Semiconductor Nanophotonics Linked to Higher Communication Speed for Supercomputers and Data Centers

May 21, 2020 by Luke James

In the world of electronics and indeed science on the whole, when something simply just works without explanation, that’s not good enough; lots of questions get asked and researchers poke and prod until they can understand the underlying mechanisms and processes at work.

And that is what Cun-Zheng Ning, a professor of electrical engineering at Arizona State University, has been doing with semiconductor nanophonics—an area of research that investigates how light and lasers act within the nanoscale of semiconductors. 

In his latest line of research, Ning and his team, led by Associate Professor Hao Sun of Tsinghua University, have explored the intricate balance of physics that governs how electrons, holes, excitons and trions coexist and mutually convert into each other to produce optical gain. 


Major Implications for Supercomputers and Data Centers 

By being able to understand the physics behind nanoscale lasers, how they interact with semiconductors, and why they work, researchers could reproduce their results and apply them to applications like high-speed communication channels for supercomputers and data centers. 

In the last five years, researchers at several United States institutions, including Ning’s team and other collaborators from Tsinghua University, have produced experimental results demonstrating that lasers can be produced in one-molecule-thin 2D materials.

What sets the work of Ning’s team apart is that their experimental results go so far as to show that lasers can be produced at room temperature despite conventional laser physics suggesting that a laser could not be generated with such a low amount of power being applied to the 2D semiconductor. 

So, why was Ning’s team able to do this? For the past three years, he and his team have been trying to find the answer and in doing so, they have stumbled across a new discovery. 


Discovering A New Optical Gain Method

Optical gain—a material’s ability to amplify photons—is the underlying concept of all lasers. To produce optical gain, electrons are injected into a semiconductor, and semiconductors that are able to provide optical gain at low carrier density levels are critically important for energy-efficient nanolasers. However, current lasers are based on traditional materials that need extremely high-density levels above the Mott transition to achieve optical gain. 

When an electrical current is applied to a semiconductor material, negatively charged electrons and positively charged particles called holes are produced. In a conventional semiconductor, when electron and holes reach a sufficient density, they form an electron-hole gas and optical gain takes place. However, the 2D material that Ning’s team studied a few years ago managed to achieve optical gain before the required density level was reached. 


Creating Optical Gain in 2D Semiconductors

In a bid to understand why this occurred, Ning’s team, in collaboration with Tsinghua University, stumbled across a process that creates optical gain in 2D semiconductors. 

“While studying the fundamental optical processes of how a trion can emit a photon [a particle of light] or absorb a photon, we discovered that optical gain can exist when we have sufficient trion population,” Ning said. “Furthermore, the threshold value for the existence of such optical gain can be arbitrarily small, only limited by our measurement system.” Trions are formed when excitons—a tightly-bound electron-hole pair—bind to another electron or hole. 

In Ning's experiment, the team measured optical gain at density levels four to five orders of magnitude -- 10,000 to 100,000 times -- smaller than those in conventional semiconductors that power optoelectronic devices, like barcode scanners and lasers used in telecommunications tools.


2D material illustration.

Ning's team placed a single layer of 2D material on a carefully designed substrate with gold as a back-gate to control the number of electrons in the material. A laser is then used to create excitons, some of which form trions with the pre-existing electrons and reflected light is then monitored. Image credited to Cun-Zheng Ning

A ’Game-changing’ Development for Energy-efficient Photonics

Ning’s interest in a phenomenon known as the Mott transition, an unresolved mystery in physics about how excitons form trions and conduct electricity, led him to his discovery. The Mott transition is the point where a material reaches the Mott density and changes from an insulator to a conductor, at which point optical gain first occurs.

However, the power needed to achieve Mott transition is more than what is desirable for the future of efficient computing. Without new low power nanolaser capabilities like the ones that Ning’s team is researching, Ning claims that it would require a small power station to operate a single supercomputer.

He adds, "If optical gain can be achieved with excitonic complexes below the Mott transition, at low levels of power input, future amplifiers and lasers could be made that would require a small amount of driving power,” something that would be “game-changing” for energy-efficient photonics and provide an alternative to semiconductor materials—these are limited in their ability to create and maintain a sufficient number of excitons. 

As Ning observed in previous experiments with 2D materials, an optical gain can be achieved earlier than previously thought. Now the team has discovered a mechanism that could make it work.

“Because of the thinness of the materials, electrons and holes attract each other hundreds of times stronger than in conventional semiconductors,” Ning said. “Such strong charge interactions make excitons and trions very stable even at room temperatures.” Ning’s team could, therefore, explore the balance of the electrons, holes, excitons, and trions, as well as control their conversion to achieve very-low-density optical gain." 


Laying the Foundation for Future Supercomputers and Data Centers

Ning says that his team is not yet sure if this is the mechanism that led to the production of nanolasers in 2017 and work is still ongoing to solve that mystery. Although the development is interesting, Ning admits that his team is only at the fundamental science level but is nevertheless hopeful that “there are new opportunities to make real-world devices out of these observations.”

Before this, the team must study the new mechanism at different temperatures and explore how it can be used to intentionally create nanolasers. 

“The next step is to design lasers that can operate specifically using the new mechanisms of optical gain,” Ning said. “The long-term dream is to combine lasers and electronic devices in a single integrated platform, to enable a supercomputer or data center on a chip. For such future applications, our present semiconductor lasers are still too large to be integrated with electronic devices,” he added.