Silicon Brillouin laser uses soundwaves to amplify light

11 Jun 2018

Yale uses light-sound interactions to create strongest amplification mechanism in silicon - promising new types of lasers.

Scientists at Yale University, New Haven, Ct, US, have created what they are calling “a new type of silicon laser”, which uses sound waves to amplify light. The work, which has implications for photonics integrated circuits, in particular, has been reported in Science.

“We have seen an explosion of growth in silicon photonic technologies over the past few years,” said Peter Rakich, an associate professor of applied physics at Yale who led the research.

“Not only are we beginning to see these technologies enter commercial products that help our datacenters run flawlessly, we also are discovering new photonic devices and technologies that could be transformative for everything from biosensing to quantum information on a chip.”

The researchers say this sector’s rapid growth has created a pressing need for new silicon lasers to drive the new circuits – a problem that has been historically difficult due to silicon’s indirect bandgap.

Sound and vision

Nils Otterstrom, of the Rakich Lab and the study’s first author, added, “Silicon’s intrinsic properties, although very useful for many chip-scale optical technologies, make it extremely difficult to generate laser light using electrical current. It’s a problem that has blocked scientists for more than a decade. We need to find other methods to amplify light on a chip. In our case, we use a combination of light and sound waves.”

The laser design collects amplified light within a racetrack shape — trapping it in circular motion. “The racetrack design was a key part of the innovation. In this way, we can maximize the amplification of the light and provide the feedback necessary for lasing to occur,” Otterstrom said.

To amplify the light with sound, the silicon laser uses a special structure developed in the Rakich lab. “It’s essentially a nanoscale waveguide that’s is designed to tightly confine both light and sound waves and maximize their interaction,” Rakich said.

Eric Kittlaus, a co-author of the study and a graduate student in the Rakich lab, added, “What is unique about this waveguide is that there are two distinct channels for light to propagate. 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, the researchers explained, amplification of light using sound would not be possible in silicon. “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,” Rakich said. “Now, we’re able to use it for new types of laser technologies no one thought possible 10 years ago.”

Two challenges

Otterstrom said there were two main challenges in developing the new laser: “First, designing and fabricating a device where the amplification outpaces the loss, and then figuring out the counter-intuitive dynamics of this system,” he said. “What we observe is that while the system is clearly an optical laser, it also generates very coherent hypersonic waves.”

The research team said these properties may lead to a number of potential applications ranging from integrated oscillators to new schemes for encoding and decoding information.

“Using silicon, we can create a multitude of laser designs, each with unique dynamics and potential applications,” said co-author Ryan Behunin, an assistant professor at Northern Arizona University and a former member of the Rakich lab. “These new capabilities dramatically expand our ability to control and shape light in silicon photonic circuits.”