NTT Technical Review

May 2023 Vol. 21 No. 5
View from the TopHiroshi Tomiyasu, Senior Vice President, Head of Technology and Innovation General
Headquarters, NTT DATA

Front-line Researchers

Masaya Notomi, Senior Distinguished Researcher, NTT Basic Research Laboratories andProject Leader of NTT Nanophotonics Center

Rising ResearchersYasufumi Ogawa, Distinguished Researcher, NTT Network Innovation CenterFeature Articles: Plasmon Control Technology
Overview and Prospects for Research on Plasmons in Two-dimensional Semiconductor
SystemsElectrical Control of Plasmon Reflectivity in GrapheneUltrafast Optical-to-electrical Conversion Processes in Graphene

Theoretical Proposal for Edge Magnetoplasmon Crystal

Time-resolved Measurement of Ambipolar Plasmon Transport in Semiconductor Composite
Quantum Wells

Regular Articles
Ultrafast and Low-power-consumption Membrane Lasers on Si with Integrated Optical
Feedback

Global Standardization Activities


Report of the First ITU-T TSAG Meeting for the 2022–2024 Study Period

Special Report50th Anniversary of NTT Yokosuka R&D Center

External Awards

View from the Top

Aiming to Attain Global No. 1
Technological Capabilities andMaximize the Value We Provide to
Our Clients
Hiroshi Tomiyasu
Senior Vice President, Head of Technology
and Innovation General Headquarters,
NTT DATA
Abstract

NTT DATA creates new systems and value using information technology
to contribute to creating a more prosperous and harmonious society. In
accordance with the five strategies laid out in its medium-term management
plan for FY 2022 to 2025, the company aims to establish a virtuous cycle ofinvestment and growth and achieve business growth toward the Global 3rdStage. Hiroshi Tomiyasu, senior vice president and head of Technology andInnovation General Headquarters, NTT DATA, is leading efforts toward
this goal in terms of technology. We interviewed him about specific technology strategies and his belief as a top executive.
Keywords: system integration, technology strategy, human resources

Leading the Technology and Innovation
General Headquarters—Could you tell us about the technology strategies of
NTT DATA?
NTT DATA is a global company that has grownover the past few years to employ 190,000 people
worldwide. We have been striving to expand ourbusiness globally since FY 2005. In the Global 1st
Stage, we increased our global coverage; in the
Global 2nd Stage, we strived to establish a global
brand; and in the Global 3rd Stage, we are aiming tobecome one of the top-five information technology(IT) service providers in the world. As the midpoint
NTT Technical Review

Vol. 21 No. 5 May 2023of the Global 3rd Stage, during the previous mediumterm management plan for FY 2019 to 2021, we
pursued profitable growth on a global scale withconsistent belief and courage to change.
To accomplish the Global 3rd Stage, under the current medium-term management plan for FY 2022 to2025, we aim to create value by connecting people
with technology and build a sustainable future in unison with our clients. To achieve this aim, we are
pursuing the following five strategies: (i) Capitalizeon the Convergence IT & Connectivity, (ii) Strengthen
Consulting with Foresight, (iii) Evolve to an Assetbased Business Model, (iv) Enhance Advanced &Development Technology, and (v) Be the Best Place
to Work.
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View from the Top

The Technology and Innovation General Head­quarters, which I lead, plays the role of the head­quarters for engineering teams scattered throughout
Japan, the United States, and Europe. In accordancewith our technology strategies for attaining global
No. 1 technological capabilities and maximizing thevalue we provide to our clients, we are working on
globally identifying and using not only the technology that can be used now but also the technology thatwill spread in the future through the application ofadvanced technology and co-creation research anddevelopment (R&D) to solve client problems. We arefocusing on two of the five strategies of the mediumterm management plan, Evolve to an Asset-based
Business Model and Enhance Advanced &Development Technology. While identifying thetechnology areas to focus on, we are developing technology that will enable system development withhigh productivity and agility suitable for the digitalage and provide that technology as global common
assets. To continuously improve the value of such
assets, we are also promoting the effective use ofhuman resources with advanced technical skills.
—It is a big role to take on two of the five strategies.
Evolve to an Asset-based Business Model is likely to
attract a lot of attention, isn’t it?
Our business model has traditionally been system
integration (SI) by which we develop a computer
system tailored to each client’s needs. We have been
engaged in sales activities through which we receive
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Vol. 21 No. 5 May 2023

a request for proposal (RFP) from a client and develop a custom-made, one-of-a-kind system in accordance with the RFP. We will shift from this conventional SI to asset-based SI by fully demonstrating ourability to develop systems with client understanding
and advanced technologies that we have cultivatedthus far and our ability to support various enterprisesystems and industry infrastructure to connect people, enterprises, and society.
The background behind this initiative includes twochallenges we must address: securing IT humanresources and quickly responding to ever-changing
environments and client requests. Regarding the former challenge, expanding sales while maintaining theconventional SI business model necessitates executing many projects with considerable human resources. However, the global competition for IT talent isheating up, so it is not always possible to secure thosehuman resources. It is therefore important to shift
from custom-made system development, and wehave decided to pursue a business model in which the
NTT DATA Group’s technologies, knowledge, andexperience are consolidated into assets at the global
level and effectively used in multiple regions andindustries.
Regarding the latter challenge, as the use of digitaltechnology accelerates business, the conventional SI
business model cannot keep up with such speed.
Accordingly, we will evolve our business model to
one in which we proactively propose and disseminate
our products and services by leveraging our assets sothat we can acquire the business agility suited to the2View from the Top

digital age and maximize the value we provide to ourclients.
Assets can be many things, but we mean reusable
assets such as industry and business foresight, bestpractices, software, and in-house tools. The Tech­
nology and Innovation General Headquarters is
responsible for creating common global technology
assets, and we are selecting global technology focus
areas and developing industry-independent technology assets in each area. By expanding the scale ofsales, increasing the number of engineers, and building an ecosystem by strengthening partner alliancesin each area, we aim to acquire global No. 1 technological capabilities and accelerate our transformation
to an asset-based business model by taking advantage
of such capabilities.
Reorganizing R&D and technology-development
activities on the basis of Emerging, Growth, andMainstream categories—Could you explain the other strategy, Enhance
Advanced & Development Technology?
This is a key strategy for turning our technology

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into our business by strengthening our ability to use
advanced technology to gain future competitivenessand system-development technology to improve productivity. We first divided these technologies into
three categories—Emerging, Growth, and Main­
stream—in accordance with the maturity level ofeach technology and reorganized our R&D andtechnology-development activities on the basis ofthese categories.
In the Emerging category, we will search foradvanced technologies that will be available five toten years from now and conduct co-creation R&D
that examines value creation with clients by using
new technologies.
In the Growth category, we will develop technologies to create growth business and technology-focus
areas in three to five years, create technology assets,
and conduct business-feasibility tests as well as trainengineers.
In the Emerging and Growth categories, we will
begin full-fledged development of our technology
assets to achieve global business expansion from atechnological standpoint. The Innovative Optical andWireless Network (IOWN) is a priority theme forcreating new social value, and we are collaboratingwith NTT laboratories, which conduct R&D on component technologies for IOWN. In particular, we aredeveloping a common testbed so that their research
results can be commercialized and delivered to ourclients. We are also promoting a digital twin co-creation program for conducting co-creation R&D with
our partners engaged in social innovation.
In the Mainstream category, we will develop technology assets in technology-focus areas concerningcurrent business, unify our global strategy to expand
our business in Japan, the United States, and Europe,
increase delivery resources, and expand market
share. The main technology-focus areas in this category include “cloud” for system development using
various cloud services; “application development andmanagement,” which pursues highly agile systemdevelopment and productivity improvement by using
digital technologies; “cybersecurity,” which focuseson security services on the basis of the concept ofzero trust, rather than the conventional perimeter
defense; and “data and intelligence,” which intends tocreate new business value through data analysis anddata utilization by using artificial intelligence (AI).

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View from the Top—The reorganization of the Technology and Inno­
vation General Headquarters seems to be a flagshipinitiative for the business transformation of NTTDATA.
I think it is a touchstone rather than a flagship. As I
mentioned at the beginning, NTT DATA has growninto a company with 190,000 employees worldwide.
This growth process involved mergers and acquisitions, and business styles of each company, businesspractices and business processes in each country, andcharacteristics and needs of clients all differ. As westandardize, and in some cases, integrate these different practices and systems and strategically establish
NTT DATA’s identity, we must accommodate thedifferent needs of our clients in each region. This
process will take a considerable amount of time;
however, in a sense, the technologies are the sameworldwide, so they are easy to unify, therefore, we
unified and shared the technology strategies ahead of
business practices.
Of course, some things will go wrong because it is
the first time. Accordingly, we have changed ourapproach to business plans; that is, we implement
them speedily and revise them as necessary.
Specifically, we review business plans every threemonths with executives from each country in reference to the status of their implementation. Since weconstantly monitor and undergo major revisions
every three months, I do not have time to be satisfied
or dissatisfied with the progress of the business plans.
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I work with the mindset that the repetition of working
enthusiastically will lead to a job well done later.
When making this assessment, in addition to theusual indicators, one of the points to consider iswhether a project is in a busy state. I think the work
we have done to the best of our abilities while saying,“It’s so tough,” is probably that in which we lookback on later and think, “That was interesting andfulfilling.” I have had this experience myself, and I
take into consideration whether the project members
are engaged in the project enthusiastically.
Developing human resources with enthusiasm—Could you tell us what is important to you as a top
executive?
I entered NTT DATA in 1990 and studied imagerecognition systems at the R&D department for about
ten years. However, due to the company’s decision tostop developing image-recognition systems in thelight of the background of that time and future potential, I was transferred to a department related to financial systems.
I learned an important lesson from that experience.
That is, it is important not to continue a project simply by force of habit; instead, it is necessary to havea strong conviction to withdraw or change course in atimely manner. NTT DATA is now a global company
with 190,000 employees, and if we continue to workby inertia, those 190,000 employees will go in the4View from the Top

wrong direction. That is why we share NTT DATA’s
clear technology strategies for global integration withengineers around the world while promoting transformation with conviction.
It is also important to have a good sense of whether
a technology being developed will be accepted byclients and whether it is likely to become popular. It
is necessary to develop technologies that can address
issues that front-line employees found in direct communication with clients and create products thatfront-line employees can propose to clients while
confidently saying, “We can solve your problems
with this technology.”
However, engineers tend to be somewhat distantfrom the field, thus end up developing technologies
on the basis of their own ideas and proposing theresults of that development to clients. Unfortunately,
technologies and services developed through thatprocess are often dismissed by clients. For these reasons, I value timely decisions taken by the front-line
employees.
—I see that you value the perspective from the frontline. Finally, what would you like to say to people
inside and outside the company?
I strive to develop human resources with enthusiasm. As for nurturing people, it is important to havethe ability to identify who they are and what kind ofsense they have. That is because not everyone grows
up in the same time or at the same rate. If people’s
growth rates differ, so will their senses. I believe thatthe ability of top executives is determined by whether
they can give a job that matches the individuality ofeach employee. We always keep in mind that we needto assess the individuality of the person we want tonurture in terms of whether the job is too difficult or
too easy for them.
I would like to say to our partners that global competition for talent has become increasingly fierce.
The media reports on layoffs at global technology
companies may lead you to believe that the IT workforce is already saturated; the reality is different.
Information and communication technology is

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changing at a dizzying pace, and the human resources
required are changing as well. Therefore, we arecompeting to acquire human resources who can copewith that change. Moreover, with the advent of various types of AI, it is easy to imagine that all computer systems will be replaced with AI in the future.
However, it is people who are operating those systems, and computer systems and software will continue to be necessary. That is why people will continue to be important. I want to work together in taking responsibility for nurturing not only people
within our company but also all our partners.
I would like to say to our clients and prospective
clients that our highly skilled employees are ready tomeet your needs and I hope you will choose us.
Finally, I’d like young engineers to immerse themselves in the study of technology and, as the common
adage goes, “forget about eating and sleeping.” Thevalue of software and computer system technologies
is determined by whether people use them, so if you
have the chance, gain experience in the field while
you are still young and grow as an engineer.

Interviewee profile
Career highlights
H i r o s h i To m i y a s u j o i n e d N T T D ATA
Communications Systems Corporation (currently
NTT DATA Corporation) in 1990. In his career at
NTT DATA, he became general manager of the
Research and Development Headquarters in
2007, head of the Center for Applied Software
Engineering of Research and Development
Headquarters in 2011, head of the System
Architecture Department of the Infrastructure
System Sector, Technology and Innovation
General Headquarters in 2015, and head of the
System Engineering Headquarters of theTechnology and Innovation General Headquarters
in 2017. He assumed his current position in June
2020.

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Front-line Researchers

Researchers Have a Responsibility toCreate New Knowledge andTechnology and Let That
Responsibility Be Rewarding
Masaya Notomi
Senior Distinguished Researcher, NTT
Basic Research Laboratories and Project
Leader of NTT Nanophotonics Center
Abstract

NTT Nanophotonics Center is investigating photonics-electronics convergence technologies with the aim of creating lowlatency and low-energy-consumption optical computing technology. As various types of photonic information processing are
becoming possible, Masaya Notomi, a senior distinguishedresearcher at NTT Basic Research Laboratories and project leaderof NTT Nanophotonics Center, is pursuing the control of opticalproperties of materials and device applications by using artificial
nanostructures. We interviewed him about the progress of his
research activities and his attitude as a researcher.
Keywords: nanophotonics, photonics-electronics convergence, topological photonics

Pursuing new phenomena and photonic
processing devices by using
integrated nanophotonics—Our last interview was in 2020. Can you tell us
about the progress of the research you are conducting?
Since joining NTT, I have been pursuing the discovery of new phenomena and development of photonic processing devices using integrated nanophotonics. More specifically, I have been engaged increating new nanophotonics structures by using stateNTT Technical Review

Vol. 21 No. 5 May 2023

of-the-art nano/microfabrication techniques, discovering new physical phenomena manifested in thosestructures, and developing technologies to fabricate
photonics-electronics convergence information-processing chips with ultrahigh speed and ultralowenergy consumption.
Modern photonics technology is the driving force
behind high-capacity information transmission suchas long-distance optical-fiber communications andserver-to-server communications within datacenters.
Research is trending toward applying the high-speed
and large-capacity properties of light to the circuits
inside devices, and the ultimate form of the application
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Front-line Researchers

AMP: amplifier
NLO: nonlinear optics

Fig. 1. Conceptual diagram of photonics-electronics convergence accelerator.

of light is the configuration of optical network circuits in computer chips and direct information processing using light. Although the creation of an optical computer has long been one of the major goals of
researchers in the field of optics, in a world wherecomplementary metal-oxide semiconductor (CMOS)
electronic circuit technology is gaining prominence,the significance of using light for arithmetic processing has not been recognized in terms of ease of implementation and cost.
As the limits of miniaturization and integration of
CMOS technology (the so-called “end of Moore’slaw”) gradually approach, expectations for arithmetic
processing taking advantage of the high-speed natureof light are growing. Integrated nanophotonics technology makes it possible to achieve such processing.
With nanophotonics technology, highly advanced
semiconductor nano/microfabrication techniques areused to create nanometer-scale structures that are thesame size as (or smaller than) the wavelength of light,
which enables miniaturization of devices and creation of new optical properties that would be unfeasible with conventional optical devices. An example
result of the application of nanophotonics technology
is a photonic crystal, which is an artificial structure inwhich refractive index is periodically modulated on asubmicron scale. We have used this crystal to fabricate ultrasmall devices and devices with ultralowenergy consumption.
Although optical arithmetic processing is anticipated, it is currently difficult to execute various general-purpose processing operations on optical circuits
only. Therefore, we focused on combining the complex digital signal processing and large-capacity
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memory of electronic circuit technology with opticalcircuit technology that carries out the processing thatlight excels at to accelerate specific arithmetic processing operations, namely, functionalization of the
optical circuit as an accelerator. In addition to digital
signal processing, analog signal processing plays animportant role in machine learning, which is increasingly in demand, and the value of using light in analog signal processing is being recognized. We havetherefore set a goal of creating a photonics-electronics convergence accelerator that combines CMOS
electronics and nanophotonics, as shown in Fig. 1.
This accelerator will enable (i) light-speed operations
exploiting optical propagation (especially opticalinterference), (ii) improvement in the efficiency ofphotoelectric conversion and digital-to-analog conversion through the fusion of photonics and electronics at the arithmetic processing level, and (iii) generation of effective nonlinear optical properties by using
optical transistor technology.
—You are involved in research with high academic
and social expectations, right?
Under the title of “Development of a photonicselectronics convergence computing infrastructure
utilizing spatial, temporal, and wavelength degrees of
freedom,” our research on developing a low-latency
and low-energy-consumption computing accelerator
was selected as a research project of the Japan Science and Technology Agency’s Core Research for
Evolutionary Science and Technology (JST-CREST)in 2021 in conjunction with the National Institute ofAdvanced Industrial Science and Technology (AIST),
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Front-line Researchers

Nagoya University, Kyoto University, and Kyushu
University. In this research project, we are pursuing
photonics-electronics convergence information processing, which combines transmission using opticaltechnology with CMOS technology to enable highspeed signal processing that takes advantage of theproperties of light. On the basis of the photonicselectronics convergence technology to be implemented around 2030, which is the commercialization
target of the Innovative Optical and Wireless Network (IOWN), we are considering the application ofthis information processing to a computer system thatuses optical signals for certain information-processing operations in the chip.
Two key points must be addressed in this research
project. The first is to solve the problem regarding the
integration of CMOS circuits and optical circuits.
The key to this integration is to create high-density
optical-electronic interfaces by reducing the size andenergy consumption of electrical-to-optical converters (EO conversion), such as optical modulators, andoptical-to-electrical converters (OE conversion),such as photodetectors. We have built the world’s
first femtofarad-level-capacitance photoelectric conversion device using photonic-crystal technology anddemonstrated that by suppressing capacitance, photoelectric conversion is possible with very little energy
consumption. I believe that this technology will
enable the first photonics-electronics convergencecomputing system that seamlessly fuses light andelectricity to achieve both high computing power andlow power consumption.
Another important aspect of this highly efficientphotoelectric conversion technology is the ability to
add new functions to light. By using integrated nanophotonics technology, we developed a nanophotodetector-nanomodulator integrated device (OEO-conversion device), a photonics-electronics-converged
device with both OE conversion and EO conversion
functions and is very small (10×15 µm2). We havesucceeded in operating this device as an optical transistor. Since optical transistors can execute nonlinear
processing, which has conventionally been difficult
with light, we believe this device can be used for
nonlinear processing, as shown in Fig. 1, and be a key
device in photonics-electronics convergence neuralnetwork (NN) processing.

