Nano-Products: Tiny Whispering Gallery

Contrary to popular belief, nanotechnology is not something that is far away in the future as nanoparticles are already found in anti-odor socks, makeup, makeup remover, sunscreen, anti-graffiti paint, home pregnancy tests, plastic beer bottles, anti-bacterial doorknobs, plastic bags for storing vegetables, and more than 800 other products.

How safe are these products and the flood of new ones about to spill out of labs across the world? A group of researchers at Washington University is devising instruments and protocols to assess the impact of nanoparticles on the environment and human health before they are sent to market

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As part of this effort, a team led by Lan Yang, Ph.D., assistant professor of electrical and systems engineering, has devised a sensor on a chip that can not only detect but also measure single particles. They expect 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, an improved version of a sensor called a whispering-gallery microresonator, is described in the December 13 edition of Nature Photonics's advanced online publication.

Whispering galleriesThe sensor belongs to a class of devices charmingly called whispering-gallery-mode resonators.

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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.

The reason is the sound bounces along the wall of the gallery with very little loss of energy and so can be heard at a great distance.

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However, if you speak at normal volume, what you say can no longer be understood. The sound travels around the dome more than once, and the recirculating signal gets mixed up and garbled.

Whispering-gallery microresonatorsIn 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.

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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.

Source-Eurekalert
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