Tiny ring laser accurately detects and counts nanoparticles

Washington University
June 27, 2011

A microlaser no bigger than a pinprick can accurately detect and count individual viruses, the particles that jumpstart cloud formation or those that contaminate the air we breathe.

A tiny doughnut-shaped laser is the latest marvel of silicon microminiaturization, but instead of manipulating bits it detects very small particles. Small particles play a big — and largely unnoticed — role in our everyday lives. Virus particles make us sick, salt particles trigger cloud formation, and soot particles sift deep into our lungs and make it harder to breathe.

The sensor belongs to a category called whispering gallery resonators, which work like the famous whispering gallery in St. Paul’s Cathedral in London, where someone on the one side of the dome can hear a message spoken to the wall by someone on the other side. Unlike the dome, which has resonances or sweet spots in the audible range, the sensor resonates at light frequencies.

Light traveling round the micro-laser is disturbed by a particle that lands on the ring, changing the light’s frequency. The ring can count the touch-down of as many as 800 nanoparticles before the signals begin to be lost in the noise. By exciting more than one mode in the ring, scientists can double-check the accuracy of the count. And by changing the “gain medium,” they can adapt the sensor for water rather than air. 

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