Quantum effect alters device motion

By Eric Smalley, Technology Research News

At the smallest scales, in the realm of atoms and subatomic particles, the laws of physics are different from the ones that govern our everyday world. Even a vacuum isn't really empty at that level.

A vacuum is actually seething with zero-point energy, which is created by subatomic particles constantly appearing and disappearing. Many of the particles are photons, which means much of the energy is electromagnetic like light, x-rays and radio waves.

Earlier this year, researchers at Bell Labs showed that this zero-point energy can subtly change the position of free-moving parts in tiny mechanical devices. The researchers have now shown that the quantum energy can alter the motion of a microelectromechanical systems (MEMS) oscillator.

Even though the oscillator is smaller than the head of a pin, it consists of millions of atoms and is otherwise immune to quantum mechanical effects. "This is the first time these [changes in motion] have been observed in a nonlinear oscillator due... to quantum electrodynamical effects," said Federico Capasso, head of the physical research laboratory at Bell Labs. "We're seeing quantum effects at a macroscopic level, which is always fascinating."

Oscillators are used as sensors and timers in microdevices. The results mean researchers will have to factor in zero-point energy as MEMS become smaller. "As you put more and more MEMS function on the same chip, at some point of high enough density... we'll have to contend with these effects," said Capasso.

Zero-point energy affects objects through the Casimir force, which comes into play when two parallel plates are positioned closely enough that the gap between them is smaller than some electromagnetic wavelengths. This means that some of the zero-point energy is shut out of the gap. Because there is more zero-point energy acting on the outer surfaces of the plates than the inner surfaces, the plates are drawn together.

The researchers' MEMS device consisted of a 500-micron-square, 3.5-micron thick silicon plate topped with gold that was mounted above a surface by two arms on opposite sides that allowed the plate to tilt seesaw-fashion. The plate measured half a millimeter on a side.

The researchers set the device in motion by applying an alternating current to an electrode under one end of the plate. They induced the Casimir force by lowering a 200-micron, gold-covered sphere over one end of the plate. Even though it produces a weaker Casimir force, a sphere-plate setup is easier to work with than two plates, which must be positioned precisely parallel to each other.

The closer the sphere came to the plate, the stronger the Casimir force became, and the stronger the Casimir force became, the more it slowed the device's movement.

"It's a mechanical oscillator so it has a restoring force. That's what makes it oscillate back and forth," said Capasso. "Since the Casimir force is in the opposite direction of the restoring force -- it's an attractive force -- that has a net effect of softening the spring, reducing the oscillation frequency."

It turned out that the Casimir force's effect on the oscillator was different depending on whether the sphere was moving toward or away from the plate. "This behavior is called hysteresis because... where you are depends on the previous history."

The hysteresis provides additional information that could be useful in a sensitive position sensor, said Capasso. "Depending on the response of the oscillator, I know whether I am... getting closer to an obstacle... or if I am departing."

The Casimir force could begin to affect MEMS device design in about 10 years, said Capasso.

"We are opening up, in a broad sense, a new area of nanotechnology here," he said. "People are talking a lot about nanotechnology but they have largely ignored these types of forces, basically because you have to have the right surface area [and] put things close enough, but eventually there can be huge effects."

Capasso's research colleagues were Ho Bun Chan, Vladimir A. Aksyuk, Raphael N. Kleiman and David J. Bishop. They published the research in the November 19, 2001 issue of the journal Physical Review Letters. The research was funded by Lucent Technologies.

Timeline:   10 years
Funding:   Corporate
TRN Categories:   MEMS
Story Type:   News
Related Elements:  Technical paper, "Nonlinear Micromechanical Casimir Oscillator," Physical Review Letters, November 19, 2001


January 16, 2002

Page One

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Quantum effect alters device motion


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