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A critical step toward ultralow-latency andlow-energy-consumption information
processing systems—You are achieving fantastic results. Would you tell
us about the second key point in the research project?
The second key point is arithmetic operations using
optical interference. Optical interference enables
operations to be executed at the speed of light. We
proposed an optical parallel adder as an application ofbinary-decision-diagram (BDD)-based digital arithmetic. This arithmetic unit executes digital addition
through a single photoelectric conversion and opticalpropagation and is expected to achieve a latency
about one order of magnitude lower than that of
CMOS adders. In cooperation with a research groupof AIST, we fabricated a prototype of a 4-bit opticaladder and successfully demonstrated full additive
operations. Regarding the BDD-based digital arithmetic, we found that circuit size increases exponentially by using multipliers, etc. and searched for ameans to avoid this problem. We discovered that circuit size can be reduced using the above-mentioned
optical transistors at key points.
Optical interference is an analog operation, so lightis most advantageous in analog operations. A typicalexample of an analog operation that can be executedat the speed of light is a multiply-accumulate operation, which is the most-fundamental operation and aprocessing bottleneck in deep learning and artificialintelligence processing using modern NNs. Therefore, we are investigating a photonics-electronics
convergence NN architecture using multiply-accumulate operations exploiting optical interference.
We plan to execute a multiply-accumulate operation with an optical circuit using optical interference
and execute nonlinear processing called activation,
which is necessary at key points in NN processing,with an optical transistor. We then envision a scenario
in which we integrate this high-speed analog computing circuitry using light with a CMOS processor byusing highly efficient photoelectric conversion technology. In this scenario, the conversion of electrical
digital signals to optical analog signals is important.
We discovered that digital-to-analog conversion withultralow latency is possible with the above-mentioned linear-interference logic gate technology andsuccessfully demonstrated circuit operation in collaboration with AIST.
To develop component technologies in accordancewith the above-mentioned scenario, NTT and AIST
8

Front-line Researchers(a) Topological photonic crystal
loaded with phase-change material(b) Non-Hermitian photonic crystal
loaded with graphene

Fig. 2. Structural diagram of a photonic crystal.

developed a prototype optical circuit using advanced
silicon-photonics integration technology and areverifying the principle on which the prototype isbased. At the same time, a research group at Kyushu
University, which has expertise in computer architecture, has been studying an architecture for a photonics-electronics convergence computing system having scalability for large-scale operations achieved byintegrating the above-mentioned multiply-accumulate operation unit with CMOS arithmetic circuits.
This research project aims to develop a photonicselectronics convergence accelerator that can operate
cooperatively with CMOS by making the most of thecharacteristics of light-speed operation. We expectthat this development will be an important step
toward implementing an ultralow-latency and lowenergy-consumption information-processing systemthat is required in the post-Moore era.
—I heard that you have achieved new results concerning topological photonics, which you talked
about in the previous interview.
As I reported in the previous interview, we are also
conducting basic research to discover new propertiesof light; in particular, we are working on a new field
called topological photonics. This field involves thecreation of new physical properties of light by applying the concept of topology to photonic crystals,
which we have been researching for many years, andis currently the subject of research activities worldwide. We are particularly focused on developing a
dynamically reconfigurable optical-topology system.
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In topological photonics, new properties are constantly being discovered, and if we can dynamicallycontrol these properties, we expect to be able toachieve operations that are currently impossible withconventional optical devices and circuits. We aim topropose two methods for controlling light throughoptical topological phase transition and dynamicchange in optical topology: (i) loading and modifying
a nanophotonic structure with functional nanomaterials and (ii) using the peculiar properties of non-Hermitian optical periodic systems using the imaginary
part of the refractive index*1.
Regarding the technology for controlling a nanophotonics structure by adding nanomaterials, we areinvestigating a method of loading a photonic crystal
with a phase-change material, the optical propertiesof which change significantly with temperature andother factors. By patterning germanium-antimonytelluride (GST)*2, an excellent phase-change material, and loading it on a photonic crystal, as shown in
Fig. 2(a), we found through theoretical analysis that
it is possible to induce an optical topological phasetransition through a phase change of GST. To demonstrate this theoretical phenomenon, we developed
technology for fabricating nanostructures using film
deposition and high-precision alignment techniquesand succeeded in fabricating silicon topological*1 Imaginary part of the refractive index: The imaginary part of therefractive index is expressed in complex numbers. A non-Hermitian photonic crystal uses the imaginary part of the refractive
index in its optical periodic system.
*2 GST: A compound consisting of germanium (Ge), antimony (Sb),and tellurium (Te) in the formula Ge2Sb2Te5.

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Front-line Researchers

photonic-crystal structures loaded with submicronscale GST. We are currently attempting to induce an
optical topological phase transition through the crystalline amorphous phase transition of GST.
Thus far, we have developed a laser using an ultracompact current-injection photonic crystal nanoresonator. Using this laser, we recently fabricated—as aworld’s first—a coupled system in which two nanolasers are integrated, and used this system to generate a
special light state called an exceptional point peculiar
to non-Hermitian optics. We also succeeded inobserving direct emission of light from this exceptional point. We proposed a method for fabricating a
new type of photonic crystal called a non-Hermitian
photonic crystal, in which the imaginary part of therefractive index is periodically modulated. As shownin Fig. 2(b), this fabrication process involves patterning graphene*3 loaded on a photonic crystal withsubmicron periodicity. We clarified that an exceptional point can be formed on this structure.
We are currently conducting experiments related togenerating optical topological-phase transitions andforming topological singularities, which have been
initially observed, to obtain quantitative data. For
non-Hermitian photonic crystals, we fabricateddevices that can control the imaginary part of therefractive index using a technique for selectively
loading graphene and researching the control of
exceptional points and topological singularities.
Although research on topological photonics andnon-Hermitian optics is still in the exploratory basicresearch stage, a variety of unknown properties thatdiffer from those of conventional materials have been
discovered, and exciting research and development isbeing conducted worldwide. Singularities with peculiar properties, such as exceptional points and topological singularities, have been reported, and we areconducting our research with the expectation that wewill find new means of optical control that will use
those properties.
Having light execute computation—I understand that your research on optical transistors was the impetus for the IOWN concept.
I am glad that our research on photoelectric conversion and optical transistors based on nanophotonics
was one of the triggers that hatched the IOWN concept, which aims to introduce light into all areas ofcommunication networks. That said, it was a happy
accident. I am quite interested in having light execute
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computation (in addition to communication), and I
believe that light can do a lot more. This idea may
sound a bit far-fetched, but in the brain, computations
are executed as a result of signals propagating
through the NN in the brain. I thought that light,
rather than electricity, was more appropriate for thosecomputations. Computing with optical networks canbe much faster, so things that were previously
unimaginable become possible. In other words, I
hope that the pursuit of having light execute computation will lead to the technological elements that will
form the foundation of IOWN.
Realizing this idea of having light execute computation through the development of an optical integrated circuit was up for internal discussion during
our research on photonic crystals, but we thought itwould be a difficult target to achieve. In fact, our collaboration with experts in computer science at JSTCREST led to the actual consideration of the idea. As
we began to work with experts in completely different fields, what had previously been a vague idea
became a concrete research topic. Through that experience, I realized—even more strongly—that it isimportant to look at other fields and other worlds.
—What do you think researchers are to society? And
what would you like to say to future generations ofresearchers?
It is our responsibility as researchers to create newknowledge and technologies, and I believe that sucha responsibility can be rewarding. If there are no
more researchers, current technologies will be unusable or not even exist. In that sense, researchers arenecessary for society and to move it forward.
To have my responsibility be rewarding, I try toensure enough time for my research. Routine tasks
come up constantly, but I allocate my time wisely. I
think it is important to set aside half a day, or even afull day, for me to concentrate and think freely without being overwhelmed by routine tasks. Having said
that, I’m currently in a difficult situation as a
researcher because I do not have enough time tospare. If I become aware that I’m in such a situation,
I try to find such time, even if I have to push myself.
Finally, to all researchers, remember when you firstjoined a lab at university and the feeling you had
when you were conducting experiments and research
for the first time. The research and experiments musthave been exciting, right? Today’s researchers are*3 Graphene: A crystal composed of a single atomic layer of carbon.

10Front-line Researchers

really busy every day, and I think it is easy to forget
what we were like back then, but let’s not forget the
fun of pursuit and create that fun by ourselves. Exciting research requires preparation. I want you to conduct research in a manner that enables you to continue
with that fun.
To students who aspire to become researchers, I
feel that Japan today has too few students going on to
doctoral programs. In Europe and the United States,
a doctoral degree is recognized as a status not only for
researchers but also for engineers. Therefore, if you
aim to become a researcher or engineer who can play
an active role in the world, you should obtain a doctoral degree. When you decide what you want to be/
do in the future, don’t look at society from a shortsighted perspective but from a long-term perspective.

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Interviewee profile
Masaya Notomi received a B.E., M.E., andPh.D. in applied physics from the University ofTokyo in 1986, 1988, and 1997. In 1988, hejoined NTT. Since then, his research interest has
been to control the optical properties of materials/
devices by artificial nanostructures (quantum
wires/dots and photonic crystals). He is also a
professor at Tokyo Institute of Technology. He
received the IEEE Laser and Electro-Optics
Society Distinguished Lecturer Award in 2006,
JSPS prize in 2009, Japan Academy Medal in2009, and the Commendation by Japanese
Minister of Education, Culture, Sports, Scienceand Technology in 2010. He served as a memberof the National University Corporation Evaluation
Committee in the Japanese government. He is anIEEE Fellow and member of the Japanese Societyof Applied Physics, American Physical Society,and Optica.

11

Rising Researchers

Network Virtualization Technology
for an NFV Infrastructure to Drive
Innovation in the Network
Yasufumi Ogawa
Distinguished Researcher, NTT Network
Innovation Center
Abstract

As network services become increasingly diversified, the difficulty of predicting and dealing with spikes and fluctuations indata communications traffic is becoming apparent. However,studies are now underway at many telecom operators on ways ofsolving this problem by applying network virtualization technology for a network functions virtualization (NFV) infrastructure(software virtualization) to network services. In this article, wetalked with Distinguished Researcher Yasufumi Ogawa about
network virtualization technology for an NFV infrastructure.
Keywords: virtualization, NFV infrastructure, open source community

Achieving a low-cost, high-performance network
through the virtualization of network software—Mr. Ogawa, please explain to us the meaning of
network virtualization technology for a network functions virtualization (NFV) infrastructure.
Before I talk about network virtualization technology for an NFV infrastructure, I would first like toexplain “virtualization infrastructure.” Virtualization
infrastructure refers to an infrastructure system andtechnology that can flexibly allocate the computerresources needed to meet a variety of requirements byusing virtualization technology. For example, a highperformance machine would be necessary to run
artificial intelligence or deep learning programs thathave attracted much attention in recent years. For anindividual, this would require that the parts and comNTT Technical Review

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ponents making up the machine be procured,arranged, and assembled on one’s own. However, thisprocess can be simplified by using a virtual machine.
In other words, a virtual machine enables an individual to freely use a high-performance machine bytuning and occupying a portion of a huge actualmachine in a datacenter for one’s own purposes. At
this time, a variety of requirements may come intoplay such as robust security functions and high network performance, but a machine that instantly meetsthese requirements can be virtually prepared by simple operations such as mouse clicks and menu selection. This is a technical overview of virtualization
infrastructure.
Network virtualization technology for an NFV
infrastructure focuses more attention on network
functions. It can be called, in general, NFV. In thisregard, the core functions of a large-scale network
12

Rising Researchers

Tacker overview
1. Lifecycle management function for virtualized resources using ETSI NFV standards-based virtualized
network function (VNF) package and representational state transferful application programming
interfaces (Restful APIs)
2. Extendibility for connecting to diverse VNFs and virtualized infrastructure managers (VIMs) as a
generic virtual network functions manager (G-VNFM)

1. Standard APIs for managing
several types of computerresources

2. Support fordiverse VNFs
and VIMs

Fig. 1. Overview of OpenStack Tacker.

have extremely strict requirements in terms of quality
and performance, but with conventional technology,
these could only be met by using expensive dedicatedequipment. On the other hand, network virtualization
technology for an NFV infrastructure enables such
expensive dedicated equipment to be replaced withgeneral-purpose servers and software, which enables,in turn, the construction of a large-scale network in a
simple and low-cost manner. Specifically, implementing software that performs the functions ofdedicated equipment in existing general-purpose
servers makes it possible to construct the equivalentof such dedicated equipment without any physicalconstruction work. This approach offers great advantages for network operators who are concerned with
reducing network development costs and shortening
development periods.
—What are you doing to achieve network virtualization technology for an NFV infrastructure?
Although elemental technologies such as design
techniques for virtual machines themselves and theconcepts behind virtual networks have already been
established, the current state of affairs is that there aremany software development projects in progress.
And while implementations are actively moving forward in the open source community and elsewhere,
there is still much to be done in meeting requirements
such as quality and availability to achieve practical
networks. For this reason, NFV standards specified
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by the European Telecommunications Standards
Institute (ETSI), a standardization organization centered in Europe whose members include telecom
operators in many countries, are being implemented
and provided in software called OpenStack Tacker as
an initiative for creating new value (Fig. 1).
Tacker is one of the major OpenStack projects thataims to achieve NFV through virtualization infrastructure software. By replacing this with a version
fully compliant with ETSI NFV standards, our goal is
to be the first in the world to construct a practical andhigh-quality network infrastructure system. Although
the standardization of NFV was already in motion
when I began my research, the standards themselves
have been evolving to keep up with an industry andrequirements that change from year to year. At ETSI,
there is a specific update plan for NFV standards, andat present, standards first drafted in Phase 1 havereached Phase 4.
In terms of specific activities, I am currently serving as a Project Team Leader (PTL) in the OpenStack
Tacker project, and in addition to providing sourcecode that I write myself, I coordinate decisions on
project development policy and take part in Tacker
promotional activities. My duties, however, go
beyond simply ensuring conformance to ETSI NFVstandards targeted by Tacker—I am also involved inproviding feedback on problems discovered during
development. Furthermore, in addition to making
contributions as a developer, I contribute to the
expansion of network virtualization technology itself
13

Rising Researchers

from the standpoint of practical development byinteracting with a diverse range of players such as
telecom operators and vendors both inside and outside Japan.
—What difficulties come up in the research of network virtualization technology for an NFV infrastructure?
In OpenStack, each project conducts development
work independently, and as part of that work, a PTL
has discretionary powers such as deciding development policy, formulating rules, and recommending
candidates for becoming core members. Moreover, asa project representative, I am also involved in theoverall administration of OpenStack. There is a lot of
hard work that comes with these activities. For example, I coordinate the extraction of requirements andfeedback related to requests for improvements for
input to ETSI members, and in the case of a functionthat NTT would like to include as a requirement, I
must make the rounds in obtaining approval fromeach member. As a specific activity to this end, I promote members that will have a significant say atmeetings (core members) from within NTT andamong corporate partners with which we closely
exchange opinions. I am also involved in providing
patches to source code, contributing reviews on
improving the quality of patches from developers,
sharing knowledge through presentations at international conferences such as OpenInfra Summit, andcoordinating activities at OpenStack developer meetings (Project Teams Gathering). It is through efforts
like these that I would like to disseminate a variety oftechnologies in society.
Moreover, at the stage of technology implementation, interconnectivity, etc., any problems that occur
must be immediately dealt with. The development oflarge-scale software like OpenStack makes use of atechnique called continuous integration/continuous
delivery (CI/CD) as a form of quality management toguarantee the operation of the product itself. Many
steps in this technique from source code testing tooperation checking are automated. Nevertheless, in
actual development, there is a greater than 90% probability that some problems will occur such as design
defects in the tests themselves or compatibility issues
in libraries due to long-term development. Of course,
dealing with these problems may seem like a straightforward issue, but to maintain this sense of normalcy,
they must be understood with a broad range of knowledge about software design in existing code. AddiNTT Technical Review

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tionally, when a problem arises, its cause must be
analyzed and corrected promptly, so we make a continuous effort to maintain advanced software development skills. Today, as well, new implementations
are being incorporated in OpenStack one after another, and keeping up with such a massive code base andguaranteeing its quality is a painstaking effort. On theother hand, I feel that taking on such a challenging
task is very rewarding.
Our goal is to provide services that meet theneeds of network operators—What kind of world can be achieved through network virtualization technology for an NFV infrastructure?
As network services continue to diversify, network
operators are finding it increasingly difficult to predict and respond to spikes and fluctuations in data
communications traffic. As a result, we are entering asituation in which ensuring the reliability of communications as a social infrastructure at the time of alarge-scale disaster or other calamity is difficult.
There is a particular need to deal quickly with suchproblems in the mobile network in which the smartphone has become an essential part of daily life.
Similarly, in the so-called fixed network that has been
supporting telephone services up to now, the conventional way of thinking has been to increase earnings
through long-term use of dedicated equipment developed at high cost. At present, however, this model nolonger holds due to shortening of the development
period and the time to provision as well as shorteningof the service lifecycle. Against this background, I
believe that we can fundamentally solve a variety ofproblems and be of great assistance to many network
operators by applying network virtualization technology for an NFV infrastructure to these network services to enable speedy and flexible service provisionthat keeps implementation and maintenance costs
down.
In addition, providing a virtualization infrastructure
system conforming to ETSI NFV standards has theadvantage of ensuring interoperability with a variety
of products. Today, large-scale systems are flooded
with products having vendor-specific connectivity inwhich virtualized network function (VNF) testing
costs and periods have ballooned and become a problem. Our goal, therefore, is to reduce testing costs ona scale of several billion yen by performing standardized implementation of virtualization infrastructure
14

Rising Researchers

Goal of OpenStack Tacker• Since VNFM is an area that does not fully conform to standards, products with vendor-specific connectivity
are widespread resulting in high VNF testing costs after procurement.
• Promote carrier-led development of OpenStack Tacker (OSS) and use as a reference implementation(standard and de facto implementation)
Before deployment:

After deployment:

VNFM (vendor-specific)

Low interoperability ofvendor-specific VNFM
for connecting with
VNFs and VIMs

After procurement, control
testing from NFVorchestrator (NFVO) to
VNFs must be performed
for each VNFM resulting
in ballooning costs.

