Whispering-gallery sensor can measure a single nanoparticle

Laura S. Marshall, laura.marshall@laurin.com

Nanoparticles can be found nowadays in approximately 800 products, from sunscreen to anti-graffiti paint, plastic beer bottles and home pregnancy tests, and researchers are working on ways to assess how these particles affect human health and the environment.

A new sensor on a chip could help in this effort. The sensor, devised by Dr. Lan Yang, assistant professor of electrical and systems engineering at Washington University, and her research team not only can detect single particles but also can measure them. Yang and her team predict that the sensor, an improved version of a whispering-gallery microresonator, will be able to measure particles smaller than 100 nm in diameter, about the size of a virus particle. Their work appeared in the advance online publication of Nature Photonics on Dec. 13, 2009.

In architecture, a whispering gallery is a space beneath a circular or elliptical area such as a dome in which, if a person stands at one focus and whispers, someone at the focus on the opposite end of the gallery can hear what is said because the line of sound emanates directly between the focal points. At normal speaking volume, the sound circulates around the dome more than once, and the signal is garbled.

On a much smaller scale, laser light can be coupled into a circular waveguide: a glass ring, for example. When the light strikes the boundary of the ring at a grazing angle, it is reflected back into the ring. The light wave can travel around the ring several times before it is absorbed, as long as the frequency of the light fits perfectly into the circumference of the ring. This perfect-match resonance is called a whispering-gallery mode.

The faint outer edge of the light wave penetrates the surface of the glass ring and allows the resonator to function as a particle detector: When a particle attaches to the ring, the light wave is disturbed and the resonant frequency is changed. The degree of change can be used to measure the size of the particle.

But microresonators have two major problems, Yang said. One is that the resonant frequency can be affected by vibration or temperature changes, not just the presence of particles. The other is that the frequency shift depends on the spot at which the particle attaches to the ring. If a particle lands on a node (the dark blue areas reflected on the base of the pedestal in Figures 1 and 2), it will cause less of a disturbance to the light wave, appearing smaller than it would if it landed on an anti-node (the red spots visible on the base).

Particles that land on the resonator disturb a light wave circulating in the torus (whose nodes and anti-nodes are visible on the torus’s base). These disturbances provide information about the particles’ sizes. The pink line receding into the distance is an optical fiber through which light is coupled into and out of the torus.

The solution: a self-referring sensing scheme possible only in an exceptionally good resonator with virtually no optical flaws. To achieve this, Yang and colleagues developed a microresonator with a quality factor, or Q value – a measure of microscopic imperfections that sap energy from the resonating mode – of about 100 million, which means that light circles the ring many, many times. Because recirculation increases the interaction of the light wave with particles on the ring’s surface, mode splitting becomes a viable approach to particle detection.

Each whispering-gallery mode is actually two modes, as the light travels both clockwise and counterclockwise around the resonator, usually with the same frequency. A particle that attaches to a resonator functions as a scattering center that couples energy between the two modes, which rearrange themselves so that the particle lies on a node of one and an anti-node of the other. This disturbs one wave much more than the other and splits the mode. The split mode is easily seen in the team’s ultrahigh-Q resonator, but it can’t be resolved in a low-Q resonator.

New high-Q microresonators could be mass-produced on silicon wafers; each torus is 20 to 30 µm across. In this image, two particles (bright spots) have landed on the closest microresonator and are acting as scattering centers, disturbing the light waves in the torus. This allows the particles to be detected and measured. Images courtesy of Jiangang Zhu and Jingyang Gan, Washington University in St. Louis.

Vibration and temperature changes aren’t an issue with mode-splitting sensors; neither is the particle-location problem. The clockwise and counterclockwise light waves share the same resonator and therefore the same noise; any jiggle that biases one will bias the other by the same amount. And the mode split will still vary with the location of the particle, but the ratio of the mode split and the difference between the breadth of the two modes will be dependent solely upon the size of the particle.