A New Spin on Silicon

Photon spin determines direction of current in silicon device

2 min read
A New Spin on Silicon
Photo: University of Pennsylvania

A group of scientists has stumbled upon a previously unknown characteristic of silicon, one that could make for faster, optical computers.

“We did nothing special,” says Ritesh Agarwal, a materials science professor at the University of Pennsylvania whose Nanoscale Phase-Change and Photonic Group made the discovery. “It’s a phenomenon which was basically crying out loud to be observed.”

What the group found was that the surface of a silicon nanowire was sensitive to the polarization of laser light. In other words, when a laser beam hit the silicon near where it was attached to a metal electrode, it produced a current, and the direction in which that current flowed depended on the polarization of the laser beam. If the photons are polarized clockwise, the current flows in one direction. Reverse that so they’re counterclockwise, and the current runs the other way.

That behavior is known in other, heavier materials than in silicon, and in fact Agarwal’s group was studying it in some new topological insulators they’d developed, which conduct electricity on their surfaces but act an insulators deeper in. In order to have a control for their experiment, they tried it on silicon nanowires, where they expected nothing would happen. They were completely surprised when it did. They describe the work in an online paper in the journal Science.

The effect is due to a quantum mechanical property of electrons called spin. Researchers are working on spintronic computers, which would be faster than conventional computers because at the quantum level, electrons can have more states than just the on and off of digital computing, thus they can encode more information. Agarwal says the same thing is true for optical computers that rely on light and photodetectors to transmit data. Adding the direction of the current into the mix increases coding possibilities. “It’s an extra degree of freedom, so you can add more information,” he says.

The effect comes from the arrangement of the silicon atoms, and doesn’t require creating any special nanoscale features. The team had been using nanowires, because they were easier to work with. But if they can align electrodes in the right orientation on a sheet of silicon, they should get the same effect, Agarwal says. That means it shouldn’t be difficult to add the effect to future generations of computer chips.

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