G-VNFM(standardized)

Standardized VNFM
(eliminates
vendor-specificcharacteristics)
Control testing fromNFVO to VNFs is
unnecessary thereby
reducing testing costs.

OpenStack Tacker• Reference implementation• Standardizes APIs

Vendor A

Vendor B

Vendor C

Standard APIs

Fig. 2. OpenStack Tacker usage plan and goal.

technology via Tacker to guarantee interoperability
and eliminate vendor-specific characteristics (Fig. 2).
Furthermore, the Innovative Optical and Wireless
Network (IOWN) vision proposed by NTT includes
an initiative to develop new server infrastructuretechnology that can both satisfy service requests andreduce power consumption by combining diverse
types of devices as a disaggregated computing platform. In terms of software technology in such a lower-layer region, I believe that Tacker can be used tocontrol computing resources on the controlled sideand virtual resources installed as software in particular. In this regard, we are endeavoring in our researchto achieve a world in which network virtualization
technology for an NFV infrastructure can be provided
as an infrastructure service incorporating a security
and data-centric platform integral to IOWN in lowerlayer software.
—What are some important ways of thinking inresearch and development (R&D)?
As a researcher, I place value on what personallydrives me and gives me satisfaction. In softwaredevelopment that I am involved in, a development
team has a deadline to meet for each project, and to
improve quality while satisfying as many requirements as possible within such an environment, I am
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sometimes forced as a project leader to pick andchoose. At NTT laboratories, I am thoroughly
involved in software development and I consider
software to be a product of “distress” and “compromise.” In short, it is not simply a matter of selecting apath to “compromise.” What I want to do is to makethe results of our efforts even more useful by placing
importance on the “distress” involved in searchingfor a method that can be achieved in some way. In thisway, I believe we can provide services that willplease many people and that will provide a sense ofaccomplishment in R&D not just for me but for allteam members as well.
I also place importance on doing work that can bepassed on to the next generation. For example, thework that I am doing has the possibility of being
passed on to someone else in the future, so in mydaily R&D work, I must keep in mind the process ofcontinuously revising and upgrading software design
while writing program source code in an orderly andclear manner. By so doing, I am not just creating
something that will run well as a function. Rather, I
am accumulating many things that anyone can understand just by reading and that can be easily taken
over. This is not only for the present—my aim here is
to pass the baton on to future R&D.

15

Rising Researchers—Mr. Ogawa, can you leave us with a message forresearchers, students, and business partners?
The NTT Network Innovation Center that I belong
to is a newly established organization based on the
IOWN vision. Its origin can be found in the extensive
history of NTT’s R&D on telecommunications technologies over many decades, and in my mind, it’s anorganization that aims to consolidate those achievements toward an even greater goal. I feel that it covers
a wide range of research fields from the core network
to the access network and that it employs a large number of professionals especially in the practical implementation phase. For example, in network server
software, the field that I specialize in, we are involvedin the development of various types of applications
used by NTT operating companies while having otherduties too at standardization bodies and open software communities. I believe that the great strength of
R&D at NTT is the many professionals in each field,
the use of diverse connections including NTT Group
companies, and an environment in which ideas andopinions can be easily exchanged. In addition, NTT
has much influence not only within the NTT Groupbut also with other companies and communities bothin Japan and overseas. There are many people thathave listened positively to our proposals and become
good allies, so for researchers who would like to be

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globally active, I think that NTT laboratories can be avery attractive place to work.
Our work in the development of network infrastructure technology using open source software(OSS) is very practical in nature within the practicaldevelopment phase of R&D. Additionally, since weengage in daily discussions on future network technology and become involved with a variety of companies and organizations, we feel that our efforts are
extremely worthwhile having a great impact on society. International conferences such as OpenInfra
Summit, moreover, give us the opportunity to play anactive role at gatherings of prominent researchers
from around the world. In addition, standardization
organizations and other open source communities canhave a great impact on the world since they enable
many companies to come together to create standards, specifications, and de facto products in a cooperative manner. To those individuals who would liketo take part in such a global community as a softwaredeveloper or who have an interest in large-scale
themes such as creating the network infrastructure of
the future, I sincerely hope that you will join us in our
activities.

Interviewee profile
Yasufumi Ogawa graduated from the Graduate
School of Engineering, Osaka University in 2003
and joined NTT the same year. He has belonged
to NTT Network Innovation Center since 2021
and has been a distinguished researcher at NTT
Network Innovation Center since 2022. He is
engaged in the research and development of network virtualization technology for an NFV infrastructure. He has spoken at many international
conferences including OpenInfra Summit 2020and DPDK Summit 2019. He serves as a project
leader in the OpenStack community.

16

Feature Articles: Plasmon Control Technology

Overview and Prospects for Researchon Plasmons in Two-dimensional
Semiconductor Systems
Norio Kumada and Kazuhide Kumakura
Abstract

Plasmons, which are collective oscillations of electric charges, have a wide range of applications insensors and other technologies. Plasmons have also been attracting attention as potential new information carriers due to their ability to be generated by light and propagate within a region that exceeds thediffraction limit of light. This article presents an overview of the research on plasmon control and basic
physical properties exploiting plasmons at NTT Basic Research Laboratories. Future prospects are also
discussed.
Keywords: plasmonics, graphene, semiconductor

1. Plasmons

2. Plasmonics

At elevated temperatures, electrons in a gas canseparate from atomic nuclei, resulting in the formation of positive and negative ions and constituting astate of matter referred to as plasma. Examples ofnatural plasma can be observed in phenomena such as
lightning and auroras. Artificial gas plasma can also
be generated through the application of high temperatures or high voltages for purposes such as welding,semiconductor processing, and nuclear fusion. Plasma can also exist in solids, derived from free electrons in metals and semiconductors. In solids,
because of the high charge density, the Coulomb
energy is higher than the thermal energy at ordinary
temperatures, causing the behavior of solid plasma todiffer significantly from that of gas plasmas. Whenthe electron density distribution in a solid becomes
nonequilibrium, an electric field is generated, and theacceleration of electrons in that field tends to restore
a uniform distribution of electron density. Thus, anoscillation analogous to the longitudinal waves in aspring occurs in the charge density with overshooting
and retraction due to inertia (Fig. 1). This plasma
oscillation, when treated as a quantum, is referred toas a plasmon.

One characteristic of plasmons is their confinement
to regions that are narrower than the diffraction
limit*1 of light at the same frequency. Technology thatapplies this characteristic in nanoscale regions isreferred to as plasmonics (Fig. 2). One example ofplasmonics is in biosensors, which use the sensitivityof plasmons to the state of the metal surface innanoparticles. Adhesion of target molecules to themetal surface causes detectable changes in plasmon
frequency and attenuation. Other applications include
surface-enhanced (or tip-enhanced) Raman spectroscopy, which uses the enhanced plasmon electric field
in metal nanoparticles or at the tip of needles, andhighly efficient photoelectric conversion. Plasmonics
has also been attracting attention in nanophotonics
and metamaterials. Nanophotonics deals with theconversion of light to plasmons and the control ofplasmon propagation in nanoscale regions. Metamaterials deals with the design of optical properties ofmaterials using plasmonic electric fields in periodic
structures that are smaller than the wavelength of light.

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*1 Diffraction limit: A limit on optical resolution that is caused by
the diffraction of light and is proportional to wavelength.

17

Feature Articles

Fig. 1. Longitudinal waves in plasmons (upper) and in a spring (lower).

Biosensor

Highly efficientphotoelectric conversion

Metamaterial

Plasmonics
Nanophotonics

Tip-enhanced
Raman spectroscopy

Fig. 2. Example applications of plasmonics.

3. Plasmons in two-dimensional
semiconductor systems
Research and application of plasmonics commonlyuse plasmons that appear on metal surfaces. However, such plasmons have a limited application range
due to significant losses and a lack of controllability.
In contrast, NTT Basic Research Laboratories focuses on plasmons in two-dimensionally confined electron systems in semiconductor heterostructures andin graphene, a single atomic layer of carbon. The
major advantage of two-dimensional (2D) electron
systems is the tunability of plasmon characteristics
through adjustment of an external electric field. This
enables electrical control of plasmon properties.
Since the Coulomb interaction in semiconductors is
weak compared with metals due to the lower electron
density, plasmons in a 2D electron system propagate
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Vol. 21 No. 5 May 2023

at a slower velocity with a shorter wavelength, akinto compression waves in springs with a weak spring
constant. The tunability and tighter confinement
resulting from shorter wavelength make 2D electron
systems ideal for applications in which metal surface
plasmons are not feasible. Furthermore, applicationof a magnetic field perpendicular to the 2D plane
generates edge magnetoplasmons, which are uniqueplasmons that propagate in one dimension along theedge of the electron system (Fig. 3).
4. Research initiatives by NTT Basic
Research Laboratories
Research at NTT Basic Research Laboratories
approaches plasmons in 2D electron systems fromboth applied and fundamental science perspectives.
Specifics are explained in the respective Feature
18

Feature Articles

Pr

Pr
o

op

pa

ag

ga

ati
on

tio
n

B

Graphene plasmon

Edge magnetoplasmon

Fig. 3. Illustration of a graphene plasmon and edge magnetoplasmon.

Articles, and a brief overview is given here.
4.1 Active spatial control of terahertz plasmons ingraphene
Plasmons in graphene feature low loss in the range
from terahertz to near-infrared frequencies and a tunable wavelength with the charge density. The firstarticle presents the results of plasmon excitations at
designated frequencies and locations by manipulating the spatial-charge-density profile in grapheneusing patterned gates [1, 2]. The goal of this researchis to construct dynamically controllable plasmonic
circuits on graphene.
4.2 U ltrafast optical-to-electrical conversion ingraphene
The second article describes the achievement of theworld’s fastest zero-bias operation of 220 GHz for agraphene photodetector based on the photothermoelectric effect*2 [3, 4]. Photodetectors are key devices
in optical communication equipment and sensors, etc.
This achievement demonstrates the high potential ofgraphene as a material for ultrabroadband and ultrafast photodetectors. Application of this achievement
would also enable on-chip plasmon excitation anddetection. Combining this achievement with the plasmon circuit described in the first article would enable
dynamic control of plasmon propagation.
4.3 T heoretical proposal for edge magnetoplasmon crystals
The results of a theoretical analysis of phenomena
and functionality in the propagation of edge magnetoplasmons through a network in a periodic 2D artificial crystal structure [5, 6] are presented in the third
article. The objectives are to understand the propagation of edge magnetoplasmons with a representation
close to that of light and apply the knowledge toimprove the manipulation of light.

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4.4 P
 lasmon propagation in a 2D electron-holesystemMeasurement of the plasmon-propagation characteristics to determine the state of a system is presented in the fourth article. Time-resolved transportmeasurement of edge magnetoplasmons is applied toa semiconductor heterostructure that includes 2D
electron and hole systems [7, 8]. This techniqueenables the clarification of the physical properties,
such as high-frequency propagation velocity anddamping mechanism, in these systems. The objective
is future application of the measurement method totopological quantum computation.
5. Prospects for the future
Since graphene was first fabricated by exfoliationfrom graphite in 2005, it became possible to fabricate
materials that do not exist in nature by exfoliating
various atomic-layer 2D materials and stacking them.
These new materials are expected to reveal new plasmon functions and elucidate physical properties
using plasmons. NTT Basic Research Laboratories
has been conducting research on plasmons in 2D
semiconductors from both basic and applied perspectives. We aim to continuously produce high-impact
results through collaboration among researchers
involved in these studies.

*2 Photothermoelectric effect: The generation of a voltage whenlight absorbed by a substance is converted to heat, creating athermal gradient in the substance. This effect can be used to convert optical signals to electrical signals.

19

Feature Articles

References
[1]

[2]

[3]

[4]

N. Kumada, “Electrical Control of Plasmon Reflectivity in Graphene,” NTT Technical Review, Vol. 21, No. 5, pp. 21–24, May 2023.
h t t p s : / / nt t - r e v i e w. j p / a r c h i v e / n t t t e c h n i c a l . p h p ? c o n te n ts =ntr202305fa2.html
N. H. Tu, K. Yoshioka, S. Sasaki, M. Takamura, K. Muraki, and N.
Kumada, “Active Spatial Control of Terahertz Plasmons in Graphene,” Commun. Mater., Vol. 1, Article no. 7, 2020. https://doi.
org/10.1038/s43246-019-0002-9
K. Yoshioka, T. Wakamura, and N. Kumada, “Ultrafast Optical-toelectrical Conversion Processes in Graphene,” NTT Technical
Review, Vol. 21, No. 5, pp. 25–29, May 2023.
h t t p s : / / nt t - r e v i e w. j p / a r c h i v e / n t t t e c h n i c a l . p h p ? c o n te n ts =ntr202305fa3.html
K. Yoshioka, T. Wakamura, M. Hashisaka, K. Watanabe, T. Taniguchi,
and N. Kumada, “Ultrafast Intrinsic Optical-to-electrical Conversion
Dynamics in a Graphene Photodetector,” Nat. Photon., Vol. 16, No.

Norio Kumada

Distinguished Researcher and Group Leader,
Quantum Solid-State Physics Research Group,
Quantum Science and Technology Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Tohoku University, Miyagi, in 1998, 2000,
and 2003. He joined NTT Basic Research Laboratories in 2003. Since then, he has been engaged
in the study of highly correlated electronic states
confined in two dimensions. From 2013 to 2014,
he was a visiting scientist at CEA Saclay, France.
He received the Young Scientist Award of the
Physical Society of Japan in 2008 and the Young
Scientists’ Prize from the Minister of Education,
Culture, Sports, Science and Technology in2012. He is a member of the Physical Society ofJapan.

NTT Technical Review

Vol. 21 No. 5 May 2023

[5]

[6]
[7]

[8]

10, pp. 718–723, 2022. https://doi.org/10.1038/s41566-022-01058-z
K. Sasaki, “Theoretical Proposal for Edge Magnetoplasmon Crystal,”
NTT Technical Review, Vol. 21, No. 5, pp. 30–33, May 2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c o ntents=
ntr202305fa4.html
K. Sasaki, “Band Structures of Edge Magnetoplasmon Crystals,”
Phys. Rev. B, Vol. 105, No. 7, 075312, 2022. https://doi.org/10.1103/
PhysRevB.105.075312
H. Kamata, H. Irie, N. Kumada, and K. Muraki, “Time-resolved Measurement of Ambipolar Plasmon Transport in Semiconductor Composite Quantum Wells,” NTT Technical Review, Vol. 21, No. 5, pp.
34–39, May 2023.
h ttp s ://n tt- r e v ie w. jp /a r c h iv e /n ttte c h n ic a l. p h p ? c o ntents=
ntr202305fa5.html
H. Kamata, H. Irie, N. Kumada, and K. Muraki, “Time-resolved Measurement of Ambipolar Edge Magnetoplasmon Transport in InAs/
InGaSb Composite Quantum Wells,” Phys. Rev. Research, Vol. 4,033214, 2022. https://doi.org/10.1103/PhysRevResearch.4.033214

Kazuhide Kumakura

Vice President, Head of NTT Basic ResearchLaboratories.
He received a B.E., M.E., and Ph.D. in electrical engineering from Hokkaido University in
1993, 1995, and 1998. He joined NTT Basic
Research Laboratories in 1998. His principal
research fields are thin film growth and device
applications of semiconductors. From 2007 to
2008, he was a visiting scientist at Paul Drude
Institute for Solid State Electronics, Germany.
He is a member of the Japan Society of Applied
Physics, the Electrochemical Society of Japan,
and the Materials Research Society.

20Feature Articles: Plasmon Control Technology

Electrical Control of Plasmon
Reflectivity in Graphene
Norio Kumada
Abstract

Properties of plasmons in graphene can be controlled electrically. My research colleagues and I haveused this feature to verify that plasmons of desired frequencies can be excited in electrically specified
regions. This technology can be applied to plasmonic devices such as waveguides and switches.
Keywords: plasmon, graphene, electrical control

1. Graphene plasmons
Plasmons are collective oscillations of electriccharges that have shorter wavelengths than electromagnetic waves at the same frequency and can be
confined to regions below the diffraction limit. The
technology for manipulating plasmons in nanoscaleregions is referred to as plasmonics. Plasmonics has
been used in practical devices such as biosensors, as
described in the article, “Overview and Prospects for
Research on Plasmons in Two-dimensional Semiconductor Systems” in this issue [1]. While surface plasmons excited on metal surfaces are commonly used
for plasmonics experiments, they have limitations
due to significant losses and poor controllability.
These drawbacks have hindered the practical implementation of nanophotonics in which plasmons areused to transmit information in nanoscale structures.
Graphene is attracting attention as a plasmonics
material that can overcome these problems. Graphene plasmons are known to have low loss in theterahertz and mid-infrared frequency range. Anotheradvantage is that they have a shorter wavelength thanmetal surface plasmons, which enables them to be
confined in smaller regions. Specifically, electromagnetic waves converted into graphene plasmons can be
confined to regions 1/1000th the wavelength of theelectromagnetic waves. Moreover, the relationshipbetween the wavelength and frequency of graphene
plasmons varies with the charge carrier density, providing functions that are not available with metals,
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Vol. 21 No. 5 May 2023

such as electrical control of the propagation speed
and wavelength of the plasmons by gating. Theseadvantages are expected to pave the way for electrically controllable nanophotonics and new applications, such as electrically tunable metamaterials*1.
2. Electrical control of graphene plasmons
The relationship between the graphene plasmon
wavelength (λ) and frequency ( f ) varies with the
charge carrier density (n): λ ∝ √n̄/f 2. This indicatesthat the wavelength decreases as the carrier density
decreases at a constant frequency. Therefore, plasmons are reflected at an interface where the charge
density changes abruptly, akin to how light is reflected at an interface between media with different
refractive indices. Since the carrier density can bevaried electrically using a gate electrode, electricallycontrollable plasmonic devices and circuits can be
implemented in principle (Fig. 1(a)). While a theoryof such active control of graphene plasmons wasreported in 2011, it is yet to be verified experimentally because of the technical difficulties in correctly
implementing the theoretical concept. For effective
control, it is necessary to induce a sharp change incarrier density within a region that is comparably
shorter than the plasmon wavelength. However,
implementing this using conventional metal gate
*1 Metamaterial: An artificial material the optical properties ofwhich are designed by plasmon electric fields in periodic structures that are smaller than the wavelength of light.

