NTT and NIMS Accelerate Ultrafast Graphene Photodetection

By Kazuhiro Gomi, President & CEO of NTT Research

NTT Research is unique, but not the only organization in our corporation with ambitious long-range 1goals. Another research group within NTT, for instance, in collaboration with Japan’s National Institute for Materials Science (NIMS), recently broke new ground involving technology that may take a decade to implement. The technology is graphene photodetection, with use cases in areas such as optical sensing and optical-to-electrical (O-E) signal conversion. 

Given our PHI Lab’s optical-based approach to quantum computing, we are naturally interested to learn about any dramatic improvements in photodetection. In this case, the high performance is due to graphene, a form of carbon that consists of a single layer of atoms arranged in a lattice structure. It is an amazing and relatively new substance. Two scientists at the University of Manchester, UK, isolated and characterized graphene in 2004. Six years later they won the Nobel Prize in Physics. What followed was a kind of “graphene gold rush,” as described in this 2014 New York Times video, with scientists exploring the exceptional properties of this super-thin form of carbon, 100 times stronger than diamond and more highly conductive of heat and electricity at room temperature than any other known material.

Among graphene’s impressive optical properties, further clarified in the NTT-NIMS research, are high sensitivity and high-speed electrical response to a wide range of electromagnetic waves, from terahertz (THz) to ultraviolet (UV). That means that graphene could be used at wavelength ranges where existing semiconductor devices cannot operate. 

What our colleagues at NTT and NIMS demonstrated is the world’s fastest photodetection response. They did so by building a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz, in a condition of thermal equilibrium known as zero-bias operation. (The previous zero-bias operating bandwidth limit was 70 GHz.) Their breakthrough, summarized in this article in Nature Photonics, is attributable to two techniques: 1) a new gate structure using a zinc oxide (ZnO) thin film to suppress the resistor-capacity circuit (RC) time constant of electric circuits; and 2) an experimental system with on-chip THz spectroscopy, which eliminates the distortion of using oscilloscopes placed at a distance from the photodetector. 

As with what we do at NTT Research, this discovery is at the basic research phase, thus it is far from market ready. It may take ten years for graphene photodetectors to replace existing semiconductor devices. A big problem is its cost; current devices are created by manually extracting crystals. But the collaborative NTT-NIMS graphene R&D team is thinking ahead. Boron nitride crystal growth technology could enable mass production. And going forward, they plan to address sub-par photoelectric conversion efficiency by using plasmonics to improve the coupling between graphene and light. 
For more information about this achievement and the fascinating technology surrounding it, check out this illustrated press release.