A Laser’s Force Dampens Cantilever Action

Hank Hogan

David M. Weld and Aharon Kapitulnik of Stanford University in California recently demonstrated that even low-power lasers pack a punch. Using laser-driven radiation pressure force-feedback, they reduced the effective quality factor and temperature of a silicon nitride microcantilever by more than a factor of 15.

For cantilever-based gravity experiments, physicists at Stanford University developed a force-feedback mechanism that uses radiation pressure provided by a second laser in the setup (left). The photo above shows the cantilever to which a gold mass is attached. Reproduced courtesy of Applied Physics Letters.

The technique could be useful in atomic force microscopy — where a cantilever’s position often is determined by a laser — because it would necessitate only the addition of a second beam. “This technique does not require any modifications to the cantilever side of your apparatus,” said Weld, who is now at MIT in Cambridge, Mass.

The researchers developed the technique because the methods typically used to dampen the movement of a cantilever — piezoelectric elements or magnetic coatings, for example — would not be appropriate for their microcantilever-based gravity experiments. Instead, they decided to employ a photon-based damping method that involved phase-shifting a second laser relative to the position of the cantilever. Calculations showed that a few microwatts of laser energy would produce all the force they needed to counter the cantilever’s movements.

They used a 1310-nm diode laser from Thorlabs Inc. of Newton, N.J., to measure cantilever displacement, sending the beam through an optical fiber, a wave division multiplexer (WDM) from JDSU of Milpitas, Calif., and an aspheric lens from LightPath Technologies Inc. of Orlando, Fla., before reflecting the light off the cantilever. They detected the result with a photodiode from what is now OSI Optoelectronics of Hawthorne, Calif.

A Thorlabs 1550-nm diode laser provided force-feedback modulation based on the displacement measurement. This second beam went through the WDM and then along the same path as the first.

The investigators tested the technique on a cantilever 230 μm long, 180 μm wide and 0.34 μm thick that had a 5-μg piece of gold attached to one end. They found that they could broaden and flatten the thermally excited resonance peak, reducing the effective temperature of the cantilever from ambient to 18 K.

Although the technique could be useful for AFM, the Stanford group employed it as a means to further an ongoing experiment. “It allowed us to measure gravity at a length scale and sensitivity where nobody has measured it before,” said Weld.

Applied Physics Letters, Oct. 16, 2006, 164102.