For nanometer-sized machines, air is so thick it saps their energy. That means most contraptions must be confined to vacuum chambers. But now physicists report that a little laser light can help nanomachines operate in open air. The advance could open the way for ultrasensitive biodetectors.
Nanotechnology promises to combine the best properties of mechanical and electronic devices. For example, tiny oscillating silicon beams might be used to precisely select the frequency of a radio signal, and microscopic swiveling mirrors might direct light pulses from an array of optical fibers. The machine parts could be built into microchips, allowing easy electronic control. But so far such nanoelectromechanical structures, or NEMS, have remained cloistered inside vacuum chambers that limit their potential applications.
A carefully applied laser can crack that problem, report Harold Craighead, Lidija Sekaric, and colleagues at Cornell University in Ithaca, New York. They studied a square of silicon that was 2 micrometers wide and was suspended by a pair of 200-nanometer-thick silicon beams so that it could move up and down like a trampoline. The researchers shined a laser through the square and bounced it off the chip below. The incoming and reflected light waves canceled out in the plane of the square, but reinforced each other just above and below it. So if the square moved up or down, it absorbed light, heated, and expanded. That effectively stiffened the silicon, boosted the square's energy, and allowed it to vibrate at a precise frequency in air, the researchers report in the 30 September issue of Applied Physics Letters.
The device could be very useful for detecting the presence of a single bacterium or several viruses because they would change the vibration frequency if they stuck to the square, says Andrew Cleland of University of California, Santa Barbara. "If you use NEMS devices in vacuum, you'll have to look at things that are dead," Cleland says. The Cornell team still has to prove it can track the tiny frequency changes, says Dan Rugar of IBM's Almaden Research Center in San Jose, California, but the laser technique is a welcome advance. "In these NEMS devices," Rugar says, "anything you can do to boost the signal is important."