Researchers Harness the Power of Graphene for On-chip All-optical Processing

October 27, 2021 by Ikimi .O

The discovery of a new phase of graphene—and subsequent topological circulators—could shift the way information is routed and processed on a chip.

One component that is essential in all-optical processing is the circulator. However, these circulators' size and bandwidth limit how they can be used. This is especially true as designs become increasingly miniaturized. 

Now, researchers are exploring the capabilities of topological circulators as a solution to shrinking board sizes in optical processing systems. Since these devices are ultra-subwavelength and broadband, topological circulators meet miniaturization needs in a host of applications, including quantum and classical computing systems.

Recently, Purdue University published research on how topological circulators can address some existing challenges of on-chip all-optical processing. 


Topological circulator made using a newly-discovered phase of graphene

Topological circulator made using a newly-discovered phase of graphene. Image used courtesy of Zubin Jacob and Purdue University

The Roadblocks to On-chip All-optical Processing

All-optical processing involves processing optical signals in the absence of optical-electrical conversion. This technology, which is becoming more commonplace, opens up the possibility for enhanced signal processing and computing capabilities, including optically transparent networks. 

However, some common challenges affect on-chip all-optical processing, including optical losses, power consumption, system coupling, wavelength tuning, and application/platform development.


All-optical logic gates on-chip based on surface plasmon polaritons

All-optical logic gates on-chip based on surface plasmon polaritons. Image used courtesy of Minzioni et al.


Other on-chip all-optical processing-related challenges include:

  • A tradeoff between low power and high processing speed
  • Complex fabrication techniques
  • System scalability
  • Optical transistor limitations
  • Inefficient light-control-light

Conventional circulators only fortify these roadblocks. 


Circulators Role in All-optical Processing

Circulators are passive devices that can significantly improve the reliability, performance, and stability of electrical systems. Although it is not compulsory to incorporate these devices into every design, they are essential to some applications, such as one-port amplifiers and all-optical signal processing. 

Generally, the operation of conventional circulators depends on the distinctive properties of microwave ferrites. These ferromagnetic materials, comprising magnetic domains (or Weiss domains) and measuring 1 to 100µm, exhibit different behaviors under static and alternating fields. For instance, the absence of an external magnetic field leads to the random orientation of Weiss domains, resulting in zero magnetization. However, applying external magnetic fields can cause significant ferrite saturation magnetization.


Planar model for Weiss domains

Planar model for Weiss domains. Image used courtesy of Philips Semiconductor


Some common circulators constructed for ferromagnetic materials include:

  • Field displacement
  • Phase shift
  • Faraday rotation
  • Ring
  • Junction
  • Edge guided mode
  • Lumped circuit circulators

These conventional circulators are bulky, inefficient, and power-intensive. The devices barely meet the fundamental requirement for all-optical functions, making them unsuitable for large-scale on-chip applications with a low operating range light intensity of 10 kW cm−2. 

These devices also employ a high-Q cavity-based optical bistable switch, trading off low power consumption for high-speed operation. The complex fabrication techniques of these devices can lead to difficulties in mass production. Finally, the differences in input and output wavelengths of on-chip all-optical processing circulator-based logic gates can result in back reflection, leading to device malfunction.

Researchers are considering three aspects to adequately solve these challenges—including integration, materials modification, and fabrication technologies. Notably, several studies with promising solutions are currently underway.


Leveraging Graphene for a Topological Circulator

A team of researchers at Purdue University recently published a paper discussing how they exploited the capabilities of graphene to develop topological circulators for all-optical processing. The researchers developed these circulators by using gapless edge plasmons with high immunity to back-scattering. 

Throughout the years, researchers' understanding of graphene has evolved significantly. Today, graphene—the thinnest material in the world—is used to support edge-based unidirectional electromagnetic waves.

The researchers at Purdue first studied light interaction at a microscopic level. The team found that the viscous fluid in graphene can uphold one-way electromagnetic waves on the edge. In the published research article, the researchers describe these "edge waves" as a new topological phase of matter, similar to a solid-to-liquid transition. 

When graphene is in this new phase, light travels on the edge in a single direction. Using this finding, the team then created topological circulators to act as one-way routers of signals. 


A 3-port ultra-subwavelength topological circulator

A 3-port ultra-subwavelength topological circulator. Image used courtesy of Van Mechelen et al.

While in a topological edge state, the circulators proved robust and highly immune to back-scattering. In addition to its small footprint (the so-called smallest in the world), this circulator is said to offer unprecedented improvements in information routing and interconnection between classical and quantum computing systems.