21

Feature Articles

n2 (< n1)

Carrier density n1
Injection

Transmission

Reflection(a) Plasmon reflection at carrier density boundary
Reflectivity varies with the carrier density difference.

w/o metal gate
Injection
Reflection

w/ metal gate
Transmission(b) Plasmon reflection due to the presence/absenceof the metal gate
Uncontrollable plasmon reflection occurs at theboundary regardless of the carrier density profile.

Fig. 1. Graphene plasmon reflection.

(a) Sample structure(b) Plasmon measurement by terahertz spectroscopy

SiO2: silicon dioxide
Al2O3: aluminium oxideFig. 2. Sample structure and measurement method.

structures is challenging as the plasmon electric field
distribution results in a substantial change at theboundary between regions with and without the gate
resulting from the screening effect*2. This effectcauses plasmon reflection at the boundary (Fig. 1(b)),
which is determined solely by the presence orabsence of the metal gate and not by the carrier density profile of the graphene, thus rendering plasmon
reflection uncontrollable.
3. Demonstration of graphene plasmon control
The results presented in this article indicate that theconfinement and reflection of graphene plasmons canbe controlled electrically by using a high-resistivity
zinc oxide (ZnO) thin film instead of metal as the gate
NTT Technical Review

Vol. 21 No. 5 May 2023

material, thus avoiding the uncontrollable reflection
caused by the metal gate [2]. The screening effect thatis inherent in metal gates can be suppressed when theresistance of the gate electrode is high enough that the
charges in the electrode cannot follow the oscillatory
electric field at plasmon frequencies. To attain thiscondition, my colleagues and I used 20-nm-thickZnO with high resistivity that was attained by adjusting the growth temperature as the gate material. The
sample used in the experiments had a two-layer gateconsisting of a ZnO thin film processed into strips of
2-µm width and 4-µm spacing on a low-doped silicon(Si) substrate (Fig. 2(a)), enabling independent control*2 Screening effect: A phenomenon in which free electrons in a
metal move when an external electric field is applied, cancelingout the electric field inside the metal.

22

Feature Articles

nSi = 0nZnO = 0

1-Transmission (%)

Carrier density
modulation

1-Transmission (%)

1-Transmission (%)

Uniform carrier density

Frequency (THz)
Frequency (THz)

Frequency (THz)

CNP: charge neutrality point

Fig. 3. Spectrum changes due to charge density modulation.

of the carrier density of graphene on the ZnO gate andSi gate (nZnO and nSi). The plasmonic response in thissample was examined by irradiating terahertz lightwith an electric field perpendicular to the strips andmeasuring the transmission spectrum (Fig. 2(b)).
Figure 3 shows the extinction spectra of this sample in the terahertz range. When the carrier density is
uniform, the absorption of terahertz light by grapheneincreases monotonically with decreasing frequency.
This is typical behavior observed in pristine graphene, indicating the absence of uncontrolled plasmonic reflections caused by the ZnO gate. However,a peak appears in the spectrum when the ZnO gate
and Si gate are adjusted such that the carrier density
in either region is set to zero (at the charge neutrality
point). Because plasmons are not excited in theregion at the charge neutrality point, the spectrum
peak can be attributed to plasmon resonance in a cavity formed in a region where the charge density is nonzero. We also succeeded in changing the resonancefrequency by using the gate. This shows that it ispossible to excite plasmons at the desired locationand frequency by adjusting the gate voltages. ThisNTT Technical Review

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active spatial control of plasmon excitation can beapplied to waveguides, switches, and resonators.
Continuously tuning of the plasmon reflectance atthe boundary is also possible by varying the difference between nZnO and nSi. Figure 4 shows spectra
for several nZnO values at a constant nSi. As the density difference is increased from a uniform condition,
the resonance peak increases with increasing reflectivity. The experimentally obtained plasmon reflectivity (red line in Fig. 4(b)) is consistent with FresnZnO – √¯¯nSi
nel’s law R = √used for light reflection.
n
¯
+
nSi
√ ZnO √¯This continuous control of plasmon reflectance canbe applied to plasmon modulators and splitters.
4. Future prospects
These achievements provide a platform for implementing the theoretical concept for electrically controllable plasmon circuit. For future work, we plan toadvance to experiments on directed plasmon propagation with controlled velocity and phase.

23Uniform condition

Reflectivity

1-Transmission (%)

Feature Articles

15%|nZnO| (×1012 cm−2)

Frequency (THz)(a) Spectra for several nZnO
at a fixed nSi values(b) Plasmon reflectivity as a functionof nZnO at a fixed nSi

Fig. 4. Plasmon reflection control.

References[1]

N. Kumada and K. Kumakura, “Overview and Prospects for Researchon Plasmons in Two-dimensional Semiconductor Systems,” NTT
Technical Review, Vol. 21, No. 5, pp. 17–20, May 2023. https://ntt-[2]

review.jp/archive/ntttechnical.php?contents=ntr202305fa1.html
N. H. Tu, K. Yoshioka, S. Sasaki, M. Takamura, K. Muraki, and N.
Kumada, “Active Spatial Control of Terahertz Plasmons in Graphene,” Commun. Mater., Vol. 1, Article no. 7, 2020. https://doi.
org/10.1038/s43246-019-0002-9

Norio Kumada

Distinguished Researcher and Group Leader,
Quantum Solid-State Physics Research Group,
Quantum Science and Technology Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Tohoku University, Miyagi, in 1998, 2000,
and 2003. He joined NTT Basic Research Laboratories in 2003. Since then, he has been engaged
in the study of highly correlated electronic states
confined in two dimensions. From 2013 to 2014,
he was a visiting scientist at CEA Saclay, France.
He received the Young Scientist Award of the
Physical Society of Japan in 2008 and the Young
Scientists’ Prize from the Minister of Education,
Culture, Sports, Science and Technology in2012. He is a member of the Physical Society ofJapan.

NTT Technical Review

Vol. 21 No. 5 May 2023

24

Feature Articles: Plasmon Control Technology

Ultrafast Optical-to-electrical
Conversion Processes in Graphene
Katsumasa Yoshioka, Taro Wakamura,and Norio Kumada
Abstract

Graphene photodetectors have gained significant interest due to their predicted high sensitivity andrapid electrical response to broadband light, offering the potential for superior performance comparedwith conventional semiconductor devices. However, the experimental evaluation of the operation speed
under a zero-bias voltage has been hindered by challenges associated with device architecture and measurement instrumentation, restricting the characterization of graphene’s intrinsic dynamics and clarification of its operating mechanism. This article presents an ultrafast graphene photodetector developed at
NTT laboratories, detailing the measurement technology and operating mechanism.
Keywords: graphene, photodetector, terahertz electronics

1. Graphene as an optical-to-electricalconversion device
Photodetectors (PDs) play a crucial role in opticalinformation communications and sensors by converting optical signals into electrical signals. Developinga PD that operates at high speeds for broadband optical signals creates new opportunities for advanced
information processing through optical sensors andreceivers working across a wide range of frequencies.
This requirement can be satisfied with graphene, apromising material that exhibits high efficiency with
its ability to absorb 2.3% of incident light with asingle atomic layer and operate in an ultrawide frequency band, from terahertz to ultraviolet light [1].
The photothermoelectric (PTE) effect enables graphene PDs to operate at zero bias, which is essential
for low power consumption and improved signal-tonoise ratio. However, previous research on graphene
PDs has limited the experimental operating speed
under zero bias to around 70 GHz [2], far from thetheoretical expectation of 200 GHz. The limitations
in device structure and measurement systems havehindered the investigation of the intrinsic response ofgraphene and the clarification of its operating mechanism, leading to a lack of design guidelines for maxiNTT Technical Review

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mizing the performance of graphene PDs. Therefore,
the demonstration of 200-GHz operation speed andthe elucidation of the physical properties of graphene,
including the optical-to-electrical (OE) conversion
mechanism, is crucial for further developments ingraphene PDs.
2. Ultrafast photocurrent detection based on anovel device architecture andmeasurement approach
Ultrafast OE conversion in a graphene PD requiresa device design that facilitates prompt tracking ofincident light signals and a measurement techniquecapable of high-speed photocurrent detection. To
meet these requirements, we used a zinc oxide thinfilm as a gate material to eliminate the current delaycaused by capacitive coupling (Fig. 1(a)) and usedon-chip terahertz spectroscopy [3] for photocurrent
readout (Fig. 1(b)). The OE conversion efficiency ofgraphene PDs using the PTE effect varies significantly with the charge density. Thus, a gate structure
must be integrated into the device. Because the gateelectrode must possess high conductivity, gold iscommonly used as the gate material; however, thislimits the operating speed to below 100 GHz due to25

Feature Articles

(a)

Drain

SourceA(b)
ZnO gate

Pump light

Photocurrent

Probe light

Drain electrode

Graphene
PD

Source electrodePC switch(a) Schematic of a graphene PD. Graphene is sandwiched between hexagonal boron nitride(hBN) as an insulator, and connected to metal source and drain electrodes. After being covered
with aluminum oxide (Al2O3) insulating film on the surface, zinc oxide (ZnO) is deposited on top
as a gate electrode. (b) Microscope photograph of an on-chip terahertz-spectroscopy circuit.
When pump light is applied to the graphene PD, the resulting photocurrent is propagated throughthe drain electrode and detected by illuminating the PC switch with probe light.

Fig. 1. Measurement system for ultrafast OE conversion.

unwanted capacitive coupling. We addressed thisissue by using a zinc oxide thin film, which enables
control of the high-frequency response by controlling
growth conditions. This material possesses a unique
combination of being a good conductor for directcurrent signals while being transparent at terahertz
frequencies [4]. As a result, we successfully eliminated the capacitive coupling-induced current delay.
The high-speed oscilloscopes commonly used to readout the photocurrent struggle to measure response
faster than 100 GHz because of the bandwidth limitation of the electronics. However, we overcame thischallenge by using on-chip terahertz spectroscopy,
which enabled us to measure the ultrafast response of
the graphene PD with a measurement bandwidth
approaching 1 THz through the detection of photocurrent using an on-chip photoconductive (PC)switch. The photocurrent generated by irradiating thegraphene PD with a pump pulse (femtosecond laser
pulse) propagates through the drain electrode to thePC switch (Fig. 1(b)). Because the PC switch
becomes conductive when irradiated by the probe
pulse (also femtosecond laser pulse), the signal flows
to the ammeter only when the generated photocurrent
and probe pulse overlap in time. Therefore, measurNTT Technical Review

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ing the photocurrent waveform with extremely hightime resolution is possible by constantly changing thetime difference between the pump and probe pulse.
Our innovative device architecture and measurement
approach have enabled us to explore the ultrafast OE
conversion mechanisms in graphene, which was previously unachievable using conventional methods.
3. Overview of the OE conversion processes
in graphene
The photocurrent waveform of the fastest response
among measured signals is shown in Fig. 2(a). The
various frequency components of the photocurrent
obtained by the Fourier transform (Fig. 2(b)) showthat the 3-dB bandwidth of the graphene PD reaches
220 GHz, more than three times the 70 GHz reported
previously and exceeding the theoretical expectationof 200 GHz. This is the first successful extraction of
the intrinsic response of graphene unimpeded bydevice structure or measurement method. To investigate the physical process that determines the dynamical response of the photocurrent, we first focused on
the decay time of the photocurrent by fabricating
multiple devices with different graphene mobilities.
26

Feature Articles

(b)

1.0
0.8

1
4

Current (arb. unit)

Current (arb. unit)

(a)

0.6
0.4
0.2

2

0.1
4
2

0.01
4

0.0

2

−5

0

5

10

15

20

0.001

25

3

4 5 6 78

0.1Time (ps)23

4 5 6 781

2

Frequency (THz)(a) Time evolution of photocurrent due to optical pulse illumination. (b) The magnitude of each frequency component of thephotocurrent obtained by the Fourier transform. The operating speed is 220 GHz, where the current magnitude drops by 3 dB.

Fig. 2. Verification of operating speed of 220 GHz.

Seebeck coefficient (µV/K)

(a)
300
200
100
0

(b)

Decay time (ps)

8

6

4

5

0
0.0

0.4

0.8

1.2

Mobility ×105 (cm2/Vs)(a) Seebeck coefficient (an indicator of sensitivity). Sensitivity becomes higher when the electron
scattering in graphene is small and mobility is high. (b) Decay time (inverse of operating speed).
As mobility increases, the scattering decreases and decay time increases (speed decreases).

Fig. 3. Tradeoff between detection sensitivity and operating speed.

Figure 3 shows that lower graphene mobility is associated with shorter decay times. This result suggeststhat the decay of the photocurrent in graphene iscaused by a decrease in the electron temperature in
NTT Technical Review

Vol. 21 No. 5 May 2023

graphene, which is increased by light irradiation.
Because of collisions between acoustic phonons anddefects, a sample with lower mobility and more
defects loses more energy in a single collision event,
27

Feature Articles

Charge density(a)High: 1.4×10 /m2

Low: ~0/m2

16

Current (arb. unit)

1.0
0.8
0.6
0.4
0.2
0.0
−10

−5

0

5

10

15

20

25

Time (ps)

(c)

(b)

Peak shift (ps)

5 µm

15 µm

4
3
2
1
0
0.5

1.0

1.5

2.0

2.5

LowCharge densityHigh(a) As the carrier density in graphene decreases, the time the current peaks after light irradiation becomes 4 pslater. (b) A schematic of the measurement configuration. The current is extracted from the lower drain electrode.
(c) The peak shift does not depend on the length or spot position of the graphene.

Fig. 4. Dependence of photocurrent-response time on charge density.

resulting in faster decay times. However, the Seebeckcoefficient, which corresponds to PD sensitivity,
increases as mobility increases (Fig. 3), so there is atradeoff between bandwidth and sensitivity in graphene PDs. Therefore, it is necessary to select optimal graphene mobility in accordance with the intended application.
To identify the mechanism that determines the photocurrent generation and propagation response(Fig. 4(a)), we investigated the dependence of thephotocurrent on the charge density. The results indicate that a decrease in charge density produced adelay in the peak position of about 4 ps. Such a significant peak shift is unprecedented and provides
important clues for understanding the OE conversion
processes. We can consider two possible causes: 1)
the propagation time of the photocurrent throughgraphene changes or 2) the time for photocurrent
NTT Technical Review

Vol. 21 No. 5 May 2023

generation after light irradiation changes. To test thefirst possibility, we measured the peak shift for various device lengths and the irradiation position of thepump light (Fig. 4(b)). All the results fall on a single
curve (Fig. 4(c)), indicating that the photocurrent
propagation time in graphene is too short to be measured, so the cause of the peak shift must come fromthe change in the photocurrent-generation time. The
ultrashort propagation time in a graphene PD resultsfrom its gapless nature. The absence of a bandgap ingraphene allows the electric field created by lightirradiation to propagate at the speed of light, instantly
displacing the electrons near the electrode and generating an immediate flow of current without the needfor propagation. The time variation in photocurrent
generation is due to the PTE effect, which transforms
the electron temperature into a voltage. This meansthat the non-equilibrium electron state created by the28

Feature Articles

light irradiation quickly settles into a Fermi-Dirac
distribution through intraband electron scattering.
Our results also indicate that the time it takes for thermal equilibrium is highly dependent on the charge
density of the graphene. These findings provide acomprehensive understanding of the ultrafast OE
conversion processes in graphene, including the generation, propagation, and decay of photocurrent uponlight irradiation [5].

ate, control the propagation of, and detect graphene
plasmons in the terahertz range. This opens up the
possibility for on-chip handling of plasmon signals,
bringing us closer to the goal of creating plasmonic
circuits.
References[1]

4. Future development
The results presented in this article indicate thehigh potential of graphene as a material for broadband, high-speed PDs. However, our fabrication
process of manually exfoliating graphene fromgraphite is not suitable for mass production. Whilethe quality of graphene deposited over a large area
has been historically inferior to that obtained throughexfoliation, the gap is closing as deposition techniques continue to improve. Future studies will focus
on evaluating PDs that use large-area graphene tofacilitate mass production. Additionally, research in
the field of creating novel materials by stacking graphene and other two-dimensional materials could beapplied to further enhance the operating speed ofPDs. The results of this study could be used to gener-

Katsumasa Yoshioka

Researcher, Quantum Solid-State Physics
Research Group, Quantum Science and Technology Laboratory, NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Yokohama National University, Kanagawa,in 2014, 2016, and 2019. He joined NTT Basic
Research Laboratories in 2019. Since then, he
has been engaged in the study of on-chip THz
spectroscopy of two-dimensional electronic systems. He is a member of the Physical Society ofJapan and the Japan Society of Applied Physics.

Taro Wakamura

Research Scientist, Quantum Solid-State Physics Research Group, Quantum Science andTechnology Laboratory, NTT Basic ResearchLaboratories.
He received a B.E., M. Sc., and Ph.D. in science from the University of Tokyo in 2010, 2012,
and 2015. He joined NTT Basic Research Laboratories in 2019 and constructed a semi-automatic heterostructure fabrication system with novel
two-dimensional materials including grapheneand transition-metal dichalcogenides (TMDs).
His current interests are broad, and he has
recently been working on nonreciprocal transport
in noncentrosymmetric TMDs. In addition to his
own projects, he also contributes to collaborative
works for developing novel PDs based on twodimensional materials such as graphene. He is amember of the Physical Society of Japan.

