Novel 3D rectification effect for high-speed,
high-sensitivity plasmonic THz detector
Authors：Akira Satou1, Takumi Negoro1,2, Kenichi Narita1,2,Tomotaka Hosotani1,2, Koichi Tamura1,2, Chao Tang1,3, Tsung-Tse Lin1,2,Paul-Etienne Retaux1,4, Yuma Takida5, Hiroaki Minamide5, Tetsuya
Suemitsu6 and Taiichi Otsuji1
1

Research Institute of Electrical Communication, Tohoku University, Japan2

34

School of Engineering, Tohoku University, Japan

Frontier Research Institute for Interdisciplinary Sciences, Tohoku University

Department of Electrical and Computer Engineering, Ecole Nationale Supérieure del’Electroniqueet de ses Applications, France56RIKEN Center for Advanced Photonics, RIKEN, JapanNew Industry Creation Hatchery Center, Tohoku University, JapanFigure 1: A 3D rectification effect in an asymmetric dual-grating-gate plasmonic THz(THz) detector.

1. Pre-reading
Two-dimensional (2D) plasmons, collective charge density quanta of 2D electrons
excited in the electron channel of a semiconductor field-effect transistor, have attracted
attention as an operation principle to realize a room-temperature-operating, high-speed,
high-sensitivity terahertz (THz) detector (plasmonic detector). Due to their strong
nonlinear rectification effect associated with the fluid nature and their fast response notlimited by the electron transit time, a high detection performance exceeding
conventional electronic and optical devices is expected.
A research group led by Associate Professor Akira Satou at Research Institute ofElectrical Communication, Tohoku University in collaboration with Professor Tetsuya
Suemitsu at New Industry Creation Hatchery Center, Tohoku University, and a researchteam led by Dr. Hiroaki Minamide, Team Leader, at RIKEN Center for Advanced
Photonics have fabricated a plasmonic THz detector based on an indium phosphorus
compound-semiconductor high-electron-mobility transistor (HEMT) and evaluatedTHz detection characteristics at room temperature. As a result, by applying an abnormal
strong positive bias to the gate terminal of the transistor and reading out the detection
signal from the gate terminal, instead of the conventional drain terminal with a normal
gate bias level, a novel detection principle, “plasmonic three-dimensional (3D)rectification effect”, was discovered that superimposes a gate-to-channel diode current
nonlinearity onto the plasmonic hydrodynamic nonlinear rectification effect (Fig. 1).
By employing this novel THz detection scheme, we have succeeded in a significant
enhancement of the THz detection responsivity outperforming the conventional
detectors by more than one order of magnitude, and we have also succeeded in thedevelopment of a device modeling manifesting the operation principle with quantitative
agreement with experimental data obtained. Furthermore, it has been demonstrated that,by following this operation principle, the output impedance of the device can bematched to the standard high-speed transmission line impedance of 50 Ω, and that theproblem of waveform distortion due to multiple reflections in conventional drainreadout configurations can be dramatically eliminated in the detection of high-speed
modulated signals.
These are groundbreaking achievements that pave the way to the realization of
next-generation 6G & 7G ultrahigh-speed wireless communications.

