A third wave emerges in integrated circuits

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Optical fibers are our global nervous system, carrying terabytes of data on the planet in the blink of an eye.

While this information travels at the speed of light around the world, the energy of light waves that bounce inside the silica and polymer fibers creates very small vibrations that lead to packets of light. return of acoustic sounds or waves, called "phonons".

This feedback causes the scattering of light, a phenomenon known as "Brillouin scattering".

For most electronics and communications industries, this dispersion of light is a nuisance, reducing signal strength. But for a group of emerging scientists, this feedback process is being adapted to develop a new generation of integrated circuits promising to revolutionize our 5G and broadband networks, our sensors, our satellite communications, our radar systems, our defense systems and even radio astronomy.

"It is not an exaggeration to say that a renaissance of research is underway in this process," said Professor Ben Eggleton, director of the Nano Institute of the University of Sydney and co-author of the research. an article of synthesis published today in Photonic Nature.

"The application of this interaction between light and sound on a chip offers the opportunity of a third revolution in integrated circuits."

The discoveries of microelectronics after the Second World War constituted the first wave of integrated circuits, which led to the omnipresence of electronic devices based on silicon chips, such as the mobile phone. The second wave came at the turn of the century with the development of optical electronics systems that have become the backbone of huge data centers around the world.

First electricity then light. And now the third wave is with sound waves.

Professor Eggleton is a world-renowned researcher studying how to apply this photon-phonon interaction to solving concrete problems. His research team based at the Sydney Nanoscience Hub and the School of Physics has written more than 70 articles on the subject.

In collaboration with other world leaders in the field, he published today a review article in Photonic Nature describing the history and potential of what scientists call "Brillouin integrated photonics". His co-authors are Professor Christopher Poulton of the Sydney University of Technology; Professor Peter Rakich of Yale University; Professor Michael Steel from Macquarie University; and Professor Gaurav Bahl from the University of Illinois at Urbana-Champaign.

Professor Bahl said: "This document describes the rich physics resulting from an interaction as fundamental as that which exists between light and sound, which is found in all states of matter.

"Not only are we seeing immense technological applications, but also the wealth of pure scientific research made possible." Brillouin's light diffusion helps us to measure the properties of materials, to transform the way light and sound move through materials, cool small objects, measure space, time and inertia, and even carry optical information. "

Professor Poulton said, "The big breakthrough in this area is the simultaneous control of light and sound waves on very small scales.

"This type of control is incredibly difficult, especially because the two types of waves have extremely different velocities." The tremendous advances in manufacturing and theory presented in this paper demonstrate that this problem can be solved and powerful interactions between light and sound Brillouin scattering can now be operated on a single chip, opening the door to a multitude of applications linking optics and electronics. "

Professor Steel said, "One of the fascinating aspects of the integrated Brillouin technology is its breadth, from fundamental discoveries in quantum-level sound-light interactions to very practical devices, such as flexible filters in the field. mobile communications. "

The French physicist Leon Brillouin predicted the scattering of light caused by his interaction with acoustic phonons in 1922.

Background information

In the 1960s and 1970s, an interesting process was discovered, which makes it possible to create an improved feedback loop between photons (light) and phonons (sound). This is known as stimulated Brillouin scattering (SBS).

In this SBS process, the sound and light waves are "coupled", a process reinforced by the fact that the wavelengths of light and sound are similar, although their speeds differ by several orders of magnitude: the light moves 100,000 times faster than the sound, which is why you see lightning before hearing the thunder.

But why would you increase the power of this Brillouin feedback effect?

"Managing information on a microchip can consume a lot of energy and produce a lot of heat," said Professor Eggleton.

"As we increasingly depend on optical data, the process of interacting light with microelectronic systems has become problematic.The SBS process offers us a completely new way to integrate optical information into a chip environment using sound waves as a buffer to slow data without the heat produced by electronic systems.

"In addition, integrated circuits using the SBS offer the opportunity to replace flight and navigation system components that can be 100 or 1000 times heavier – it will not be a trivial feat."

Reduce complexity

The question of how to contain the light-sound interaction process was the stumbling block, but as Professor Eggleton and his colleagues pointed out in Photonic Nature Today, the last decade has been marked by considerable progress.

In 2017, researchers Birgit Stiller and Moritz Merklein of the Eggleton Group of the University of Sydney announced the world's first transfer of light information to acoustic information on chip. To emphasize the difference between the speed of light and that of sound, this has been called "lightning storage in thunder".

Dr. Amol Choudhary developed this work in 2018, developing an on-chip information retrieval technique that eliminated the need for bulky processing systems.

"Above all, it is about reducing the complexity of these systems so that we can develop a general conceptual framework for a complete integrated system," said Professor Eggleton.

Industry and government are increasingly interested in deploying these systems.

Sydney Nano recently signed a partnership with the Royal Australian Air Force to work with its Jericho Plan program to revolutionize RAAF's detection capabilities. Companies such as Lockheed Martin and Harris Corporation are also working with the Eggleton Group.

The challenges ahead

Before commercially deploying this integrated system at the scale of a chip, there are obstacles to overcome, but the gain in terms of size, weight and power (SWAP) will be worth the effort said Professor Eggleton.

The first challenge is to develop an architecture integrating microwave and radio frequency processors with opto-acoustic interactions. As the results of the Eggleton Group show, great progress has been made in this direction.

Another challenge is to reduce "noise" (or interference) in the system caused by unwanted light scattering that deteriorates the signal-to-noise ratio. One proposal is to have chips operating at cryogenic temperatures close to absolute zero. Although this would have significant practical implications, it could also involve quantum processes, allowing better control of the photon-phonon interaction.

There is also an ongoing investigation into the most appropriate materials on which to build these integrated systems. Silicon has its obvious attractions since most microelectronics are built using this cheap and plentiful material.

However, the silica used in the optical fibers when coupled to the silicon substrate means that information may leak due to the similarity of the materials.

Finding enough elastic and non-elastic materials to contain the light and sound waves while allowing them to interact is one of the suggested paths. Some research groups use chalcogenide, a low-rigidity, high refractive index flexible glass substrate that can confine optical and elastic waves.

Professor Steel from Macquarie University, co-author of the journal, said, "At this point, all hardware systems have their strengths and weaknesses, and it's still a successful area of ​​research.

Professor Eggleton said: "This new paradigm in signal processing using light and sound waves opens new perspectives for basic research and technological advances."