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Integrated optics

Sounding out lasing in silicon

08 Jun 2018 Anna Demming
Acoustic modes enable lasing in silicon.
Acoustic modes enable lasing in silicon. Courtesy: istock/fotojog

Combining photonics and electronics would bring the advantages of speedy optical data transfer to the increasingly miniaturized world of electronics. Ideally photonics and electronics elements would be seamlessly integrated but many traditional photonics components are tricky to replicate in silicon, a fundamental challenge being a source of light on-chip. Now researchers at Yale University, Arizona University and the University of Texas at Austin show how this hurdle may be tackled using a silicon Brillouin laser.

When a material is strained, its refractive index changes due to “Brillouin scattering” from strain-induced deformations. In a crystal this strain may be due to acoustic vibrations in response to the electric field from an intense beam of light – stimulated Brillouin scattering. Put this scenario in a ring structure where the optical gain from stimulated scattering overcomes roundtrip loss and you have a Brillouin laser.

While the power and flexibility of Brillouin lasers has already attracted notice, as Peter Rakich, and Nils Otterstrom at Yale University and their co-authors point out in a report of their work, Brillouin interactions are markedly weak in conventional silicon photonic waveguides. The trick with this latest work was devising a silicon system with “unusually large Brillouin coupling”.

Narrowing in on Brillouin lasing in silicon

For the Brillouin lasing silicon system the researchers fabricated a racetrack structure from single crystal silicon on insulator. They then removed the insulator under the two long edges and it is these two suspended waveguide sections that produce large intermodal Brillouin gain.

Investigations of the system by injecting continuous wave pump light into an asymmetric cavity mode and analysing the “Stokes” scattered light emitted into a symmetric cavity mode revealed a 3% slope efficiency of input versus output power.

Another notable feature of the system’s response was the spectral compression – the curve describing the spectra of the Stokes emission narrowed by a factor of around a thousand to a line width with a resolution-limited value of just 20. This kind of monochromatic single-frequency output is a typical feature of lasers. Despite a high degree of acoustic spatial damping and lack of phonon feedback spectral compression occurred as long as the temporal acoustic dissipation rate was lower than that of the optical field.

Full details are reported in Science

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