Scientists devise sensor that can detect single nanoparticle and take its measurement

Washington, December 19 (ANI): A group of researchers at Washington University has devised a sensor on a chip that can not only detect but also measure a single nanoparticle.

The research, led by Lan Yang, assistant professor of electrical and systems engineering, expect that the sensor will be able to measure nanoparticles smaller than 100 nanometers in diameter (about the size of a virus particle) on the fly.

The new sensor belongs to a class of devices called whispering-gallery-mode resonators.

One famous whispering gallery is St. Paul's Cathedral in London. If you stand under the dome close to the wall and speak softly to the wall, someone on the opposite side of the gallery is able to hear what you say.

In a miniature version of a whispering gallery, laser light is coupled into a circular "waveguide," such as a glass ring.

When the light strikes the boundary of the ring at a grazing angle it is reflected back into the ring.

The light wave can make many trips around the ring before it is absorbed, but only frequencies of light that fit perfectly into the circumference of the ring can do so.

If the circumference is a whole number of wavelengths, the light waves superimpose perfectly each trip around.

This perfect match between the frequency and the circumference is called a resonance, or whispering-gallery mode.

The glass resonator can serve as a particle detector because the faint outer edge of the light wave, called its "evanescent tail," penetrates the ring's surface, probing the surroundings.

So, when a particle attaches to the ring, it disturbs the light wave, changing the resonant frequency.

This change can be used to measure the size of the particle.

There are two problems with these microresonators, according to Yang.

One is that they are finicky. Lots of things can shift the resonant frequency, including vibration or temperature changes.

The other is that the frequency shift depends on where the particle lands on the ring.

The way around these problems is a self-referring sensing scheme possible only in an exceptionally good resonator, one with virtually no optical flaws.

Yang's lab uses surface tension to achieve the necessary perfection. The microresonators are etched out of glass layers on silicon wafers by techniques borrowed from the integrated circuit industry.

These techniques allow the rings to be mass-produced, but leave them with rough surfaces.

In a crucial finishing step, the microresonators are reheated with a pulsed laser until the glass reflows. Surface tension then pulls the rings into smooth toruses.

"Nature helps us create the perfect structure," said Yang. (ANI)