RF & Microwave

Sound as 'third wave' for integrated circuits

05 September 2019
Conceptual illustration of integrated circuit incorporating stimulated Brillouin scattering devices. Source: Nature Photonics

The first two waves in integrated circuitry can be thought of as microelectronics, which led to the ubiquity of silicon chip-based electronic devices; and optical electronics, which have become the backbone of data centers around the world.

Now, a third wave is emerging: sound waves.

Chip-based control of light-sound interactions could transform 5G and broadband networks, satellite communications, defense industries and more. Professor Ben Eggleton, director of the University of Sydney Nano Institute, refers to a “research renaissance” into the subject currently underway.

The science is based on the phenomenon of Brillouin scattering — feedback packets of sound or acoustic waves, known as “phonons,” which are caused by the vibration of light waves within optical fibers used for data transmission. The phenomenon was named for French physicist Leon Brillouin, who first predicted it in 1922.

While most of the electronics and communications industry is plagued by this feedback, which causes light to disperse and thus reduces signal power, an emerging group of scientists sees it as an opportunity to develop a new generation of integrated circuits that can solve real-world problems.

Eggleton’s newly published review article in Nature Photonics outlines the history and potential of Brillouin integrated phononics. One of the review’s co-authors, Professor Gaurav Bahl, noted that the research offers both technological applications and pure scientific investigations. "Brillouin scattering of light helps us measure material properties, transform how light and sound move through materials, cool down small objects, measure space, time and inertia, and even transport optical information," Bahl said.

Key to the research is the process of decades-old process simulated Brillouin scattering (SBS), which couples light and sound waves to create an enhanced feedback loop. It’s found an entirely new application: slowing down data to integrate optical information into a chip environment, using sound waves as a buffer. Most importantly, it does it without the heat produced by electronic systems.

The first transfer of light to acoustic information on a chip was announced by researchers from the Eggleton group in 2017. They referred to the achievement as “storing lighting inside thunder,” as illustrated in the video below. Since then, the complexity of the system has been reduced; Eggleton noted that integrated circuits using SBS offer the opportunity to replace components in flight and navigation systems that can be 100 or 1,000 times heavier.

Before a chip-scale integrated system can be commercially deployed, several challenges will need to be overcome. These include developing an architecture that integrates microwave and RF processors with optical-acoustical interactions; reducing interference caused by unwanted light scattering; and finding the most appropriate materials for building the systems.

The search for materials that can contain the light and sound waves while also allowing them to interact has led some research groups to use chalcogenide, a soft glass substrate with a high refractive index and low stiffness. "At this stage, all material systems have their strengths and weaknesses,” said Macquarie University professor Michael Steel, another co-author of the review. “This is still an area of fruitful research."

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