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[2]

[3]

[4]

[5]

M. Romagnoli, V. Sorianello, M. Midrio, F. H. L. Koppens, C.
Huyghebaert, D. Neumaier, P. Galli, W. Templ, A. D’Errico, and A. C.
Ferrari, “Graphene-based Integrated Photonics for Next-generation
Datacom and Telecom,” Nat. Rev. Mater, Vol. 3, No. 10, pp. 392–414,
Oct. 2018. https://doi.org/10.1038/s41578-018-0040-9
S. Marconi, M. A. Giambra, A. Montanaro, V. Mišeikis, S. Soresi, S.
Tirelli, P. Galli, F. Buchali, W. Templ, C. Coletti, V. Sorianello, and M.
Romagnoli, “Photo Thermal Effect Graphene Detector Featuring 105
Gbit s−1 NRZ and 120 Gbit s−1 PAM4 Direct Detection,” Nat. Commun., Vol. 12, No. 1, p. 806, Dec. 2021. https://doi.org/10.1038/
s41467-021-21137-z
K. Yoshioka, N. Kumada, K. Muraki, and M. Hashisaka, “On-chip
Coherent Frequency-domain THz Spectroscopy for Electrical Transport,” Appl. Phys. Lett., Vol. 117, No. 16, p. 161103, Oct. 2020.
https://doi.org/10.1063/5.0024089
N. H. Tu, K. Yoshioka, S. Sasaki, M. Takamura, K. Muraki, and N.
Kumada, “Active Spatial Control of Terahertz Plasmons in Graphene,” Commun. Mater., Vol. 1, No. 1, p. 7, Dec. 2020. https://doi.
org/10.1038/s43246-019-0002-9
K. Yoshioka, T. Wakamura, M. Hashisaka, K. Watanabe, T. Taniguchi,
and N. Kumada, “Ultrafast Intrinsic Optical-to-electrical Conversion
Dynamics in a Graphene Photodetector,” Nat. Photon., Vol. 16, No.
10, pp. 718–723, Oct. 2022. https://doi.org/10.1038/s41566-02201058-z

Norio Kumada

Distinguished Researcher and Group Leader,
Quantum Solid-State Physics Research Group,
Quantum Science and Technology Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Tohoku University, Miyagi, in 1998, 2000,
and 2003. He joined NTT Basic Research Laboratories in 2003. Since then, he has been engaged
in the study of highly correlated electronic states
confined in two dimensions. From 2013 to 2014,
he was a visiting scientist at CEA Saclay, France.
He received the Young Scientist Award of the
Physical Society of Japan in 2008 and the Young
Scientists’ Prize from the Minister of Education,
Culture, Sports, Science and Technology in2012. He is a member of the Physical Society ofJapan.

29

Feature Articles: Plasmon Control Technology

Theoretical Proposal for Edge
Magnetoplasmon Crystal
Ken-ichi Sasaki
Abstract

When a static magnetic field is applied to a two-dimensional electron system, plasmons appear at theedge of the specimen and exhibit peculiar behavior, such as the wave propagation to a single direction
along the edge. The calculated results of investigating plasmon propagation when a sample is divided
into regions and a crystalline network is formed indicate that the characteristics of the integer/fractional
quantum Hall effect, which has been developed in various fields, can be reproduced as optical properties.
Keywords: plasmon, crystalline network, edge state

1. Directed wave propagation
Propagating of light to a single direction is notstraightforward because when there is a scatterer,
light is reflected backwards, reversing the directionof propagation. To take advantage of straightness oflight, we use the total reflection by materials such as
in optical fibers. It is necessary to use the interaction
between light and materials to improve the control oflight. For the electrons in materials that emit light,
however, such directed propagation can be achieved
by applying a strong magnetic field perpendicular toa two-dimensional electron system.*1 The individual
electrons in the system take on a circular trajectory,orbiting in a direction that is determined by themagnetic-field vector (cyclotron motion, Fig. 1).
As is easily visualized on the microscopic level, adjacent electrons move in opposite directions, thus cancel out on the macroscopic level, leaving substantial
current produced by many electrons only at the edgesof the specimen. This situation is referred to as anedge state and forms a propagation channel along theedge. The propagation direction in the edge state iseither clockwise or counterclockwise, depending on
the vector direction of the applied magnetic field.
What happens when the edge state is excited has beenstudied since the 1980s. An excited state is a hybridof electrons and electromagnetic field, and it propagates in a single direction along the specimen edge. It
NTT Technical Review

Vol. 21 No. 5 May 2023

is called an edge magnetoplasmon, because it is a
plasmon created at the edge by an external magnetic
field.
2. Charge fractionalization
An interesting question is what would happen if asingle system in which edge magnetoplasmons circulate is divided into two regions by microfabrication.
We divide a specimen into two parts as shown in
Fig. 2, where the upper part is region 1 and the lower
part is region 2. We assume that edge magnetoplasmons exist on the edges of regions 1 and 2 and thatthe plasmons propagate counterclockwise. In thegroove between the two specimens, there are counterpropagating channels (like parallel roads or train
tracks) that interact if they are close enough to eachother through the electromagnetic interaction (Coulomb potential). Suppose we excited the magnetoplasmon in region 1. Upon reaching the coupled
region, a gradation in the charge density (positive andnegative charge pair) is formed in region 2, where
there had been nothing (Fig. 2(a)). Denoting thecharge of the edge magnetoplasmon excited in region
1 as Q, the charge generated at the entry of the*1 Two-dimensional electron system: A structure in which electrons
exist within a two-dimensional plane; a typical example is graphene, which is a planar sheet of carbon atoms that is only one
carbon atom thick.

30

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Electron motion becomes circular (cyclotron orbit) under application of a magnetic field that isperpendicular to the plane. Because the electrons are moving in opposite directions relative toadjacent electrons, the motions cancel out at the local level and there is zero net electron motionin the interior. At the edges of the specimen, however, edge states appear and a clockwise current
flows around the edges of the specimen.

Fig. 1. Conceptual diagram of a two-dimensional electron system.

(a) Propagation of an edge magnetoplasmon observed near the entrance to theregion of opposing edges of the two regions of a two-dimensional electron system
divided by microfabrication. (b) Charge fragmentation near the exit of the regionof opposing edges. (c) Example of an edge magnetoplasmon crystal.

Fig. 2. Charge-fragmentation phenomenon.

coupled region in region 2 can be denoted as ±rQ,
where r is referred to as the coupling constant andrepresents the strength of interaction between regions
1 and 2 as a value from 0 to 1. An edge magnetoplasmon with a charge of rQ then moves counterclockwise along the edge of region 2, without entering thecoupled region, and detected by detector 2. However,a plasmon with a charge of –rQ propagates throughthe coupled region in specimen 2 in the direction
opposite normal propagation, pulled by the edge
magnetoplasmon that is propagating in region 1. The
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Vol. 21 No. 5 May 2023

propagation speed is reduced by a factor of 1 − r by
1+r
the interaction. This fact corresponds to the refractive
index of the opposing region expressed as 1 + r .
1−r
When the composite pulse reaches the exit of the
opposing region, the –rQ component in region 2 loses
the attraction of the partner in region 1, and its motion
reverses to the normal direction. A charge pair ±r2Q
is then generated in region 1 and a composite pulse isagain created and propagates (Fig. 2(b)). Following
31

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Fig. 3. Propagation in an edge magnetoplasmon crystal.

the time evolution of the pulses in this manner, we seethat the charge is progressively fragmented, so thephenomenon is referred to as charge fractionalization. This highly interesting phenomenon was verified experimentally by NTT [1]. Somewhat surprisingly, the familiar reflectance and transmittance forwhen ordinary thin films are excited by light are calculated on the basis of the AC (alternating current)
input given by the angular frequency ω rather thanpulses. In other words, the magnetoplasmon is ahybrid state of light and electrons that behaves aslight.
3. Edge magnetoplasmon crystal
My current research is an extension of the problem
described above, where I considered what phenomena or capabilities appear when a two-dimensional
semiconductor is divided into many elements byusing microfabrication techniques to produce a semiconductor “crystal” network (for example, a crystal
that comprises several “atoms,” as shown in Fig. 2(c)).
Such problems are a natural direction for extended
research. In nanocarbon research, for example, there
is a history of increasing the network periodicity ofbenzene rings from C60 fullerene to planar graphenesystems. However, this is not the only motivation.
There is also understanding the electronic propertiesof two-dimensional semiconductors subjected tostrong magnetic fields from a perspective that iscloser to light and applying that understanding to new
NTT Technical Review

Vol. 21 No. 5 May 2023

areas. For example, assuming the degree of fragmentation to be the diameter of the cyclotron motion, thesituation can be regarded as a plasmon optical simulation of a quantum Hall system. One interesting
topic is the conditions under which edge magnetoplasmons appear (coupling strength and crystal
shape). I believe that the differences between the two
types of quantum Hall effects (integer and fractional)
can be understood from the viewpoint of edge magnetoplasmons. The integer Hall effect is thought tooriginate in a single-electron scenario where impurities cause scattering. The fractional Hall effect isbelieved to result from a many-body effect producedby the interaction between electrons. For edge magnetoplasmon crystals, scattering is caused by defects
in the network, and r can serve as a parameter for theelectron interaction.
I first calculated the energy band structure of anedge magnetoplasmon to understand the characteristics of propagation through the domain crystal [2].
The details are omitted here, but I used the transfer
matrix method for a periodic network without edges.
The band structure is a plot of natural frequency versus wavelength and is useful for understanding thestate of a crystal. If there is no eigenfrequency for any
wavelength within a band, it is referred to as anenergy gap within which there is no state for propagation. The calculation results for a honeycomb network of hexagonal “atoms” indicate a band structurethat includes energy gaps (Fig. 3). The size of the gap
between the two cones tends to increase with r
32

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indicating an interaction-induced energy gap. In theoriginal specimen prior to splitting, the edge magnetoplasmons appeared in the energy gap created byquantization*2 of the cyclotron orbits (Fig. 1). That
gap was not created by interaction, so gaps in edge
magnetoplasmon crystals have a different origin fromthe original edge magnetoplasmons.
Although I learned that energy gaps also appear inan edge magnetoplasmon crystal, I do not know fromthe band structure alone (calculation results obtained
under the assumption of a periodic edgeless system)
whether there are edge modes that propagate along
the network edge within those gaps. A more detailed
analysis revealed that there are edge modes (Fig. 3).
This means that edge magnetoplasmons also exist inedge magnetoplasmon crystals. Interestingly, a modethat moves in the direction opposite normal edge
propagation appears at frequencies near the band-gap
margins. It is known that edge-state propagation canbe bidirectional in the fractional quantum Hall effect.
If we consider the interaction between “atoms,” thefractional quantum Hall effect caused by the manybody effect can also be taken into account in the calculation. This can be interpreted as the appearance ofa mode that propagates in the opposite direction.
4. Conclusion and future prospects
Our understanding of the physical properties of
two-dimensional electron systems has undergone

broad development, ranging from basic research,such as linking to mathematical topology, to application. The potential of applying this understanding to
improving light manipulation is clear. However, theenergy scales of electronic states and optical states
are generally far apart (the telecommunications
wavelength band, for example), and finding a direct
link between the two is a difficult problem that
requires a new approach. Many researchers are interested in the fusion of the behaviors of light and electrons to produce new capabilities. Inspired by plasmons as a hybrid optoelectronic state, I am searchingfor such a new approach, and an edge magnetoplasmon crystal is a likely candidate for this purpose. The
quantum mechanical behavior of photons at low lightintensity has also attracted much interest. The quantum mechanics of edge magnetoplasmons has also
been extensively studied, and one topic for future
work is the quantum field theory*3 of edge magnetoplasmon crystals.
References
[1]

[2]

P. Brasseur, N. H. Tu, Y. Sekine, K. Muraki, M. Hashisaka, T. Fujisawa, and N. Kumada, “Charge Fractionalization in Artificial
Tomonaga-Luttinger Liquids with Controlled Interaction Strength,”
Phys. Rev. B, Vol. 96, No. 8, 081101(R), 2017. https://doi.org/10.1103/
PhysRevB.96.081101
K. Sasaki, “Band Structures of Edge Magnetoplasmon Crystals,”
Phys. Rev. B, Vol. 105, No. 7, 075312, 2022. https://doi.org/10.1103/
PhysRevB.105.075312

Ken-ichi Sasaki

Researcher, Low-Dimensional Nanomaterials
Research Group, Multidisciplinary Materials
Design and Science Laboratory, NTT Basic
Research Laboratories.
He received an M.S. and Ph.D. in physics fromTohoku University, Miyagi, in 1999 and 2003.
He joined NTT Basic Research Laboratories in2010 and has been studying theoretical aspects ofgraphene and carbon nanotubes. He is a memberof the Physical Society of Japan and the American Physical Society.

NTT Technical Review

Vol. 21 No. 5 May 2023

*2 Quantization: A process that results in properties having discretevalues such as one or two. An example is the number of particles,
for which fractional values do not exist. The same concept applies to energy and other such properties.
*3 Quantum field theory: A general term for computational methods
that predict the creation, annihilation, and propagation of subatomic particles (photons and electrons, etc.) in time and space.

33

Feature Articles: Plasmon Control Technology

Time-resolved Measurement of
Ambipolar Plasmon Transport in
Semiconductor Composite Quantum
Wells
Hiroshi Kamata, Hiroshi Irie, Norio Kumada,and Koji Muraki
Abstract

Two-dimensional topological insulators have attracted much attention for potential application toquantum computers on the basis of a new principle. Semiconductor composite quantum wells, in whichelectrons and holes are confined in separate layers, can serve as two-dimensional topological insulators
when their layer structure is optimized. We describe time-resolved measurements of ambipolar plasmontransport in both the electron and hole regimes separately using a single device. This technique will pave
the way for investigations of charge and spin dynamics in two-dimensional topological insulators.
Keywords: two-dimensional topological insulators, topological quantum computing, plasmonics

1. Topological quantum computing
Next-generation computers based on new operating
principles have been attracting much attention forsolving problems such as combinatorial optimization
problems, which are difficult to achieve with conventional computers. Quantum computers that executemassively parallel computation by simultaneously
encoding multiple units of information on the basis of
the principles of quantum mechanics have been
extensively investigated. Various proof-of-principle
experiments, including fundamental quantum operations such as quantum entanglement, have been demonstrated using single atoms, photons, and electron
spins to implement qubits for practical applications.
For social implementations of quantum computers,
fault-tolerant quantum computation and scalable
quantum circuits are recognized as key issues.
Topological quantum computation with quasiparticles,*1 the unique properties of which differ fromthose of natural elementary particles,*2 has been proNTT Technical Review

Vol. 21 No. 5 May 2023

posed as an alternative approach. Differently fromconventional quantum computing, topological quantum computation uses changes in the quantum stateof the system triggered by the exchange of quasiparticles as a logic gate. Calculations are executed bysequentially exchanging quasiparticles arranged inspace (Fig. 1). Since the result is determined only by
the order in which the quasiparticles are exchanged
and does not depend on the details of the trajectory ofthe quasiparticles, this qubit is expected to be faulttolerant against external noise. The two-dimensional
topological insulator is expected to serve as a platform on which quasiparticles with unique properties*1 Quasiparticle: When many particles behave as though they werea single particle, such as the case of electrons in semiconductors,
it is referred to as a quasiparticle. In two-dimensional systems,
quasiparticles that have the unique properties of exchanging two
identical particles exist and follow quantum statistics that differ
from those of both fermions and bosons.
*2 Elementary particle: The smallest unit particles that compose
natural substances and include fermions such as electrons, andbosons such as photons.

34

Feature Articles

Qubit array

Trajectory

Quasiparticle
pairs

Time

Fig. 1. Schematic image of topological quantum computation.

(a) Helical edge channels

Two-dimensional
topological insulator

Two-dimensional
electron system

Two-dimensionalhole system

Magnetic
field

Magnetic
field

Helical edge states

Chiral edge states(b) Helical edge states and chiral edge states

Fig. 2. Two-dimensional topological insulator.

can be generated.
2. Two-dimensional topological insulator
In two-dimensional topological insulators, the interior of the sample is insulating while one-dimensional conductive channels with opposite spins propagating in opposite directions (helical edge channels) areformed along the edge of the sample (Fig. 2(a)).
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Since inelastic scattering between the channels isprevented by time-reversal symmetry,*3 backscattering is suppressed at zero magnetic field if the sample
has no magnetic impurities. The quasiparticles thatare key ingredients for a topological quantum computing (Majorana quasiparticles) are expected to*3 Time-reversal symmetry: The case in which a state does notchange, even if the flow of time is reversed; time-reversal symmetry is broken by a magnetic field.

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Feature Articles

3. InAs/InGaSb composite quantum well
A composite quantum well consists of InAs and
InGaSb sandwiched between aluminum gallium antimonide (AlGaSb) barrier layers. Figure 3 shows theenergy band diagram, where electrons and holes are
confined in the InAs and InGaSb quantum wells,
respectively. When the bottom of the InAs conduction band is lower than the top of the InGaSb valenceband (band-inversion), hybridization of the electron
and hole states opens an energy gap, resulting in a
two-dimensional topological insulator [1]. When thebottom of the InAs conduction band is higher than the
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Vol. 21 No. 5 May 2023

Conduction band
Gate electrodeappear in systems where a superconductor is brought
to close proximity with the helical edge states. The
conductance of the helical edge states should bequantized as theoretically predicted. In experiments,
however, quantized conductance*4 has been observed
only in samples that have a scale of several micrometers, and the details of the dissipative transportmechanism have not been experimentally understood.
When a strong magnetic field is applied to a twodimensional electron (hole) system, the cyclotron
motion of electrons (holes) is quantized, forming
discrete energy levels called Landau levels. In thiscase, if the Fermi level is placed between the Landaulevels, the interior of the sample becomes insulating
and one-dimensional conductive channels for which
the propagating direction is determined by the magnetic-field direction and charge-carrier type areformed along the edge of the sample (chiral edge
channels). This phenomenon is called as a quantum
Hall effect, and the system is classified as a twodimensional topological insulator with broken timereversal symmetry. Helical edge states in a twodimensional topological insulator with time-reversal
symmetry can be approximately thought of as asuperposition of chiral edge states of electrons andholes moving in opposite directions, ignoring theexact spin orientation (Fig. 2(b)). At NTT Basic
Research Laboratories, we therefore focused on edgetransport of plasmons (edge magnetoplasmons) inindium arsenide/indium gallium antimonide (InAs/
InGaSb) composite quantum wells, the base materialof semiconductor two-dimensional topological insulators. Edge magnetoplasmons are collective oscillations of charges at the edges of two-dimensional
systems. By investigating their dissipation and propagation velocity, we can evaluate the characteristics ofthe edge states.