2. Background
Terahertz waves are electromagnetic waves with a frequency lying between thatof radio waves and light waves, and have unique characteristics not found in otherelectromagnetic waves, such as the fact that they overlap with the vibrational frequency
of molecules of substances, and thus almost all matter exhibits inherent absorption
characteristics of their own finger prints in this THz range. For this reason, thedevelopment of technologies utilizing THz waves is progressing rapidly in various
scientific and industrial fields, such as spectroscopy and imaging for safety and security
to see the “invisible," and ultra high-speed wireless communications. In particular, thedevelopment of 6G and 7G technologies, the next generations of ultrahigh-speed
wireless communications using THz waves, is essential for the dramatic improvementof information and communication services that are indispensable for the realization ofa near-future super-smart society. However, the development of electronic devices suchas transistors and optical devices such as lasers has been extremely difficult due to thesubstantial physical limitations of operation in the THz band. In particular, further
performance improvement is required to realize THz wave detectors that operate atroom temperature, can be miniaturized and integrated, and have high speed and highsensitivity, which are essential as a means of receiving 6G and 7G wireless signals in areal time manner.
Against this background, 2D plasmons, collective charge-density quanta of 2D
electrons excited in the electron channel of a semiconductor field-effect transistor, haveattracted attention as an operation principle to realize a room-temperature-operating,high-speed, high-sensitivity THz detector (plasmonic detector), which is difficult withconventional electronic and optical devices, due to their strong nonlinear rectification
effect associated with the fluid nature and their fast response not limited by the electrontransit time. In particular, a detector that introduces a unique transistor electrodestructure called an asymmetric dual-grating-gate structure can efficiently couple
plasmons with THz waves (Fig. 1(a)). The conventional operation principle that reads
out the photovoltage signal from the drain terminal has the advantage that the detection
signal can be enhanced by making the internal resistance very high (typically around
100 kΩ), but the output impedance cannot be matched to the standard high-speed

transmission line impedance of 50 Ω. This causes waveform distortion due to multiple
reflections of the detection signal between the signal wiring with the next stage,
severely degrading the receiver performance. In other words, there is a trade-off
between the detection sensitivity and speed performances, which has been a major
barrier to the practical application of ultrahigh-speed wireless communications.

3. Innovative research
Assoc. Prof. Satou’s research group fabricated a plasmonic detector with anasymmetric dual-grating-gate structure based on an indium phosphorus compoundsemiconductor HEMT (Fig. 1(b)) and evaluated its detection performance by a THz
pulse detection measurement at 0.8 THz at room temperature with a new method toread out a detection signal from the gate terminal of the device under the application ofan abnormal strong positive bias to the gate terminal. Under this original detection
scheme, we discovered a novel detection principle called “plasmonic 3D rectificationeffect” (Fig. 1(c)), resulting in a significant enhancement of the current detection
responsivity by more than one order of magnitude over the conventional performance(Fig. 1(d)). This novel operation principle can be explained as follows. At a highpositive gate bias voltage, a heterobarrier between the InGaAs channel and the InAlAs
barrier layer provides the exponentially increasing diode-like current nonlinearity
associated with electron tunneling. Then, the 2D plasmonic photocurrent oscillating at
the incoming THz radiation frequency as well as all the higher harmonic frequencies
are generated and introduced into the channel to induce the in-plane plasmonicrectification. This results in multiplication of the diode nonlinearity (vertical direction
to the channel) and the plasmonic nonlinearity (in-plane direction to the channel),leading to the giant “3D rectiﬁcation effect”. This significant enhancement in measured
photovoltage was successfully verified by a device modeling with a simplified
formulation of this physical picture.

Figure 2: Drastic enhancement of detector photoresponse by introduction of theinverted HEMT structure.
On the other hand, an undesired very long tail photoresponse waveform wasobserved after the output pulse waveform corresponding to an incident THz pulse wave.
Such a slow response limits the data rate of error-free operation in wirelesscommunications and must be eliminated. The cause of this tail waveform was found tobe the electron trapping by the donor levels at the InAlAs carrier-supply layer existingin the electron tunneling path between the gate and the channel in the normal HEMTstructure (Fig. 2(a)). By introducing the so-called inverted-HEMT structure (Fig. 2(b)),
where the carrier supply layer is situated under the channel layer out of the tunneling
path, The tail waveform was completely eliminated (Fig. 2(c)). This demonstrated thesuperiority of the inverted-HEMT structure for wireless communication applications.
The maximum measured current detection responsivity obtained for the device
with a normal HEMT structure (Fig. 3) was 0.49 A/W and is more than one order ofmagnitude higher than that in the conventional drain-readout. It is also higher than theresponsivity of existing electronics-based THz detectors such as Schottky barrier diodes(0.3~0.4 A/W at 0.8 THz). The output resistance of the gate-readout HEMT structure
was approximately 100 Ω at a gate bias of +2.1 V, assuring that the impedance matchingwith a 50-Ω transmission system can be achieved easily by the HEMT design
optimization which was difficult in the conventional drain-readout scheme because of
its inherently very high output resistance on the order of 100 kΩ.

