Graphene plasmons can help achieve ultrahigh-speed terahertz signal processing

For the advancement of high-speed communications and advanced sensing and imaging, the terahertz region is a promising but largely unexplored opportunity. Collaborative research conducted by NTT Corporation (NTT), the University of Tokyo, and Japan’s National Institute for Materials Science (NIMS) aims to optimize electrical signal generation and detection performance within this region to overcome the current limitations of contemporary electronics.

Background on terahertz

The terahertz region is located between light waves and radio waves, and generally refers to electromagnetic waves of 0.1 to 10 THz. Modern electronics technology was developed within the lower frequency gigahertz (GHz) region (<100 GHz), while emerging and future photonics technology can be used within the higher frequency range (>10 THz).

Attempts to optimize signal processing within the terahertz region are not new. In 2012, Carter M. Armstrong published an article in IEEE Spectrum, “The Truth About Terahertz,” which noted: “the most intense work to tame and harness the power of the terahertz regime” began around 2002, and research efforts harken back to the 1950s.

Armstrong’s article summarizes a few key challenges of operating within the terahertz region, such as the fact that “it is still exceedingly difficult to efficiently produce a useful level of power from a compact terahertz device,” and “a better understanding of material properties at terahertz frequencies, as well as general terahertz phenomenology” is needed.

He concluded: “Ultimately, we may need to apply out-of-the-box thinking to create designs and approaches that marry new device physics with unconventional techniques.”

A new approach

In their 2017 paper “Graphene plasmonics: physics and potential applications,” Fudan University researchers noted that “graphene plasmon is highly tunable and shows strong energy confinement capability. Most intriguingly, as an atom-thin layer, graphene and its plasmons are very sensitive to the immediate environment.”

And in experimentation published in the British science journal Nature Electronics in July 2024, NTT, University of Tokyo, and NIMS researchers tried a new approach to using the terahertz frequency and achieved promising results by using graphene plasmonics as a new control technique for terahertz electric signals.

Graphene plasmons confine terahertz waves into a small region, which enables the electrical control of properties such as wavelength via external voltage. The ability to control these properties and manage terahertz electric signals within circuits is vital to developing new, ultrahigh-speed communications, and imaging and sensing technologies.

Initially, however, it was unclear whether generating and controlling graphene plasmons electrically within the terahertz region was even possible.

To validate this possibility, the research group examined the propagation characteristics and controllability of graphene plasmon wave packets, as well as generation efficiency by injecting ultrashort electrical pulses into a graphene device. By doing so, three key findings became clear.

Generating and detecting ultrashort pulses

Today, it is extremely difficult to generate and detect electrical pulses within the terahertz region using existing electronics technology. To do so, the group applied on-chip terahertz spectroscopy, which combines femtosecond (1/1000 trillion second) optical pulses with a photoconductive switch to enable the generation and detection of electrical signals in a bandwidth of up to 2 THz (see Fig. 1).

It enabled successful generation, control, and measurement of 1.2-picosecond ultrashort graphene plasmon wave packets on a chip—a pulse width equivalent to the time width of the electrical pulse before injection (and the creation of the shortest electrically excited plasmon wave packet in existence). By achieving this result, the researchers confirmed that electrical signals within the terahertz range can be transmitted without distortion.

Optimizing device materials

The group 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. Since phase and amplitude control is the fundamental operation to achieving many types of signal processing, the results demonstrated an operation device capable of handling electrical signals within the terahertz range.

To do so, the researchers optimized the gate materials by selecting a gold (Au)-based gate to generate graphene plasmons with a large confinement effect, and a zinc oxide (ZnO)-based gate transparent to terahertz signals to make high-efficiency plasmon excitation possible (see Fig. 2).

By optimizing the gate electrode material, the researchers achieved a maximum conversion efficiency of 3% from electrical pulses input to graphene plasmon wave packets. This conversion efficiency exceeds the conventional reported light-to-plasmon conversation efficiency (0.06%) by several orders of magnitude—which demonstrates that graphene plasmons are inherently suitable for handling electrical signals within the terahertz region.

They further confirmed that additional metrics—confinement effect, propagation velocity, and pulse width—are significantly influenced by the gate electrode, which makes it possible to optimize a device’s structure according to its intended usage.

A look ahead

The results of this experiment strongly indicate that achieving a plasmonic device capable of electrically controlling the phase and amplitude of terahertz electrical signals in circuits is possible.

Through additional research and development, NTT, the University of Tokyo, and NIMS will now target development of even more advanced signal-processing elements, such as variable frequency filters, amplifiers, and modulators within the terahertz region.

And, while this research shows that graphene plasmons can be handled with electricity, the fact that plasmons can also be generated by light may lead to the development of new photonics-electric convergence technologies, such as those currently being developed by NTT Innovative Devices.

In the further future, the full realization of signal processing technology in the terahertz region will be vital to enabling next-generation information technology capabilities, including ultrafast computational processing.

While more study is needed, graphene plasmons present a unique path forward to achieving new frontiers within terahertz-based signal processing.