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Computing with Light and Magnets

A new way of controlling light could open the door to novel optical devices.
June 13, 2007

Manipulating light in a novel way, researchers at the United States Naval Research Laboratory, in Washington, DC, and the University of Alberta, in Canada, have demonstrated that light can be controlled with magnets in very small, transistor-like devices. Such switches could lead to fast, small, and efficient optical chips for cell phones, and optical communications.

Light switches: These microscopic gold-coated cobalt particles (shown at two magnifications, at top) can transmit light until a magnetic field is turned on, which shuts down the transmission. This happens because the magnetic field transports electrons with a certain spin orientation (indicated by the arrowheads on the circles above) from the cobalt to the gold, increasing resistance.

The advance combines insights from two nascent research fields. In plasmonics, researchers are studying ways of guiding light along very thin metal wires to allow faster communication between devices on a chip. The other field, spintronics, involves manipulating a property of electrons called spin; in the past several years, spintronics research has enabled ultradense memory in hard drives. Now the Naval Lab and University of Alberta researchers have shown that by manipulating electron spin using magnetic fields, they can turn off and on light that’s being guided through metals.

Such an on-off light switch could be used for information processing, including routing infrared light in optical communications or, modified for lower-frequency electromagnetic waves, processing radio signals in cell phones. The advance could potentially increase processing speed and decrease power consumption compared with conventional methods, says Mark Johnson, a researcher at the Naval Research Lab. Indeed, he says, it may eventually be possible to replace multiple specialized chips with a single light-based chip that can be reconfigured on command to perform different types of tasks, saving space and money. Because the switches stay in position without a continuous power supply, they could reduce power consumption.

The researchers coated microscopic cobalt particles with a very thin layer of gold. When they exposed the particles to a form of electromagnetic radiation close in frequency to visible light, the radiation was converted into another form, called a plasmonic wave, that allows it to travel through the particles. The radiation is then reemitted, although it’s slightly weakened due to resistance in the particles. The researchers found that if they expose the particles to a magnetic field, the resistance increases significantly, stopping the emission of radiation from the magnetized particles. The effect can be reversed by applying an oscillating magnetic field, which scrambles the orientation of the electrons’ spins, demagnetizing the particles.

Having demonstrated this phenomenon, the researchers’ next step is to build devices that can act as switches in a chip. For the current study, they worked with particles because particles were convenient for the experiments, but for actual devices they’ll need to switch to thin films of metal that can be patterned using photolithography to make circuits. They’ll also need a faster way of switching the light on and off. “That’s one of the challenges, but we think there are ways to do it,” Johnson says.

Another challenge will be working with magnetic fields to control very densely packed devices, in which the proximity of the fields can cause a problem with cross talk, says Shoucheng Zhang, a professor of physics, applied physics, and electrical engineering at Stanford University. “It’s very interesting new physics,” Zhang says. “It’s hard for anyone to judge the commercial potential yet. But it’s definitely something worth looking into.”

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