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Stanford breakthrough heralds super-efficient light-based computers

28 May 2015

Light can transmit more data while consuming far less energy. Now, an engineering breakthrough brings chip-scale optical data transport a little closer.

Infrared light enters this silicon structure (greatly magnified) from the left. The cut-out patterns, determined by an algorithm, route two different frequencies of this light into the pathways on the right (image: Alexander Piggott)

Optical transport uses far less energy than sending electrons through wires, and at the chip scale, light can carry more than 20 times as much data. As silicon is transparent to infra red light - the way glass is transparent to visible light - wires could be replaced by optical interconnects: silicon structures designed to carry infra red light.

Hitherto, engineers have had to design optical interconnects one at a time. Given that thousands of such linkages are needed for each electronic system, optical data transport has remained impractical.

Now, Stanford engineers believe they've broken that bottleneck by inventing what they call an inverse design algorithm. They specify what they want the optical circuit to do, and the software provides the details of how to fabricate a silicon structure to perform the task.

"We used the algorithm to design a working optical circuit and made several copies in our lab," says Stanford University electrical engineer, Jelena Vuckovic. As reported in the journal, Nature Photonics, the devices functioned flawlessly despite tiny imperfections.

"Our manufacturing processes are not nearly as precise as those at commercial fabrication plants," says Stanford graduate student Alexander Piggott. "The fact that we could build devices this robust on our equipment tells us that this technology will be easy to mass-produce at state-of-the-art facilities."

The researchers envisage other potential applications for their inverse design algorithm, including high bandwidth optical communications, compact microscopy systems and ultra-secure quantum communications.

Just as a prism refracts visible light, different silicon structures can bend infra red light in useful ways.

The Stanford algorithm designs silicon structures so slender that more than 20 of them could sit side-by-side inside the diameter of a human hair. These silicon interconnects can direct a specific frequency of infra red light to a specific location thus replacing a wired interconnect.

By loading data on these frequencies, the Stanford algorithm can create switches or conduits, or whatever else is required for the task.

Once the algorithm has calculated the proper shape for the task, engineers can use standard industrial processes to transfer that pattern onto a slice of silicon.

"Our structures look like Swiss cheese but they work better than anything we've seen before," says Vuckovic. She and Piggott have made several different types of optical interconnects and they see no limits to what their inverse design algorithm can do. In particular, they feel that they have set the stage for the next generation of faster and far more energy-efficient computers.

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