“Silicon’s intrinsic properties, although very useful for many chip-scale optical technologies, make it extremely difficult to generate laser light using electrical current,” said Yale scientist Nils Otterstrom. “It’s a problem that’s stymied scientists for more than a decade. To circumvent this issue, we need to find other methods to amplify light on a chip. In our case, we use a combination of light and sound waves.”

Interraction of physical vibration and light comes from stimulating the non-linear effect of Brillouin scattering, using physical suspension of the cavity to improve coupling.

The light is trapped around a path shaped like a running track which acts as nano-scale waveguide designed to tightly confine both light and sound waves and boost interaction. “The racetrack design was a key part of the innovation. In this way, we can maximise the amplification of the light and provide the feedback necessary for lasing to occur,” said Otterstrom.

“What’s unique about this waveguide is that there are two distinct channels for light to propagate,” said researcher Eric Kittlaus. “This allows us to shape the light-sound coupling in a way that permits remarkably robust and flexible laser designs.”

Without this type of structure, amplification of light using sound would not be possible in silicon.

According to team leader Peter Rakich: “We’ve taken light-sound interactions that were virtually absent in these optical circuits, and have transformed them into the strongest amplification mechanism in silicon. Now, we’re able to use it for new types of laser technologies no one thought possible 10 years ago.”

Challenges included fabricating a device where amplification out-paces the loss, then understanding the dynamics of the system, that were sometimes counter-intuitive, said Yale, and included appreciating that is not only generates coherent light, but coherent hypersonic waves.

Potential applications include integrated oscillators and schemes for encoding and decoding information.

The work is published as ‘A silicon Brillouin laser‘ in Science.