Figure 3: Current detection responsivity of the device.

4. Applications and perspectives
The obtained THz detection responsivity characteristics can be well qualified tobe sufficiently high to realize a room-temperature-operating THz detector fortransmission of about 100 m, which is required for receivers of next-generation 6G- &7G-class ultrahigh-speed THz wireless systems. On the other hand, as mentioned above,
there is a room for further performance enhancement in plasmonic detectors utilizing
the 3D rectification effect, for example, an improvement on the band engineering of theelectron tunneling-path heteroepitaxial layers between the cannel and the gate electrodeto maximize the diode-like nonlinearity. Then therefore, in the very near future, it isfully expected that the transmission distance of next-generation ultrahigh-speed
wireless communications 6G & 7G can be extended to the kilometer level.
These research results are published online with the title “Gate-readout and a 3D
rectification effect for giant responsivity enhancement of asymmetric dual-grating-gate
plasmonic THz detectors” in Nanophotonics, a high-impact factor journal.
The authors of this article are Akira Satou, Takumi Negoro, Kenichi Narita,
Tomotaka Hosotani, Koichi Tamura, Chao Tang, Tsung-Tse Lin, Paul-Etienne Retaux,
Yuma Takida, Hiroaki Minamide, Tetsuya Suemitsu and Taiichi Otsuji. Akira Satou is
the corresponding author of this work. Author contributions are as follows:Akira Satou: Conceptualization, Data curation, Formal analysis, Funding acquisition,
Investigation, Methodology, Project administration, Resources, Software, Supervision,Validation, Visualization, Writing – original draft, Writing – review & editing.
Takumi Negoro: Data curation, Investigation, Methodology, Validation, Visualization.
Kenichi Narita: Data curation, Investigation, Methodology, Validation, Visualization.
Tomotaka Hosotani: Data curation, Investigation, Methodology, Validation.
Koichi Tamura: Formal analysis, Investigation, Methodology, Validation, Visualization.
Chao Tang: Investigation, Methodology, Validation, Visualization, Writing – originaldraft.
Tsung-Tse Lin: Investigation, Methodology, Resources, Validation, Writing – originaldraft; Paul-Etienne Retaux: Software, Investigation, Methodology, Validation, Formalanalysis.
Yuma Takida: Investigation, Methodology, Validation, Resources.
Hiroaki Minamide: Investigation, Methodology, Validation, Resources.
Tetsuya Suemitsu: Investigation, Methodology, Validation, Resources.
Taiichi Otsuji: Conceptualization, Funding acquisition, Investigation, Methodology,Validation, Writing – original draft, Writing – review & editing.
All authors have accepted responsibility for the entire content of this manuscript andapproved its submission.
Assoc. Prof. Akira Satou’s research group is affiliated to Research Institute ofElectrical Communication, Tohoku University, Sendai 980-8577, Japan.
[Original publication]
Akira Satou, Takumi Negoro, Kenichi Narita, Tomotaka Hosotani, Koichi Tamura,
Chao Tang, Tsung-Tse Lin, Paul-Etienne Retaux, Yuma Takida, Hiroaki Minamide,
Tetsuya Suemitsu, and Taiichi Otsuji, "Gate-readout and a 3D rectification effect forgiant responsivity enhancement of asymmetric dual-grating-gate plasmonic terahertz
detectors," Nanophotonics, no. 12, issue 23, pp. 4283–4295, Nov. 9, 2023.
https://doi.org/10.1515/nanoph-2023-0256