InGaSbAlGaSbAlGaSbBand gap

Band gap

Valence band
InAs

Fig. 3. Energy band diagram.

top of the InGaSb valence band (non-band-inversion), the semiconductor has a normal band gap.
However, the charge-carrier type can be switchedbetween two-dimensional electron and hole systems
by controlling the Fermi level with a voltage applied
to the surface gate electrode. We carried out on-chiptime-resolved measurements of edge magnetoplasmons in both two-dimensional electron and holesystems using non-band-inverted composite quantum
wells.
4. Time-resolved
transport-measurement scheme
Conventionally, time- or frequency-domain transport of plasmons in the gigahertz range has beenmeasured with a sampling oscilloscope or spectrum
analyzer. However, it has been difficult to observeand evaluate the original plasmon waveform inside asample because of measurement sensitivity and distortion of high-frequency signals in the measurement
system. We succeeded in observing edge magnetoplasmon waveforms inside samples in the time
domain by using a method that is similar to pumpand-probe spectroscopy in optical measurement.
Figure 4(a) schematically shows the device structure and experimental setup used in the time-resolved
measurements. The structure has an injection gate for
exciting edge magnetoplasmons, detection gate forthe time-resolved measurement, and global gate forcontrolling the charge-carrier types and densities.
These gates are insulated from each other and fromthe semiconductor substrate by aluminum oxide(Al2O3) insulating layers so that gate voltage can beapplied independently to each gate electrode. In addition to an Ohmic contact for measuring current, there*4 Quantized conductance: Non-dissipative transport in which conductivity is quantized to e2/h, where e is the elementary charge(1.602 × 10-19 C) and h is Planck’s constant (6.626 × 10-34 J • s).

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Feature Articles

Detection
gate
Global gate

Injection-gate
voltage pulse
Plasmon waveform

Injection
gate

30 µm

Current

Constriction

Detection-gate
voltage pulse

Current
td

Time delay td

Time
Magnetic field(a) Device structure and experimental setup(b) Schematic of plasmon waveform and applied voltage pulses

Fig. 4. Time-resolved measurement scheme.

are several Ohmic contacts for stabilizing the device
under the applied high-frequency voltages. The measurements were carried out in the quantum Hall
regime under a strong magnetic field at a measurement temperature of 1.5 K.
When a voltage pulse is applied to the injection gate
(injection pulse), negative and positive pulsed edge
magnetoplasmons are excited at the rising and fallingedges of the injection pulse, respectively. The excitededge magnetoplasmons propagate along the edge ofthe device to the right or left, depending on the chirality determined by the magnetic-field direction andcharge-carrier type. For the time-resolved measurement, a short voltage pulse (detection pulse; about 80
ps in width) is applied to the detection gate, which is
30 μm from the injection gate, to temporarily modulate the local carrier density in a constriction (Fig.
4(a)). If the detection pulse coincides with the arrivalof edge magnetoplasmons on the constriction, theedge magnetoplasmons are reflected at the constriction; otherwise, the edge magnetoplasmons are transmitted through the constriction and measured at the
Ohmic contact located to the right of the constriction.
The waveform of the local edge magnetoplasmons
propagating in the device can thus be obtained bymeasuring the current as a function of the time delay
td on the detection pulse from the injection pulse(Fig. 4(b)) [2].

in Fig. 5(a), where the origin of the time axis represents the time of edge magnetoplasmon excitation.
The timing of the positive and negative pulsed waveforms corresponds to edge magnetoplasmons excitedat the rising and falling edges of the injection pulse,
and the time interval corresponds to the width of theinjection pulse. Pulsed waveforms are observed for
only one magnetic-field direction depending on the
charge-carrier type since electrons and holes have
opposite chiralities in the same magnetic-field direction. The peak delay of about 2 ns, which corresponds
to the propagation time of the edge magnetoplasmonson the propagation distance (30 μm) yields the propagation velocity.
Theoretically, the propagation velocity of edge
magnetoplasmons is proportional to the Hall conductivity when the surface of the device is covered witha metallic gate electrode. The measured propagation
velocities for edge magnetoplasmons in the electron
and hole regimes are almost identical and proportional to the Hall conductivity (Fig. 5(b)). Therefore,
the propagation of edge magnetoplasmons in both theelectron and hole regimes can be measured in thetime domain. This result indicates that the characteristics of the edge channels can be measured as the
propagation velocity of the edge magnetoplasmons.

5. Time-resolved measurement ofedge magnetoplasmons

At NTT Basic Research Laboratories, we observededge magnetoplasmon waveforms in the time domain
in both electron and hole regimes of an InAs/InGaSb
composite quantum well, which is the base materialof a two-dimensional topological insulator. Since theAn example of edge magnetoplasmon waveforms
measured in the electron and hole regimes are shown
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6. Future prospects

37

0.2

Magnetic field > 0 T
Magnetic field < 0 T

60

0.0
−0.2
0.2

Velocity (km/s)

Current (nA)

Current (nA)

Feature Articles

Electron regime

0.0
−0.2

Electrons
Holes

40

20

Hole regime
0

10
20Time delay, td (ns)

0
0

30

50

100

150

Hall conductivity (µS/m)(a) Observed edge magnetoplasmon waveforms(b) Velocity as a function of the Hall conductivity

Fig. 5. Time-resolved edge magnetoplasmon transport.

measured propagation velocity of edge magnetoplasmons reflects the properties of the edge channels, thistime-resolved measurement scheme can be used toinvestigate charge and spin dynamics and its relaxation mechanism on various materials, including
two-dimensional topological insulators. Such experiments will pave the way for developments in topological quantum computing. Furthermore, control ofthe propagation velocity and chirality of edge magnetoplasmons with gate voltage and magnetic fields are
expected to be powerful techniques for plasmonics,

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in which plasmons are used as information carriers.
References
[1]

[2]

T. Akiho, “Creating New Two-dimensional Topological Insulators for
Fault-tolerant Quantum Computing,” NTT Technical Review, Vol. 15,
No. 7, 2017.
https://www.ntt-review.jp/archive/ntttechnical.php?contents=
ntr201707fa6.html
H. Kamata, H. Irie, N. Kumada, and K. Muraki, “Time-resolved Measurement of Ambipolar Edge Magnetoplasmon Transport in InAs/
InGaSb Composite Quantum Wells,” Phys. Rev. Research, Vol. 4,033214, 2022.
https://doi.org/10.1103/PhysRevResearch.4.033214

38

Feature Articles

Hiroshi Kamata

Research Associate, Quantum Solid-State
Physics Research Group, Quantum Science andTechnology Laboratory, NTT Basic ResearchLaboratories.
He received the B.E. degree in science from
Tokyo University of Science, Chiba in 2007, andthe M.E. and Ph.D. degrees in science from
Tokyo Institute of Technology, Tokyo in 2009
and 2012, respectively. From 2012 to 2014, he
was a postdoctoral researcher at Tokyo Instituteof Technology, Tokyo. From 2014 to 2015, he
was a postdoctoral researcher at the University ofTokyo, Tokyo. From 2015 to 2018, he was a
Special Postdoctoral Researcher at RIKEN,
Saitama. From 2018 to 2020, he was a postdoctoral researcher at École normale supérieure,
Paris, France. He joined NTT Basic ResearchLaboratories in 2020. Since 2020, he has been a
PRESTO researcher, Japan Science and Technology Agency (concurrently). His current interests
are spin and charge dynamics in topologicalmaterials. He received the Young Scientist Awardof the Physical Society of Japan (JPS) in 2019.
He is a member of the JPS.

Hiroshi Irie

Senior Researcher, Quantum Solid-State Physics Research Group, Quantum Science andTechnology Laboratory, NTT Basic ResearchLaboratories.
After receiving the Ph.D. degree from University of Rochester, NY, U.S. in 2010, he joined
NTT Basic Research Laboratories (BRL) as a
research associate and later became a researcher
in 2012. In BRL, he has been engaged in the
study of quantum transport in topological materials and superconductor/semiconductor hybrid
structures. He is a member of the Japan Societyof Applied Physics, the Physical Society ofJapan, and American Physical Society.

NTT Technical Review

Vol. 21 No. 5 May 2023

Norio Kumada

Distinguished Researcher and Group Leader,
Quantum Solid-State Physics Research Group,
Quantum Science and Technology Laboratory,
NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in physics
from Tohoku University, Miyagi, in 1998, 2000,
and 2003. He joined NTT Basic Research Laboratories in 2003. Since then, he has been engaged
in the study of highly correlated electronic states
confined in two dimensions. From 2013 to 2014,
he was a visiting scientist at CEA Saclay, France.
He received the Young Scientist Award of the
Physical Society of Japan in 2008 and the Young
Scientists’ Prize from the Minister of Education,
Culture, Sports, Science and Technology in2012. He is a member of the Physical Society ofJapan.

Koji Muraki

Senior Distinguished Researcher, Quantum
Solid-State Physics Research Group, Quantum
Science and Technology Laboratory, NTT Basic
Research Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in applied physics from The University of Tokyo,
Japan, in 1989, 1991, and 1994, respectively. He
joined NTT Basic Research Laboratories in
1994. Since then, he has been engaged in thegrowth of high-mobility heterostructures and thestudy of highly correlated electronic states realized in such structures. He was a guest researcher
at Max-Planck Institute, Stuttgart, Germany during 2001–2002. He is a member of the PhysicalSociety of Japan and Japan Society of Applied
Physics.

39

Regular Articles

Ultrafast and Low-powerconsumption Membrane Lasers on Si
with Integrated Optical Feedback
Nikolaos-Panteleimon Diamantopoulos,
Suguru Yamaoka, Takuro Fujii, and Shinji Matsuo
Abstract

We developed energy-efficient membrane III-V distributed-reflector lasers on silicon-based substrates
for ultrafast short-reach communication links and neuromorphic computing applications. By leveraging
high-speed photon-photon interactions enabled by integrated optical feedback and a high-opticalconfinement membrane structure, we demonstrated record-fast directly modulated laser bandwidths andspike-processing rates with ultralow operating energies. This is a step towards our goal of reducing thecarbon footprint of information and communication technology and artificial intelligence hardware,
while keeping pace with the increasing demand of processing speeds.
Keywords: integrated photonics, membrane lasers, neuromorphic photonics.

1. Introduction
The advent of modern artificial intelligence andmachine learning (AI/ML) applications and cloudservices have led to tremendous information andcommunication technology (ICT) growth, enabling
unprecedented processing capabilities. However, as
processing and datacenter communication speeds
continue to increase, so do their power consumption
and associated CO2 (carbon dioxide) emissions [1, 2].
To support ultrahigh speeds but at reduced power
consumption, we developed membrane distributedreflector lasers with integrated optical feedback forenergy-efficient photonics-electronics convergencewithin NTT’s Innovative Optical and Wireless Network (IOWN) project [3, 4].
With our membrane lasers with integrated opticalfeedback on silicon dioxide/silicon (SiO2/Si) substrates, we could achieve unprecedented directly
modulated laser (DML) bandwidths of ~60 GHz [5,6] and spike-processing rates (i.e., inter-spike rate) of
10 GHz [7, 8] with sub-pJ/bit and ~pJ/spike laseroperating energies (see Fig. 1). We also achieved the
NTT Technical Review

Vol. 21 No. 5 May 2023

world’s fastest DML bandwidth of ~108 GHz byintegrating our membrane distributed-reflector lasers
with integrated optical feedback on silicon carbide(SiC) substrates [9–11].
2. Membrane distributed-reflector lasers withintegrated optical feedback
Our membrane laser structure for lasers fabricatedon SiO2/Si substrates is based on a distributed-reflector longitudinal design that includes a middle uniform-distributed feedback (DFB) section sandwichedwith an 80-μm-long back distributed Bragg reflector(DBR) mirror (DBR-r) and 200-μm-long front DBR
mirror (DBR-f). In this structure, the DBR-r is used
to filter one of the two DFB modes for single-mode
operation [12], and the DBR-f is used to generate
optical feedback and side-modes for enacting photonphoton dynamics [5, 6, 9–11] (see Fig. 2). Very low
operating-power consumption was achieved with ourheterogenous membrane III-V on Si technology,
which uses a thin-film (<350-nm thick) III-V layer ona low-refractive index SiO2/Si substrate with a similar
40

Regular Articles

InP: indium phosphide
LD: laser diode
MRR: micro-ring resonators

PCM: phase-change materials
Si(N): silicon nitride
VCSEL: vertical cavity surface emitting laser

Fig. 1. R ecent records of 3-dB bandwidths of DMLs [5, 6, 9–11] (left) and spike-processing rates of integrated photonic
spiking neurons [7, 8] (right).

MQW: multiple quantum well
SSC: spot-size converter

Fig. 2. Membrane distributed-reflector laser with integrated optical feedback [5–11].

low-refractive index SiO2/silicon oxide (SiOx) overcladding. This structure enables very strong transverse optical confinement, which leads to both low
operating power and high-speed dynamics. Other
advantages include cost reduction by using large Si
wafers and mature processes, co-integration capabilities with Si photonics and other integrated photonic
platforms, and coupling to fibers via SiOx-based spotsize converters.
3. Photon-photon resonance for short-reachcommunication links
Although our previously developed membrane
DMLs could achieve high (~20 GHz) bandwidths
with sub-pJ/bit energy consumptions [12], there is an
NTT Technical Review

Vol. 21 No. 5 May 2023

inherent trade-off between further DML bandwidth
improvement and power consumption since therelaxation oscillation frequency (fR) is proportional to
the square-root of the bias current (Ib) above a threshold (Ith), i.e., fR ∝ (Ib – Ith)1/2. This limitation can bealleviated by introducing a photon-photon resonance(PPR) at high frequencies (see Fig. 2) on the basis of
the optical-feedback-generated side-modes.
By using the PPR effect, we could effectively triple
the bandwidths of the membrane DML-on-Si reaching ~60 GHz [5] while maintaining the same power
consumption. This enabled us to achieve 112-Gbit/s
short-reach transmissions for datacenter applications[5, 6] (see Fig. 3), and the first 400-Gbit/s-class linkusing a single >100-GHz-bandwidth DML-on-SiC[10]. We also demonstrated the ability to use the PPR
41

Regular Articles

BER: bit error rate
E-O: electro-optical
FEC: forward error correction

HD: hard decision
NRZ: non return to zero
PAM: pulse amplitude modulation

Fig. 3. Transmission performance of membrane DML-on-Si [5, 6].

effect at operating temperatures of 50℃ and above,
which enabled us to sustain >100-Gbit/s operation for
short-reach links using a membrane DML-on-Si at
50℃ [5, 6], and more than 100-GBaud modulations
under uncooled (85℃) conditions using a membrane
DML-on-SiC [11].
4. Spiking membrane laser neurons
One of the most promising neuromorphic computing architectures in terms of energy-efficiency andscalability is the hardware implementation of spiking
neural networks (SNNs) due to the unmatched noisetolerance and event-driven capabilities of spikeinformation processing. In particular, integrated
photonics hold great promise in offering high-bandwidth and scalable on-chip SNNs by taking advantage of the tens-of-GHz speeds offered by modern
opto-electronics and the numerous parallelization
capabilities of photonics. Nevertheless, most photonic implementations of spiking neurons to datehave been limited by physical processes that operate
on nanosecond time scales or slower, leading tospike-processing rates (i.e., the factor that ultimately
defines the processing speed) of around the GHz
level or less.
By using our membrane laser structure with integrated optical feedback, we were able to demonstrate
ultrafast spiking behavior with clearly defined threshNTT Technical Review

Vol. 21 No. 5 May 2023

olds and spiking rates up to 10 GHz using 50-ps-longelectrical pulses [7, 8] (see Fig. 4), overcoming previous speed limitations. This was achieved by taking
advantage of the ultrafast photon-photon dynamics
between two longitudinal modes (see Fig. 2). In sucha case, a small input energy perturbation can temporarily excite a secondary longitudinal side-mode,
which leads to power excitability of ultrashort (~pslong) output optical pulses, when the input energy
perturbation exceeds an energy threshold. Moreover,
our strong-confinement membrane-III-V-on-SiO2/Si
structure ensured very low operating and thresholdenergies of ~pJ/spike and ~0.1 pJ/spike, respectively[7, 8].
5. Summary and future plans
To meet the increasing demands on processing
speeds of modern ICT and AI applications, while
maintaining low operating energies for a sustainable
and greener future within the IOWN project, wedeveloped ultrafast and energy-efficient membrane
distributed-reflector lasers on Si-based substrates
with integrated optical feedback. With such lasers,
we could achieve unprecedented DML bandwidths of~60 GHz and spike-processing rates of 10 GHz with
~sub-pJ/bit and ~pJ/spike, respectively.
Future developments will focus on multi-channel
DML transmitters for 800-Gbit/s systems and beyond
42Regular Articles

Vpp: peak-to-peak voltage

Fig. 4. Spiking effect in membrane lasers on Si [7, 8]. Note: oscilloscope data at ~240-ps inter-spike intervals are shown.

by taking advantage of our previously developed
techniques and technologies [13, 14]. We also plan toexpand our spiking-membrane laser technology tomulti-neuron SNN-PICs (photonic integrated circuits) and showcase their capabilities at solving practical computational tasks at unprecedented processing speeds.
References
[1]
[2]

[3]
[4]

[5]

[6]

[7]

