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Silicon photonic device to generate microwave signals

Researchers at the University of Campinas in Brazil have paved the way for a silicon photonic device that would enable optical and mechanical waves vibrating at gigahertz frequencies to interact.

“Through computer simulations, we proposed a device that could exploit a mechanism for the scattering of light by mechanical vibrations, called Brillouin scattering, and could be transposed to photonic microchips,” said Professor Gustavo Wiederhecker.

In Brillouin scattering, photons interact with elastic vibrations, which consist of phonons, at tens of GHz.

“This light scattering mechanism is easy to observe in optical fibres, which can be hundreds of kilometres long, because it’s cumulative,” Prof Wiederhecker said. “It’s harder to observe and exploit in an optomechanical device at the micrometre scale.”

To overcome this size limitation, the group developed silicon disks with a diameter of 10μm. The disks act as microcavities. The light is reflected from the edge of the material and spins around the disk cavity thousands of times over a few nanoseconds before dissipating.

As a result, the light remains in the cavity longer and thereby interacts more with the material, and the optomechanical effects are augmented.

The problem is that such a microcavity does not allow light at any arbitrary frequency to be resonant – to propagate through the cavity. “So you can’t exploit the Brillouin scattering effect in these microcavities,” Prof Wiederhecker explained.

Using computer simulations, the researchers theoretically constructed a system comprising two silicon microdisks with one cavity each.

“We show that with a laser power of about 1mW, it would be possible to observe the Brillouin scattering effect in a double-disk cavity system,” he said. “We foresee important applications for this kind of device, including stable high frequency microwave generation, narrowband signal filtering, and ultra-stable lasers.”

Generating stable high frequency microwave signals is said to be important in high speed communications. Silicon’s low acoustic attenuation at high frequencies could also enable narrowband microwave-frequency filtering, which is key for receivers used in microwave communication.

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