Silicon Emits Visible Light

A surprising sparkle of green light from a silicon chip has opened up a field of possibilities for communications devices, including exponentially shrinking the hardware needed for high-quality Internet connections.

“When I saw the green light on the camera, I was extremely puzzled,” said Dr. Christian Grillet of the University of Sydney. “We were using infrared light, not green. And besides, silicon does not transmit light at that wavelength!”

Grillet’s colleague, Dr. Christelle Monat, was in the labs of CUDOS (Centre for Ultrahigh Bandwidth Devices for Optical Systems) in the school of physics at the time. “I didn’t believe the camera. I had to look with my own eyes. It was as strange as seeing a house-brick suddenly emit light,” Monat said.

The effect was real, however. Their infrared laser was being converted to green light – light of higher energy – in a process known as third- harmonic generation.

“One could imagine that a small green light indicator could help users of numerous Internet applications. This could be used to immediately inform companies such as Skype of a problem in the clarity of their connections, thereby allowing them to fix this in real time, all without the end user even noticing,” Monat said of the discovery’s potential.

The key to this unlikely event was a regular pattern of submicroscopic airholes in the researchers’ silicon chip, creating what is known as a photonic crystal. At the time of the discovery, Monat and Grillet were assisting doctoral student Bill Corcoran with experiments on slow light, itself a very novel and surprising phenomenon.

“The experiments use specially designed photonic crystals from our colleagues at St. Andrews in Scotland. They allow us to slow the laser light used for telecommunications to one-fortieth of its usual speed,” Corcoran said. “As the light slows down, the energy from the laser is greatly concentrated. This energy can be used like traffic lights on the road to control the movement of large amounts of optical data through networks much more efficiently.”

The researchers said that converting infrared to green light adds another important tool to the impressive suite of capabilities of silicon, already the material of choice for the microelectronics industry.

“Being able to control light on a chip, along wires no wider than one-hundredth of the width of a human hair, represents the first step to realize all sorts of operations with significantly better performance than electronics alone,” Monat said. “And if we can do that in silicon, even more complex and exciting architectures become possible by integrating and marrying both the photonic and electronic worlds.”

The research team’s paper, “Green Light Emission in Silicon Through Slow-Light Enhanced Third-Harmonic Generation in Slow Light Photonic-Crystal Waveguides,” was published online in Nature Photonics on March 22.

For more information, visit: www.usyd.edu.au