Device detects subatomic-scale motion

Scientists at the National Institute of Standards and Technology (NIST) have developed a new measuring device that measures the motion of super-tiny particles traversing distances shorter than the diameter of a hydrogen atom.

Not only can the handheld device sense the atomic-scale motion of its tiny parts with unprecedented precision, but the researchers have devised a method to mass produce the highly sensitive measuring tool.

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 travelling through thin films.

In order to develop the device, the NIST researchers measured subatomic-scale motion in a gold nanoparticle. They did this by engineering a small air gap between the gold nanoparticle and a gold sheet. This gap is so small that laser light cannot penetrate it. However, the light energised 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 equivalently, its frequency, the laser light causes plasmons of a particular frequency to oscillate back and forth along the gap.

Meanwhile, as the nanoparticle moves, it changes the width of the gap and changes the frequency at which the plasmons 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 nanoparticle.

To use this motion-sensing technique in a practical device, the researchers embedded the gold nanoparticle in a microscopic-scale mechanical structure – a vibrating cantilever – that was a few micrometres 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 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 NIST scientists hope their new way of measuring motion at the nanoscale will help to further miniaturise and improve performance of many such micromechanical systems.

The team’s fabrication approach allows production of some 25,000 of the devices on a computer chip, with each device tailored to detect motion according to the needs of the manufacturer.

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