New Type of Silicon Laser

Illustration of the silicon Brillouin laser in operation. The laser is formed from nanoscale silicon structures that confine both light and sound waves. (Source: Yale Univ.)

In recent years, there has been increasing interest in trans­lating optical techno­logies such as fiber optics and free-space lasers into photonic integrated circuits. Researchers say silicon photonics are one of the leading platforms for such techno­logies, thanks to their compa­tibility with existing micro­electronics. “We’ve seen an explosion of growth in silicon photonic techno­logies the past few of years,” said Peter Rakich, an associate professor of applied physics at Yale. “Not only are we beginning to see these techno­logies enter commercial products that help our data centers 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. It’s really an exciting time for the field.”

The researchers said this rapid growth has created a pressing need for new silicon lasers to power the new circuits – a problem that has been histori­cally difficult due to silicon’s indirect bandgap. “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 Nils Otter­strom, a graduate student in the Rakich lab. “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.”

The laser design corrals 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.

“What’s unique about this wave­guide is that there are two distinct channels for light to propagate,” added Eric Kittlaus, a graduate student in the Rakich lab. “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.”

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 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.” (Source: Yale Univ.)

Reference: N. T. Otterstrom et al.: A silicon Brillouin laser, Science 360, 1113 (2018); DOI: 10.1126/science.aar6113

Link: Quantum Nonlinear Optics Group (P. Rakich), Yale University, New Haven, USA

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