Scientists at Arizona State University (ASU) have identified the process in physics that enables nanolasers to be created using 2D materials. This led them to the discovery of a mechanism for providing optical gain in 2D materials at much lower density levels than traditional semiconductors. The research could provide an alternative to conventional semiconductors and could be game-changing for energy-efficient photonic devices. In 2017, professor Cun-Zheng Ning and his collaborators at Tsinghua University produced experimental results showing that lasers could be produced in 2D materials as thin as a single layer of molecules. While other researchers had developed these lasers at cryogenic temperatures, Ning’s team produced them at room temperature. The conventional mechanism of laser physics would suggest that it would be impossible to generate a laser with a very low amount of power being pumped into a 2D semiconductor. Yet it worked in the Ning team’s experiments. To understand this phenomenon, Ning and his colleagues investigated the physics that govern how electrons, holes, excitons, and trions coexist and mutually convert into each other to produce optical gain. The researchers uncovered a new gain mechanism involving charged excitons or trions in an electrically gated 2D material well below the Mott density (the point at which a semiconductor changes from an insulator to a conductor and optical gain first occurs). After examining the signatures of optical gain and their relationship with excitons and trions, the researchers were able to clearly identify the origin of optical gain as being trionic in nature. Cun-Zheng Ning, a professor of electrical engineering in the Ira A. Fulton Schools of Engineering at Arizona State University, and collaborators from Tsinghua University in China discovered a process of physics that enables low-power nanolasers to be produced in 2D semiconductor materials. Understanding the physics behind lasers at nanoscale and how they interact with semiconductors can have major implications for high-speed communication channels for supercomputers and data centers. Courtesy of Rhonda Hitchcock-Mast/ASU. “Because of the thinness of the [2D] 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.” This allowed the researchers to explore the balance of the electrons, holes, excitons, and trions and control their conversion to achieve optical gain at very low levels of density. “When more electrons are in the trion state than their original electron state, a condition called population inversion occurs,” Ning said. “More photons can be emitted than absorbed, leading to a process called stimulated emission and optical amplification or gain. “We discovered that optical gain can exist when we have sufficient trion population,” he said. “Furthermore, the threshold value for the existence of such optical gain can be arbitrarily small, only limited by our measurement system.” Ning and his team measured optical gain at density levels four to five orders of magnitude smaller than those in conventional semiconductors. The electrical power needed to achieve Mott transition — the process by which excitons form trions and conduct electricity in semiconductor materials to the point that they reach the Mott density — is far more than what is desirable for the future of efficient computing, Ning said. Without new low-power nanolaser capabilities like the ones he is researching, it would take a small power station to operate one supercomputer. “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,” Ning said. While the team’s findings have led to a new mechanism that researchers can exploit to create low-power 2D semiconductor nanolasers, Ning said that the team is not yet sure if this is the same mechanism that led to the production of their 2017 nanolasers. The work to resolve this remaining question is ongoing. Ning’s team also plans to study how this new mechanism of optical gain works at different temperatures, and how to use it to create nanolasers for a useful purpose. The existence of optical gain at extremely low density levels is of tremendous importance for fabricating nanolasers with very low threshold and power consumption. “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.” The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-020-0278-z).
