With a tunable bandgap, black phosphorus could be used to make better optical communications circuits than graphene and even germanium. Researchers at the University of Minnesota used black phosphorus, a 2-D, stable crystalline form of phosphorus, to create optical circuits operating in the near-infrared telecommunications band. The group demonstrated data speeds up to 3 Gbps, which is equivalent to downloading an HD movie in about 30 seconds. “Even though we have already demonstrated high-speed operation with our devices, we expect higher transfer rates through further optimization,” said graduate student Nathan Youngblood. “Since we are the first to demonstrate a high-speed photodetector using black phosphorus, more work still needs to be done to determine the theoretical limits for a fully optimized device.” A photodetector that uses few-layer black phosphorus (red atoms) to sense light in the waveguide (green material). Graphene (gray atoms) is also used to tune the performance. Courtesy of the University of Minnesota College of Science and Engineering. While the existence of black phosphorus has been known for more than a century, its potential as a semiconductor has come to light only in the past year, the researchers said. Photodetectors made with black phosphorus rival comparable germanium devices, the researchers said. Germanium is considered the gold standard in on-chip photodetection, but it is difficult to grow on silicon optical circuits. Black phosphorus and other two-dimensional materials, on the other hand, can be grown separately and transferred onto any material, making them more versatile. And while graphene has proven useful in a variety of applications, its main limitation is its lack of a bandgap and high dark current. Black phosphorus, on the other hand, has a tunable bandgap that varies depending on how many atomic layers are stacked together. This means that black phosphorus can be tuned to absorb light in the visible or IR range. In their experiments, the Minnesota researchers demonstrated black phosphorus photodetectors that operated under bias with very low dark current, attaining an intrinsic responsivity at room temperature up to 135 mA/W at 11.5-nm thickness and 657 mA/W at 100-nm thickness. Additionally, black phosphorus is a direct-band semiconductor, meaning it has the potential to efficiently convert electrical signals into light. Combined with its high performance photodetection abilities, black phosphorus could also be used to generate light in an optical circuit, making it a one-stop solution for on-chip optical communication. “Black phosphorus is an extremely versatile material,” said professor Dr. Steven Koester. “It makes great transistors and photodetectors, and has the potential for light emission and other novel devices, making it an ideal platform for a new type of adaptable electronics technology.” Funding came from the U.S. Air Force Office of Scientific Research and the National Science Foundation. The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.23). For more information, visit www.umn.edu.