International Energy Agency (IEA), “Data Centres and Data
Transmission Networks,” 2022. https://www.iea.org/reports/datacentres-and-data-transmission-networks
N. C. Thompson, K. Greenewald, K. Lee, and G. F. Manso, “DeepLearning’s Diminishing Returns: The Cost of Improvement Is
Becoming Unsustainable,” IEEE Spectrum, Vol. 58, No. 10, pp.
50–55, 2021. https://spectrum.ieee.org/deep-learning-computationalcost
NTT Corporation, “NTT Technology Report for Smart World 2022,”2022. https://www.rd.ntt/e/download/NTT_TRFSW_2022_E.pdf
T. Sakamoto, N. Sato, and T. Segawa, “Photonics-electronics
Convergence Technologies for Disaggregated Computing,” NTT
Technical Review, Vol. 19, No. 7, pp. 58–64, 2021. https://www.nttreview.jp/archive/ntttechnical.php?contents=ntr202107fa8.html
N.-P. Diamantopoulos, T. Fujii, S. Yamaoka, H. Nishi, K. Takeda, T.
Tsuchizawa, T. Segawa, T. Kakitsuka, and S. Matsuo, “60 GHz
Bandwidth Directly Modulated Membrane III-V Lasers on SiO2/Si,”
Journal of Lightwave Technology, Vol. 40, No. 10, pp. 3299–3306,
2022. https://doi.org/10.1109/JLT.2022.3153648
N.-P. Diamantopoulos, S. Yamaoka, T. Fujii, H. Nishi, K. Takeda, T.
Tsuchizawa, T. Kakitsuka, and S. Matsuo, “47.5 GHz Membrane-IIIV-on-Si Directly Modulated Laser for Sub-pJ/bit 100-Gbps
Transmission,” Photonics, Vol. 8, No. 2, Article no. 31, 2021. https://doi.org/10.3390/photonics8020031
N.-P. Diamantopoulos, S. Yamaoka, T. Fujii, H. Nishi, T. Segawa, and
S. Matsuo, “Ultrafast Spiking Membrane III-V Laser Neuron on Si,”
Proc. of 2022 European Conference on Optical Communication(ECOC 2022), paper Mo3G.2, Basel, Switzerland, Sept. 2022. https://
opg.optica.org/abstract.cfm?uri=ECEOC-2022-Mo3G.2

NTT Technical Review

Vol. 21 No. 5 May 2023

[8]

N.-P. Diamantopoulos, S. Yamaoka, T. Fujii, H. Nishi, T. Segawa, and
S. Matsuo, “Stability of Spiking Effect in Membrane Laser Neurons
on Si Utilizing Optical Feedback,” Proc. of 28th International
Semiconductor Laser Conference (ISLC 2022), paper WC-02,
Matsue, Japan, Oct. 2022. https://doi.org/10.23919/ISLC52947.2022.
9943397[9] S. Yamaoka, N.-P. Diamantopoulos, H. Nishi, R. Nakao, T. Fujii, K.
Takeda, T. Hiraki, T. Tsurugaya, S. Kanazawa, H. Tanobe, T.
Kakitsuka, T. Tsuchizawa, F. Koyama, and S. Matsuo, “Directly
Modulated Membrane Lasers with 108 GHz Bandwidth on a Highthermal-conductivity Silicon Carbide Substrate,” Nature Photonics,
Vol. 15, pp. 28–35, 2021. https://doi.org/10.1038/s41566-02000700-y[10] N.-P. Diamantopoulos, H. Yamazaki, S. Yamaoka, M. Nagatani, H.
Nishi, H. Tanobe, R. Nakao, T. Fujii, K. Takeda, T. Kakitsuka, H.
Wakita, M. Ida, H. Nosaka, F. Koyama, Y. Miyamoto, and S. Matsuo,“>100-GHz Bandwidth Directly-modulated Lasers and Adaptive
Entropy Loading for Energy-efficient >300-Gbps/λ IM/DD Systems,”
Journal of Lightwave Technology, Vol. 39, No. 3, pp. 771–778, 2021.
https://doi.org/10.1109/JLT.2020.3021727[11] S. Yamaoka, N.-P. Diamantopoulos, H. Nishi, T. Fujii, K. Takeda, T.
Hiraki, S. Kanazawa, T. Kakitsuka, and S. Matsuo, “Uncooled 100GBaud Directly Modulated Membrane Lasers on SiC Substrate,”
Journal of Lightwave Technology, 2023. https://doi.org/10.1109/
JLT.2023.3239614[12] T. Fujii, K. Takeda, N.-P. Diamantopoulos, E. Kanno, K. Hasebe, H.
Nishi, R. Nakao, T. Kakitsuka, and S. Matsuo, “Heterogeneously
Integrated Membrane Lasers on Si Substrate for Low Operating
Energy Optical Links,” IEEE Journal of Selected Topics in Quantum
Electronics, Vol. 24, No. 1, Article no. 1500408, 2018. https://doi.
org/10.1109/JSTQE.2017.2778510[13] T. Fujii, K. Takeda, H. Nishi, N.-P. Diamantopoulos, T. Sato, T.
Kakitsuka, T. Tsuchizawa, and S. Matsuo, “Multiwavelength
Membrane Laser Array Using Selective Area Growth on Directly
Bonded InP on SiO2/Si,” Optica, Vol. 7, No. 7, pp. 838–846, 2020.
https://doi.org/10.1364/OPTICA.391700[14] H. Nishi, T. Fujii, N.-P. Diamantopoulos, K. Takeda, E. Kanno, T.
Kakitsuka, T. Tsuchizawa, H. Fukuda, and S. Matsuo, “Integration ofEight-channel Directly Modulated Membrane-laser Array and SiN
AWG Multiplexer on Si,” Journal of Lightwave Technology, Vol. 37,
No. 2, pp. 266–273, 2019. https://doi.org/10.1109/JLT.2018.2873742

43

Regular Articles

Nikolaos-Panteleimon Diamantopoulos

Research Scientist (Lead), Materials andDevices Laboratory, NTT Device Technology
Laboratories.
He received a B.Sc. from University of Peloponnese, Tripoli, Greece in 2009, two M.Sc.
degrees from Aston University, Birmingham, UK
and Scuola Superiore Sant’Anna Pisa (SSSUP),
Pisa, Italy in 2012 under the joint-masters Erasmus Mundus program, and a Ph.D. from Osaka
University, Japan in 2016. From 2011 to 2016, he
was involved in several EU-funded R&D projects at the Athens Information Technology (AIT)
research center, Athens, Greece. Since 2016 hehas been with NTT Device Technology Labs,
where he conducts research on integrated photonics, optical communications, digital signal
processing, neuromorphic computing systems,
space-division multiplexing, and semiconductor
lasers. He is a senior member of the Institute ofElectrical and Electronics Engineers (IEEE) andOptica.

Suguru Yamaoka

Researcher, Materials and Devices Laboratory,
NTT Device Technology Laboratories.
He received a B.E., M.E., and Ph.D. in applied
physics from Osaka City University in 2015,2017, and 2021. In 2017, he joined NTT Device
Technology Labs. His research interests include
high-speed and low-power-consumption directly
modulated semiconductor lasers. He is a memberof the Institute of Electronics, Information andCommunication Engineers (IEICE) and theJapan Society of Applied Physics (JSAP).

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Vol. 21 No. 5 May 2023

Takuro Fujii

Research Scientist (Lead), Materials andDevices Laboratory, NTT Device Technology
Laboratories and NTT Nanophotonics Center.
He received a B.E. and M.E. in system design
engineering from Keio University, Kanagawa, in2010 and 2012. In 2012, he joined NTT Photonics Laboratories, where he has been researching
metalorganic vapor phase epitaxy growth of
III-V semiconductors and the development of
III-V semiconductors and III-V semiconductor
lasers on Si for photonic integrated circuits. He
was the recipient of the Young Scientist Presentation Award from JSAP in 2014. He is a memberof IEICE, JSAP, and IEEE.

Shinji Matsuo

Senior Distinguished Researcher, Materials
and Devices Laboratory, NTT Device Technology Laboratories and NTT Nanophotonics Center.
He received a B.E. and M.E. in electrical engineering from Hiroshima University in 1986 and1988, and Ph.D. in electronics and applied physics from Tokyo Institute of Technology in 2008.
In 1988, he joined NTT Optoelectronics Laboratories, where he researched photonic functional
devices using MQW pin modulators and
VCSELs. In 1997, he researched optical networks using wavelength division multiplexing
technologies at NTT Network Innovation Laboratories. Since 2000, he has been researching
high-speed tunable optical filters and lasers at
NTT Photonics Laboratories and NTT Device
Technology Laboratories. He is a member of
JSAP and IEICE, and a fellow of IEEE andOptica.

44

Global Standardization Activities

Report of the First ITU-T TSAG
Meeting for the 2022–2024 Study
PeriodNoriyuki Araki and Jiro Nagao
Abstract

The first Telecommunication Standardization Advisory Group (TSAG) meeting for the 2022–2024
study period of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) was held from 12 to 16 December 2022 in a hybrid format at ITU Headquarters in Geneva
and online. The main outcomes of this TSAG meeting, including the establishment of a Focus Group on
Metaverse, are reported in this article.
Keywords: ITU-T, TSAG, standardization

1. Introduction
The Telecommunication Standardization Advisory
Group (TSAG) reviews the standardization activitiesof all Study Groups (SGs) of the International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) and reviews their working methods, meeting rules, and procedures forcooperation with other standardization bodies. On thebasis of an analysis of standardization issues to beaddressed by ITU-T in the future, TSAG discusses
and proposes an SG structure for the next study
period to the World Telecommunication Standardization Assembly (WTSA) [1], which is held every four
years.
The first TSAG meeting for the 2022–2024 study
period of ITU-T was held from 12 to 16 December
2022 in a hybrid format at the ITU Headquarters in
Geneva and online, with approximately 257 participants from 52 countries. From Japan, the ICT Standardization Division, Global Strategy Bureau, Ministry of Internal Affairs and Communications (MIC)
headed the Japanese delegation, which consisted of
13 in-person participants including Mr. Seizo Onoe,then director-elect of the Telecommunication Standardization Bureau, and participants from companies
and organizations (National Institute of Information
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and Communications Technology (NICT), the Telecommunication Technology Committee (TTC),
NEC, NTT, NTT DOCOMO, OKI, Hitachi), and 8
remote participants from the MIC, KDDI, NICT,
Hitachi, Fujitsu, and the ITU Association of Japan.
On the first day of the plenary, the TSAG Management Team proposed a new organizational structure
for TSAG meetings, which was approved. The new
structure introduces two new Working Parties (WPs),
which had not been established before: two Rapporteur Groups (RGs) under WP1, Working Methods(RG-WM) and WTSA Preparations (RG-WTSA),
and two RGs under WP2, Working Programme andRestructuring, SG work, SG Coordination (RGWPR) and Industry Engagement, Metrics (RG-IEM)(Fig. 1). Members of the TSAG Management Teamare listed in Table 1. The TSAG chairman is Mr.
Abdurahman M. Al Hassan from Saudi Arabia, Ms.
Miho Naganuma from NEC was appointed as one of
the 10 TSAG vice-chairmen and rapporteur for RGWPR, and Mr. Shigeru Miyake of Hitachi was agreed
to continue as the liaison officer with the International Organization for Standardization/International
Electrotechnical Commission (ISO/IEC) Joint Technical Committee 1 (JTC1).

45

Global Standardization Activities

TSAG Plenary
Chairman: Mr. Abdurahman M. AL HASSAN
10 TSAG vice-chairmen

Rapporteur SOPStrategic & Operational Plan
Rapporteur: Mr. Víctor MARTÍNEZ
VANEGAS
JCA-AHF
Accessibility, Human Factors
Chairman: Ms. Andrea SAKS
JCA-DCC
Digital COVID Certificate
Chairman: Mr. Heung-Youl YOUM
Coordination, Representatives
Representatives to IEC-ISO-ITU-T SPCG:
Ms. Miho NAGANUMA, Mr. Per FRÖJDH,Mr. Ajit JILLAVENKATESA, Mr. Olivier
DUBUISSON, Mr. Zhicheng QU
ISCG and Inter-Sector Coordination: Mr.
Abdurahman M. AL HASSAN, Mr.
Dominique WÜRGES, Mr. Noah LUO
Coordination with CITS: Mr. Paul NAJARIAN
Liaison officer to IETF: Mr. Scott MANSFIELD

WP1Working Methods and related WTSA
preparations (WP-WMW)
Chairman: Mr. Tobias KAUFMANN
Vice-chairman: Ms. Minah LEE

WP2
Industry Engagement, Work Programme,
Restructuring (WP-IEWPR)
Chairman: Ms. Gaëlle MARTIN-COCHER
Vice-chairman: Mr. Guy-Michel KOUAKOU

RG
Working Methods (RG-WM)
Rapporteur: Mr. Olivier DUBUISSON
Associate rapporteur on e-meetings:
Mr. Phil RUSHTONRGWork Programme and Restructuring,
SG work, SG Coordination (RG-WPR)
Rapporteur: Ms. Miho NAGANUMA
Associate rapporteur on restructuring:
Mr. Greg RATTA

RG
WTSA Preparations (RG-WTSA)
Rapporteur: Ms. Fang LI
Associate rapporteur on WTSA
Guidelines: Mr. Isaac BOATENG
Associate rapporteur on streamlining
resolutions: Mr. Evgeny TONKIKH

RG
Industry Engagement, Metrics (RG-IEM)
Rapporteur: Mr. Glenn PARSONSAssociate rapporteur on emerging
technologies: Mr. Arnaud TADDEIAssociate rapporteur on metrics: Mr. Noah LUO

Coordination, Representatives
Liaison officer to ISO/IEC JTC1: Mr. Shigeru
MIYAKE
Liaison officer to IEC/SMB/SG12:
Mr. Olivier DUBUISSON

CITS: Collaboration on ITS Communication Standards
IETF: Internet Engineering Task Force
ISCG: Inter-Sector Coordination Group on issues of mutual interest
SPCG: Standardization Programme Coordination Group

Fig. 1. Structure of TSAG.

2. Establishment of new Focus Groups and Joint
Coordination Activities
2.1 Proposal for a new Focus Group on Metaverse
When the Focus Group (FG) on Metaverse (FGMV) was proposed by ITU-T SG16 (Multimedia), a
key point for discussion was which organization
should be the parent SG. Establishing an ad hocgroup (AHG) for creating Terms of Reference (ToR)
was proposed on the first day of the plenary and Ms.
Gaelle Martin-Cocher (Canada), vice-chairman of
TSAG, was elected as the AHG leader. FG-MV wasagreed in the plenary to be newly established under
TSAG. Regarding the opinion that the term metaverse should not be used from a trademark point ofview, the ITU legal department was consulted, and it
was confirmed that there was no problem as it is not
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about selling a product.
2.2 E
 stablishment of Joint Coordination Activity
on Machine Learning (ITU-T JCA-ML)
The request was made by a liaison from SG13
(Next Generation Networks) and was endorsed on thefirst day of the plenary because of the wide range ofissues related to ML among the SGs. The need forcollaboration with ISO/IEC and other standards
developing organizations (SDOs) dealing with similar ML-related standardization was noted to avoid
duplication of efforts.
2.3 E
 stablishment of JCA on Quantum Key Distribution Network (JCA-QKDN)The FG on Quantum Information Technology for
Networks (FG-QIT4N) ended its activities in 2022.
46

Global Standardization Activities

Table 1. TSAG Management Team.
Name

Country

Contact

Abdurahman M. AL HASSAN
Chairman

Saudi Arabia

National Cyber Security Authority (NCA)

Khalid AL-HMOUD
Vice-chairman

JordanTelecommunications Regulatory Commission (TRC)

Ulugbek AZIMOV
Vice-chairman

Uzbekistan

Ministry for Development of Information Technologies and Communications

Isaac BOATENG
Vice-chairman

Ghana

National Communications Authority

Olivier DUBUISSON
Vice-chairman

FranceOrangeTobias KAUFMANN
Vice-chairman

Germany

Federal Network Agency for Electricity, Gas, Telecommunications, Post and
Railway (BNetzA)

Guy-Michel KOUAKOU
Vice-chairman

Côte d’Ivoire

ARTCI

Fang LI
Vice-chairman

China

CAICT, MIIT

Gaelle MARTIN-COCHER
Vice-chairman

CanadaInterDigital Canada Ltd.

Víctor Manuel MARTÍNEZ VANEGAS
Vice-chairman

MexicoInstituto Federal de Telecomunicaciones (IFETEL)

Miho NAGANUMA
Vice-chairman

Japan

NEC Corporation

Tobias KAUFMANN
WP1 chairman

Germany

Federal Network Agency for Electricity, Gas, Telecommunications, Post and
Railway (BNetzA)

Minah LEE
WP1 vice-chairman

Korea (Rep. of)

Ministry of Science and ICT, TTA

Gaelle MARTIN-COCHERWP2 chairman

CanadaInterDigital Canada Ltd.

