency, jostled by the random motion of their molecules at room temperature. Even though the amplitude of the vibration was tiny — moving subatomic distances — it was easy to detect with the new plasmonic technique.
Similar, though typically larger, mechanical structures are commonly used for both scientific measurements and practical sensors; for example, detecting motion and orientation in cars and smartphones. The team's fabrication approach allows production of about 25,000 of the devices on a computer chip, with each device tailored to detect motion according to the needs of the manufacturer (Figure 2).