Harnessing Stimulated Brillouin Scattering for CMOS-Compatible, High-Performance Optical Circuits

Brillouin scattering has caused trouble for fiber communications at telecom wavelengths in the past. But stimulated Brillouin scattering (SBS)—plus a new method of integrating arsenic trisulfide onto silicon-based chips—might be the key to a new generation of ultra-high-performance optical circuits.

In principle, using optics to communicate should be much easier than using electronic components. We've been using optics to communicate for quite awhile and have even developed fiber optic networks because photons can transfer data exceptionally quickly. Unfortunately, there have been various impediments to light ousting electricity when it comes to the scale of nanometer computing. 

Most of our modern electronic technology, such as cell phones and smart watches, use integrated circuits embedded on a silicon semiconductor material. As the versatility of silicon increased, research possibilities broadened and the potential of optical processing gained the attention of quite a few research institutions.

Recently, the international research group CUDOS led by Senior researcher Alvaro Bedoya and PhD Candidate Blair Morrison worked in a collaborative effort with physicists from the Australian Institute for Nanoscience and Technology, RMIT, and the Australian National University to overcome silicon processing limitations. The result was a new CMOS-compatible platform with integrated nonlinear high-Brillouin-gain chalcogenide glass.

Building a Hybrid Platform with Stimulated Brillouin Scattering (SBS)

In the last few years, stimulated Brillouin scattering has seen development as a mechanism for photonics and optical processing and is now considered to be one of the best types of scattering for nonlinear optics. SBS occurs when high-intensity light, such as a laser, travels through a medium and causes acoustic vibrations in the medium.

A representation of forward-scattered (Raman) and backward-scattered (Brillouin) light. Image courtesy of Mohammad Abdelnaby Hasan

The team's hybrid platform consists of optical circuits made from As₂S₃ (arsenic trisulfide) chalcogenide glass connected to a silicon-based material. The chalcogenide glass was chosen due to its ability to function efficiently as a stimulated Brillouin scattering material. The process has been known to cause complications for fiber optics communications that span over a large distance. However, this phenomenon does offer use in a few practical applications, namely in microwave photonic filters.

The team’s silicon platform (silicon wafer) was manufactured in a research laboratory from Belgium. The team then took the wafer and thermally deposited arsenic trisulfide chalcogenide glass using the laser physics laboratory at ANU. The product was then taken to the RMIT lab to undergo lithography and plasma etching to form waveguides and finally tested at AINST.

A New Kind of Integrated Optical Circuit

The hybrid platform tests yielded promising results. First, they tested their spiral waveguide running a series of pump-probe SBS tests, where they managed to achieve an increase in Brillouin gain by nearly 50% over previous chalcogenide wavelengths.

The team then placed the waveguide onto the embedded area where they completed the fabrication of an integrated optical circuit which achieved Brillouin lasing, potentially for the first time ever in a planar integrated circuit.

Figure (a) shows an example of the As₂S₃ circuit created by the team. Image courtesy of OSA.org

One of the most notable aspects of the design was that it offered the prospect for their hybrid silicon platform to perform optical processing in a CMOS compatible platform while retaining the benefits of both silicon and chalcogenide glass.

In the future, the platform could be developed to combine active components such as detectors and modulators, leading to ultra high-performance devices that could outperform traditional RF devices.

“The breakthrough here is this realization that we can actually interface, we can integrate that glass onto silicon and we can interface from silicon to the glass very efficiently – we can harness the best of both worlds,” said Professor Eggleton, the director of CUDOS.

The original research article is available through Optica Publishing.