NTT Corporation, along with The University of Tokyo and National Institute for Materials Science (NIMS), showed that graphene plasmon wave packets can be generated, manipulated and read out on-chip using terahertz electronics.
The team managed to electrically generate and control graphene plasmon wave packets with a pulse width of 1.2 picoseconds. This result shows that the phase and amplitude of a terahertz signal can be controlled electrically by using graphene plasmons. It enables terahertz signal processing, a method different from conventional electrical circuit technology using transistors and is expected to contribute to realizing ultrahigh-speed signal processing in the future.
The terahertz region represents a largely unexplored but promising opportunity for high-speed communications. However, it is difficult to generate and detect electrical pulses in this region using contemporary electronics technology. The team of scientists, however, applied on-chip spectroscopy (combining optical pulses with a photoconductive switch) to enable the generation and detection of electrical signals in a bandwidth of up to 2 THz.
The research group evaluated the propagation characteristics and controllability of graphene plasmon wave packets and the plasmon generation efficiency by injecting ultrashort input pulses in the THz region generated using laser pulses into a graphene device. As a result, the following three points became clear for the first time in this experiment:
(1) the team succeeded in generating, controlling, and measuring 1.2 picosecond ultrashort graphene plasmon wave packets on a chip. This pulse width is equivalent to the time width of the input pulse before injection, and it is the shortest electrically excited plasmon wave packet in existence. This indicates that electrical signals in the THz range can be transmitted without distortion.
(2) the team found that the phase and amplitude of the plasmon wave packet can be controlled by electrically modulating the charge density of graphene with a gate. Phase and amplitude control is the basic operation to realize all kinds of signal processing, which means that they have demonstrated a new device operation to handle electrical signals in the THz range.
(3) By optimizing the gate electrode material, the team reached a maximum conversion efficiency of 35% from input pulses to graphene plasmon wave packets. This value exceeds the conventional light-to-plasmon conversion efficiency by several orders of magnitude, making graphene plasmons inherently suitable for handling electrical signals in the THz region. Furthermore, they clarified that not only the conversion efficiency but also the confinement effect, propagation velocity, and pulse width are significantly influenced by the gate electrode. These findings make it possible to optimize the device structure according to the intended purpose.
This announcement supports NTT’s leading R&D of photonic-electronic devices. In March, NTT announced NTT Innovative Devices, an operating company
dedicated to the further development of photonic-electronic hardware for ultra-fast, sustainable, next-gen communications infrastructure.
The results of this work show that a plasmonic device capable of electrically controlling the phase and amplitude of THz electrical signals in circuits can be implemented. By developing this technology, the scientists aim to realize more advanced signal processing elements such as variable frequency filters, amplifiers, and modulators in the THz region. This research shows that graphene plasmons can be handled with electricity, but considering that plasmons can also be generated by light, it may lead to the development of new photonics-electric convergence technologies.
The team expects that further development of signal processing technology in the THz region will contribute to significant improvements in information and communications, as well as computational processing speed in the future.