Guy-Michel KOUAKOUWP2 vice-chairman

Côte d’Ivoire

ARTCI

The establishment of this JCA was proposed by
China and approved because the issues related to
QKDN are diverse among the SGs and it is necessary
to fulfil a mutual coordination function in a forum
such as the JCA.
3. Main results of WP1
3.1 RG-WM
The RG-WM discusses the work methods in ITU-T
and the revision of the ITU-T A-series Recommendations (Organization of the work of ITU-T). At thismeeting, the activities of the AHG on Governanceand Management of E-meetings (AHG-GME),
chaired by Mr. Phil Rushton (UK), were reported.
This AHG has been the most active group since thelast TSAG meeting. He reported that the AHG has
met four times since the last TSAG meeting and compiled a list of issues and guidelines for holding
e-meetings. After reflecting the revised comments
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from each country in the editing session, the Supplement 4 to ITU-T A-series Recommendations (A.
Suppl. 4) “Supplement on guidelines for remote participation” was agreed upon.
This meeting also agreed on a proposal from Telecom Italia and others to revise A. Suppl. 2 “Guidelines on interoperability experiments and proof-ofconcept events” and discussed the Recommendations
on working methods and the future course of action
for the supplement documents.
3.2 RG-WTSA
The RG-WTSA discusses the consolidation, simplification, and streamlining of Resolutions for
WTSA-24. At this meeting, Canada proposed thecreation of a one-pager to help WTSA session chairmen work more efficiently by providing a concise
summary and reference at hand of the useful guidance found in the A-series Recommendations andResolution 1. It is envisioned that the one-pager will47

Global Standardization Activities

be developed as an RG-WTSA guideline under RGWTSA. The proposal was continued for further discussion, as there were different opinions from eachcountry as to whether it is useful or not necessary.
There was also a proposal from Russia for the integration and simplification/streamlining of WTSA andPlenipotentiary Conference (PP) Resolutions, and it
was agreed to continue the activities for the integration and simplification/streamlining in accordancewith the instructions from PP-22, taking into consideration how it should be achieved.
In other discussions, it was agreed to develop guidance on “Principles for Reviewing WTSA Resolutions” and guidelines for the preparation of WTSA
resolutions.
4. Main results of WP2
4.1 RG-WPR
The RG-WPR reviews work plans, structures, SG
work, and coordination, including the review ofactivity reports of all SGs, approval of proposed taskorganization by SGs, coordination of regional groups,
coordination among SGs, other SDOs and sectors,
and coordination on matters related to SMART (scientific monitoring and reliable telecommunication)submarine cable systems, IMT (International Mobile
Telecommunications)-2020, and climate change.
In this meeting, the status reports of each SG wereintroduced and the activities of other SDOs werereported. The action plan for the SG restructuring
analysis was discussed with the U.S. participant as
the editor, which aims at a thorough review of potential restructuring options for ITU-T on the basis of theempirical analysis, with a view to approving the SG
restructuring proposal at WTSA-24. SG restructuring
is an important issue that TSAG needs to address. To
move forward with the action plan, the definition of
key performance indicators (KPIs)/metrics to be collected and analyzed is to be clarified, the priorities ofthe various KPIs/metrics to be collected and the timing of their implementation are to be identified, and a
project plan for conducting the analysis related to SG
restructuring (Gantt chart to WTSA-24) was devel-

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oped and agreed upon.
4.2 RG-IEM
The RG-IEM discusses measures to promote theparticipation of industry in ITU-T. At this meeting,Canada proposed to add to the ToR of this RG arequirement to promote participation of next-generation personnel from industry in ITU-T and thoroughly review the current industry involvement process,including the current chief experience officer/chief
technical officer (CXO/CTO) meeting-coordination
process. There were various opinions such as from alegal perspective, and it is important to consider the
obligation to participate as well as acquiring rights
through participation, and while some said that
attending the CXO/CTO meetings was a very positive experience, other commented that there is an
inherent problem of not attracting people from industry to ITU-T. Therefore, it is important to solve thisfundamental problem so that the participation of
future engineers will increase.
There was also a proposal from China to send acircular including PP resolutions to organizations thatare not members of ITU-T to encourage participationof industry. Russia, Saudi Arabia, and others
expressed their agreement with the proposal, butthere were some who thought that it was a premature
idea, and the proposal was finally included as part of
the action plan for the future.
5. Future plans
The 2nd TSAG meeting for this study period isscheduled to be held in Geneva from May 30 to June2, 2023. Due to the short period until the next WTSA,
TSAG meetings are scheduled to be held approximately every six months, with the third TSAG meeting scheduled for January 2024. It was proposed thatthe final two meetings of TSAG for this study period
be held in conjunction with the Inter-regional Meetings.
Reference[1]

WTSA-20, https://www.itu.int/en/ITU-T/wtsa20/Pages/default.aspx

48

Global Standardization Activities

Noriyuki Araki

Director, Standardization Office, Research andDevelopment Planning Department, NTT Corporation.
He received a B.E. and M.E. in electrical andelectronic engineering from Sophia University,
Tokyo, in 1993 and 1995. He joined NTT AccessNetwork Service Systems Laboratories in 1995,
where he researched and developed operationand maintenance systems for optical fiber cable
networks. He has been contributing to standardization efforts in ITU-T SG6 since 2006. He was
the rapporteur of Question 6 of ITU-T SG6 from2006 to 2008 and rapporteur of Question 17 of
ITU-T SG15 from 2008 to 2012. He also served
as the chairman of the ITU-T Focus Group on
Disaster Relief Systems and Network Resilienceand Recovery. He was the vice-chairman of
ITU-T SG15 from 2013 to 2022. He also contributes to the activities of International Electrotechnical Commission (IEC) Technical Committee
86 (fiber optic systems). He received the ITU-AJ
award from the ITU Association of Japan in
2017. He is a member of the Institute of Electronics, Information and Communication Engineers(IEICE).

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Vol. 21 No. 5 May 2023

Jiro Nagao

Senior Manager, Standardization Office,Research and Development Planning Department, NTT Corporation.
He received a Ph.D. in information sciencefrom Nagoya University, Aichi, in 2007 andjoined NTT the same year. From 2007 to 2011, he
was engaged in research and development ofimage processing and content distribution technology. From 2012 to 2017, he worked for NTT
Communications, serving as the technical leaderof commercial video streaming services. From2017 to 2021, he was engaged in research anddevelopment of immersive media and presentation technology at NTT Service Evolution Laboratories. Since 2019, he has contributed to theinternational standardization efforts on immersive live experience (ILE) of ITU-T SG16. Heserved as an editor of ITU-T H.430.4 (ex H.
ILE-MMT) and H.430.5 (ex H.ILE-PE) from2019 to 2020. He received the ITU Associationof Japan Encouragement Award in 2021. He iscurrently an associate rapporteur of ITU-T SG16
Q8 (Immersive Live Experience, since 2022) andthe leader of ILE Sub Working Group of theTelecommunication Technology Committee(since 2020). He is a member of the Institute ofElectrical and Electronics Engineers (IEEE),IEICE, and the Japanese Society of Medical
Imaging Technology (JAMIT).

49

Special Report

50th Anniversary of NTT Yokosuka
R&D Center
Tomoyoshi Oono, Kei Karasawa, Takahiro Hamada,
Hiroyuki Aihara, Yasushi Okano, Kazuo Hirata,
Hirofumi Abe, Minako Izawa, Kyoko Suzawa,
Kouichirou Higuchi, and Chika Kitahara
Abstract

NTT Yokosuka R&D Center was established in November 1972 as the third research laboratory of the
Nippon Telegraph and Telephone Public Corporation (the predecessor of NTT), and it celebrated its 50th
anniversary in 2022. To commemorate this anniversary, the Center held a commemorative event on
December 9, 2022. At the event, Mr. Shinjiro Koizumi, a member of the Japanese House of Representatives, and Mr. Katsuaki Kamiji, mayor of Yokosuka City, gave their greetings, and many executives from
Yokosuka-based companies and organizations attended the event, which reaffirmed the bond between
the Center and the community.
Keywords: regional cooperation, NTT, Yokosuka

1. Introduction
Blessed with good weather, the NTT Yokosuka
R&D Center 50th Anniversary Lecture was held on
December 9, 2022. The event was attended byapproximately 100 guests, mainly companies andorganizations based in the Yokosuka Research Park
(YRP), the nearby neighborhoods of Yokosuka City,
as well as alumni of NTT Yokosuka R&D Center. It
provided an opportunity to introduce the past andpresent achievements of the Center and reaffirm ourcommitment to deepening regional ties and contributing to social development through regional cooperation (Photos 1, 2).
2. Commemorative speeches
In the first commemorative speech, Tomoyoshi
Oono, senior vice president, head of NTT Service
Innovation Laboratory Group, introduced Yokosuka
R&D Center at the beginning of his lecture andshowed a commemorative video (Photo 3).
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NTT Yokosuka R&D Center was opened in
November 1972 by then President Shigeru Yonezawaof the Nippon Telegraph and Telephone Public Corporation with the declaration that it would “conduct
research under five themes: information-processing
systems for data communications, large-capacitytransmission systems, ocean-communications systems, development and application of satellite communications, and research and practical applicationof mobile communications.”
These themes spread throughout the world in theform of the popularization of the Japanese-madegeneral-purpose mainframe-computer series “DIPS(Dendenkosha Information Processing System),” theconstruction of optical backbone communications
networks, the launch of Japan’s first domestically
produced communications satellite, and the development of mobile communications that started from carphones and led to smartphones. Another technologythat has flourished at Yokosuka R&D Center isimage-encoding technology. Image-coding technology began with the research and development of50

Special Report

Photo 1. Reception.

Photo 4. IEEE Milestone (G3FAX).

Photo 2. 50th anniversary event-management executives.

Photo 5. IEEE Milestone (LSP).

Photo 3. M
 r. Oono, senior vice president, head of NTT
Service Innovation Laboratory Group.

tion of LSP as the voice-coding standard of the U.S.
federal government in 1991 as well as its inclusion in
the 3rd Generation Partnership Project (3GPP) and
3GPP2 standards for third-generation cell phones in
1999. As of December 2022, 41 Japanese companies
have been awarded IEEE Milestones in recognitionof technologies and achievements—such as theTokaido Shinkansen bullet train, automatic ticket
gates for train stations, solar cells, VHS (Video Home
System) video, and car-navigation systems for automobiles—that have provided the foundation for modernization.
In the 1990s, NTT’s image-and-video coding technology was standardized. For example, in 1990,
when NTT held the chair of the ITU-T (International
Telecommunication Union - Telecommunication
Standardization Sector) working group (WG), H.261
was internationally standardized for video coding.
Around that time, when NTT was leading international standardization as the chair of the International
Organization for Standardization/International Electrotechnical Commission (ISO/IEC) Joint TechnicalCommittee 1/Subcommittee 29, MPEG (Moving

facsimile (fax) machines and culminated in the establishment of the G3FAX international standard in
1980, which was awarded an IEEE Milestone in 2012
(Photo 4).
In 2014, our high-compression speech-coding technology, called Line Spectrum Pair (LSP) and developed in 1975, was recognized with an IEEE Milestone (Photo 5). This award resulted from the adopNTT Technical Review

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Special Report

Photo 6. US Emmy Award for Technology Development.

Photo 8. M
 r. Koizumi, member of the House ofRepresentatives.

Photo 7. Mr. Kamiji, mayor of Yokosuka City.

Photo 9. M
 r. Kinoshita, vice president, head of NTT
Human Informatics Laboratories.

Picture Experts Group)-2, which is the basis of digital
broadcasting, received the US Emmy Award forTechnology Development (Photo 6). In 1994, when
NTT held the chair of the ISO WG, the JPEG (Joint
Photographic Experts Group) still-image-coding system was also standardized.
Next to speak, Mr. Kamiji expressed his expectations for an approach to solving problems through thefusion of cyberspace and physical space taking thepost COVID-19 pandemic era into consideration(Photo 7). After that speech, focusing on Yokosuka,
Mr. Koizumi gave a speech on the concept of developing human resources to deal with cybersecurity,
which is an imminent threat in cyberspace (Photo 8).
Next, Shingo Kinoshita, vice president, head of
NTT Human Informatics Laboratories, who was representing the Yokosuka R&D Center, introduced theresults of the united challenge of NTT laboratories
targeting the Tokyo 2020 Olympics, namely, themost-recent achievements of the Yokosuka R&D
Center (Photo 9). Following him, five young

researchers talked about their efforts targeting theupcoming Expo 2025 Osaka, Kansai, Japan (Photos
10–14).
The presentations by young researchers on theirefforts targeting the Expo 2025 were well received by
the guests, who were especially impressed to see
young researchers working energetically on theirresearch. NTT Senior Executive Vice President Katsu­
hiko Kawazoe, concluded the speeches by thanking
all the guests for their attendance, and the speeches
ended on a high note (Photo 15).

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3. Demonstration exhibitions
Before the start of the speeches and during thesocial gathering, two demonstrations from eachresearch laboratory—ones selected because they
seemed to be easy for the guests to understand—were
conducted. The details of each demonstration arelisted in Table 1. Many of the guests who wereunable to attend the NTT R&D Forum took the52

Special Report

Photo 10. M r. Muraoka, researcher at NTT Human
Informatics Laboratories.

Photo 13. M
 s. Miura, researcher at NTT Network
Innovation Laboratories.

Photo 11. M
 s. Kozuka, researcher at NTT Computer andData Science Laboratories.

Photo 14. M
 r. Nagano, researcher at NTT Access Network
Service Systems Laboratories.

Photo 12. M
 r. Sano, researcher at NTT Social Informatics
Laboratories.

Photo 15. K
 atsuhiko Kawazoe, NTT senior executive vicepresident.

opportunity to experience in detail the latest researchof NTT laboratories (Photo 16).

Planning, everyone enjoyed talking about the olddays with their alumni.

4. Social gathering

5. Concluding remarks

At the social gathering, after a toast given by Atsuko Oka, NTT executive vice president, head of R&D

The previous 40th anniversary was held in a moodof self-restraint, partly because it was right after the

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Special Report

Table 1. Demonstration exhibitions.
1F Exhibition Hall (dynamic exhibitions)

In front of 10F Lecture Hall (static exhibitions)

Human Informatics Laboratories

Remote World (piano-scale performance)

Metaverse in the IOWN Era

Social Informatics Laboratories

Secure Optical Transport Network Technology

Social Well-being

Computer & Data Science
Laboratories

Personalized Sound Zone

MediaGnosis

Network Innovation Laboratories Ultra-low-latency Video-transmission Technology

Digital Coherent Optical Transmission Technology

Access Network Service
Systems Laboratories

Future Wireless Infrastructure Technologies for
Realization of 6G/IOWN Concept

Extreme NaaSIOWN: Innovative Optical and Wireless Network
NaaS: network as a service

Photo 16. Exhibition area.

Photo 18. A scene from the social gathering.

Great East Japan Earthquake. This time, the COVID19 epidemic was still spreading, so we paid close
attention to infection-control measures, such as asking all attendees to take antigen tests before participating. We are confident that this commemorative
event significantly contributed to increasing the presence of NTT Yokosuka R&D Center in the YRP area
as well as Yokosuka City through strengthening ties
and laying the foundation for future collaborations
that will produce significant achievements.
Photo 17. A tsuko Oka, NTT executive vice president, headof R&D Planning.

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Special Report

Authors: (from left in the top layer) Tomoyoshi Oono, senior vice president, head of
NTT Service Innovation Laboratory Group; Kei Karasawa, general manager ofPlanning Department, NTT Service Innovation Laboratory Group; Takahiro
Hamada, director of Research Promotion, NTT Service Innovation Laboratory
Group; Hiroyuki Aihara, general affairs manager, NTT Service Innovation Laboratory Group; (from left in the middle layer) Yasushi Okano, business support manager, NTT Service Innovation Laboratory Group; Kazuo Hirata, research promotion
division manager, NTT Service Innovation Laboratory Group; Hirofumi Abe,
information strategy section manager, NTT Service Innovation Laboratory Group;
Minako Izawa, information strategy section manager, NTT Service Innovation
Laboratory Group; (from left in the bottom layer) Kyoko Suzawa, research promotion division manager, NTT Service Innovation Laboratory Group; Kouichirou
Higuchi, general affairs department chief, NTT Service Innovation Laboratory
Group; Chika Kitahara, General Affairs Department staff, NTT Service Innovation
Laboratory Group.

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55

External Awards
Best SEIP Paper Award
Winners: Shinobu Saito, NTT Computer and Data Science Laboratories; Kenji Takahashi, NTT Security; Yasuyuki Hamada, NTT
Security; Jan Vermeulen, NTT Americas
Date: December 7, 2022
Organization: 29th Asia-Pacific Software Engineering Conference(APSEC 2022)
For “RP2A: Rare Process-Pattern Analysis - Identifying Potential
Problem Process-Patterns by Analyzing System Execution Data.”
Published as: S. Saito, K. Takahashi, Y. Hamada, and J. Vermeulen,“RP2A: Rare Process-Pattern Analysis - Identifying Potential Problem Process-Patterns by Analyzing System Execution Data,” Proc. of
APSEC 2022, pp. 432–436, Virtual, Dec. 2022.
Electronics Society Activity Testimonial
Winner: Koji Azuma, NTT Basic Research Laboratories
Date: March 7, 2023
Organization: The Institute of Electronics, Information and Communication Engineers (IEICE) Electronics Society
For contribution as a co-chair of the 12th Quantum Information
Technology Symposium steering committee.
Young Researcher’s Award
Winner: Shimpei Shimizu, NTT Network Innovation Laboratories
Date: March 9, 2023
Organization: IEICE
For “Bandwidth Extension of PPLN-based Optical Parametric
Amplification.”
Published as: S. Shimizu, T. Kobayashi, T. Kazama, T. Umeki, M.

NTT Technical Review

Vol. 21 No. 5 May 2023

Nakamura, K. Embutsu, T. Kashiwazaki, K. Watanabe, and Y.
Miyamoto, “Bandwidth Extension of PPLN-based Optical Parametric Amplification,” Proc. of the 2022 IEICE Society Conference,
B-10-28, Virtual, Sept. 2022.
Awaya Prize Young Researcher Award
Winner: Tatsuya Kako, NTT Computer and Data Science Laboratories
Date: March 16, 2023
Organization: Acoustical Society of Japan
For “Simulation of Open-back Enclosure for Open Ear Hearable
Device.”Published as: T. Kako, H. Chiba, and K. Kobayashi, “Simulation ofOpen-back Enclosure for Open Ear Hearable Device,” Proc. of 2022
Autumn Meeting of the Acoustical Society of Japan, pp. 417–418,
Hokkaido, Japan, Sept. 2022.
Best Paper Award
Winners: Kei Fujimoto, NTT Network Innovation Center; Ko
Natori, NTT Network Innovation Center; Masashi Kaneko, NTT
Network Innovation Center; Akinori Shiraga, NTT Network Innovation Center
Date: April 10, 2023
Organization: IEICE
For “Energy-efficient KBP: Kernel Enhancements for Low-latency and Energy-efficient Networking.”
Published as: K. Fujimoto, K. Natori, M. Kaneko, and A. Shiraga,“Energy-efficient KBP: Kernel Enhancements for Low-latency andEnergy-efficient Networking,” IEICE Trans. Commun., Vol. E105-B,
No. 9, pp. 1039–1052, Sept. 2022.

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