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Levitating Nanomachines

Reversing the direction of a quantum force could prevent nano devices from sticking together–and even make them levitate.
August 14, 2007

As mechanical devices shrink down to the nanoscale, they fall victim to a strange quantum effect that makes their moving parts stick together. But theoretical physicists at the University of St. Andrews, in Scotland, have found a way to turn that effect against itself, producing completely frictionless nanomachines.

Current microelectromechanical systems–used in accelerometers, car air-bag triggers, and optical switches that transfer light from one fiber to another–have moving parts separated by about one micrometer. But that distance will soon shrink to a few hundred nanometers. At such short distances, a quantum-mechanical effect known as the Casimir force–which is too weak to be seen at distances greater than a micrometer–becomes significant. “If you have nanoelectromechanical systems [NEMS] with mobile parts, the Casimir force will be attractive, and the parts will stick together,” says Ricardo Decca, associate professor of physics at Indiana University-Purdue University Indianapolis. “This causes friction, and these devices will not move.”

St. Andrews researchers Ulf Leonhardt and Thomas Philbin have calculated that a specially engineered material called a perfect lens can reverse the direction of the Casimir effect. Sandwiching such a lens between NEMS parts would eliminate friction by making the parts repel each other instead of sticking together. Leonhardt and Philbin’s calculations–which will appear in the New Journal of Physics–suggest that the repulsive force could even be made strong enough to make the parts levitate. Such manipulations of the Casimir effect could lead to frictionless NEMS devices with tiny mirrors and metal plates that pivot easily on their anchors or that are suspended in air.

Perfect lenses are nothing like conventional glass lenses. Instead, they are made of metal carved into a repetitive lattice-like structure. Glass lenses such as the ones used in cameras can’t resolve details of an object that are smaller than the wavelength of the light bouncing off them. Perfect lenses do not have that limitation. And they bend–or refract–light in a direction opposite to that in which ordinary materials like water and glass bend it. Researchers predict that perfect lenses could lead to higher-density DVDs, ultrahigh-resolution microscopes that can image nanoscale objects, and faster fiber-optic communications.

The technology to make such optical lenses is only a few years old. But Philbin says it’s already good enough to produce lenses that can convert the Casimir attraction between objects into repulsion. Leonhardt and Philbin’s calculations show that such lenses should also be sufficient to levitate an aluminum foil 500 nanometers thick. “In theory, if you could build the right kind of lens, you could levitate heavier objects,” Philbin says.

According to Umar Mohideen, a physics professor at the University of California, Riverside, who was one of the first to measure the Casimir force accurately, “Anything that can tailor the [Casimir] force is useful from an engineering point of view, since the force is such a big parameter.” But making the perfect lenses will not be easy, he cautions, because the technology is new and still faces many challenges.

Perfect lenses have to be painstakingly designed and fabricated to work with specific wavelengths of light. Only within the past year have researchers designed perfect lenses that work at wavelengths as small as a micrometer, Mohideen says. To reverse the Casimir force in a NEMS device, a perfect lens will have to work at even shorter wavelengths–that is, those corresponding to the nanometer distances between parts. Designing and manufacturing such a lens is not a trivial task, says Jordan Maclay, chief scientist at Quantum Fields, based in Richland Center, WI. Controlling the spacing between the lens and the parts of a NEMS device will also be a challenge, Maclay says.

Making perfect lenses with the right properties could take years. And once those lenses are in hand, says Mohideen, researchers will still face the challenge of doing experiments to verify the new theory. But so far, using perfect lenses is the only technique proposed to reverse the Casimir force, Decca says. The promise of frictionless nanosensors and switches might make the method worth testing.

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