Detecting Motion, No Matter How Small

Hank Hogan

A virus isn’t big. A few years ago, a nanomechanical sensor found that one tipped the scale at a few attograms. Such instruments now have the potential to sense mass an order of magnitude smaller, on the scale of zeptograms. However, small-scale sensing requires the challenging task of measuring the motion of miniature mechanical elements. Recently, a team from the European nanotechnology research center IMEC of Leuven and from Ghent University, both in Belgium, reported on a novel way to register nanoscale movement.

This scanning electron micrograph of a nanomotion detector shows light traveling down a waveguide (straight structure running from lower left to upper right), and evanescent optical coupling sends some of it into a parallel microresonator. Because the waveguide and resonator are freestanding, their separation changes and is detected by monitoring the power in versus the power out of the waveguide. Images reprinted with permission of Applied Physics Letters.

The technique exploits evanescent optical coupling between two waveguides. The tunneling rate of photons from one waveguide to another strongly depends on the separation of the two and, thus, can provide a way to accurately monitor the position and motion of one relative to another.

Separation changes

In a demonstration of the idea, the researchers fabricated waveguides in silicon on insulator material, creating one 10 μm long, 400 nm wide and 220 nm thick. Along the length of this sensing waveguide they constructed an identically sized resonator that acted like another waveguide, with a 300-nm separation between the two. As with the sensing waveguide, the resonator was freestanding and clamped at each end.

The level of evanescent optical coupling and the sensitivity of the nanomotion detector depend upon the separation between the waveguides, as shown in this plot comparing a 300- and a 150-nm separation.

In experiments, 1550-nm light from a Photonetics tunable laser entered one end of the sensing wave- guide, and the power at the other end was measured with a fast Bookham New Focus detector. Any change in power meant that movement had occurred between the fiber and the resonator.

Graduate student and lead author Iwijn de Vlaminck said their results showed that the technique’s sensitivity was much higher than conventional optical methods, with the responsivity depending upon the waveguide separation. A smaller distance is expected to produce even better results. “Decreasing the width of the photonic structures can further enhance the interaction between the resonator and main waveguide,” de Vlaminck said.

The technique does demand a high-resolution fabrication process to construct the device. However, progress in this area and the widespread availability of such a process make this requirement not burdensome.

In the future, de Vlaminck noted, the method could be used in mass- and force-sensing applications where sensitivity is of utmost importance, such as in atomic and magnetic resonance force microscopy. Variations could be employed in signal processing applications through the use of mechanical filters. Finally, he said, the approach could be used for nanomotion detection in harsh conditions.

Applied Physics Letters, June 4, 2007, 233116.