Samco-Interview

Dr. Satoshi Iwamoto
Professor, Research Center for Advanced Science and Technology,
The University of Tokyo
In this interview, we visit the University of Tokyo’s Research Center for
Advanced Science and Technology to talk with Dr. Satoshi Iwamoto
about his research on topological photonics and other topics.
BackgroundMar. 2002 Completed Doctoral program in Applied Physics, Graduate School of Engineering,
The University of Tokyo (Doctor of Engineering)
Sep. 2002 Research Associate, Institute of Industrial Science, The University of Tokyo
Jul. 2003 Lecturer, Institute of Industrial Science, The University of Tokyo
Sep. 2003 Lecturer, Research Center for Advanced Science and Technology, The Universityof Tokyo
Oct. 2007 Associate Professor, Research Center for Advanced Science and Technology, TheUniversity of Tokyo
Apr. 2009 Associate Professor, Institute of Industrial Science, The University of Tokyo
Apr. 2019 Professor, Institute of Industrial Science, The University of Tokyo
May 2019 Professor, Research Center for Advanced Science and Technology, The Universityof TokyoAwards
2000 Presentation Encouragement Award, Japan Society of Applied Physics
2005 International Conference on Solid State Devices and Materials Best Paper Award
2008 Activity Merit Award, Electronics Society, The Institute of Electronics, Information andCommunication Engineers (awarded in 2008, 2012 and 2019)2012 Young Scientist Award, Commendation for Science and Technology by the Minister ofEducation, Culture, Sports, Science and Technology
2017 APEX/JJAP Editorial Contribution Award (awarded in 2017 and 2020)2018 The 23rd Microoptics Conference, Best Paper Award (awarded in 2018 and 2019 (24th))
2020 The 44th Laser Society of Japan / Distinguished Paper Award (Review Paper Section)(awarded in 2020 and 2021 (45th))
Selected as OSA Fellow (currently OPTICA Fellow) in 2021
2022 DOCOMO Mobile Science Award for Excellence in Basic Science

Realizing a waveguide in which light travels in only one direction
is a very challenging research goal we are aiming for.
▶ Please introduce your current
research.
Our laboratory is working on photonicnanostructures, which are microstructures,
mainly photonic crystals, for the control oflight. Currently, we are focusing on threethemes. First, topological photonics,second, diamond nanophotonics, andthird, quantum nanophotonics. Topologicalphotonics is an attempt to explore newfunctions and applications by rethinking
the control and propagation of lightbased on a new concept of topology.
The behavior of electromagnetic waves,including light, is described by Maxwell’sequations, and photonics technology,
which is widely used in communications
and optoelectronics, has developed on
this basis. However, there are still someissues to be solved in optical waveguides,
which are necessary for small integrated
optical circuits like electronic circuits. For
example, structural defects and losses

vol.120 / 2023. JAN.

occur when optical waveguides are bent
abruptly. In topological photonics, which is
our focus, the concept of topology can beused so that even if there is a defect on the
optical waveguide, light can travel throughit without detecting the flaw. It may also be
possible to realize the function of an opticalisolator, like a diode, in which light travels
in only one direction, with a small device.
Realizing a waveguide in which light travels
in only one direction is a very challenging
research goal we are aiming for.
Diamond nanophotonics is the study ofcontrolling light by fabricating photonic
structures using diamonds insteadof materials such as Si or compound
semiconductors. This is part of the“Development of Quantum Interfaces forConstruction of Quantum ComputationalNetworks,” which is Goal 6 of theMoonshot R&D Program led by the Cabinet
Office and for which Professor Hideo
Kosaka of Yokohama National University
is the project manager. Although diamondnanophotonics research is flourishingoverseas, it is not very advanced inJapan. Our presentation at the upcoming
conference will probably be the first paper
on diamond nanophotonics in Japan. We
are finally getting to the point where wecan fabricate hollow structures such as airbridges.
Finally, we are working on quantum
nanophotonics. This is related to thediamond nanophotonics I mentioned
earlier, but it involves research related tothe development of basic technologies
for quantum interfaces. For example, in
the Kosaka project, we are working on theconcept of converting superconducting
qubits into light in order to connect aquantum computer to another quantum
computer. Our mission is to develop
quantum nanophotonics and to develop
devices that serve as interfaces using
photonic crystal nanocavities and otherdevices. This may be a dream-like
technology that we are looking forward toseeing its future developments.
▶ Please tell us more about topologicalphotonics.
Topology is a branch of mathematics
that studies and understands the shapeof “things” in the big picture. A ball anda doughnut are well-known examples.
Even if a ball is continuously deformed,
it cannot be deformed into a doughnutwithout a “discontinuous” operation suchas drilling a hole. In this case, the ball andthe doughnut are topologically different.
In the field of condensed matter physics,
new materials such as quantum spin Hall
insulators and topological insulators havebeen discovered based on the topological1

Samco-Interview

Figure 1:
Valley photonic crystal waveguideunderstanding of the quantum Hall effect.
In these materials, electrons flow in one
direction at the boundary (edge) where two
topologically different structures meet.
This is called the topological edge state.
In 2008, British physicist Duncan Holden
and his co-authors first showed that this
topological edge state can occur in lightpropagating through periodic structuresin the same way as electrons in crystals.
Shortly after, in 2009, MIT demonstrated
topological edge states in an experimentusing microwaves. Microwaves have alower frequency, and there are several
materials that tend to change their
response to electromagnetic waves whena magnetic field is applied. Therefore, it iseasier to achieve topological edge states
than with light. However, the realizationof the topological edge state with light,which transmits electromagnetic waves
in only one direction, as I mentioned at
the beginning of this article, is still a way
off. If we can achieve this, we will be able
to create a compact optical isolator thatphysically travels in only one direction even
if it has defects, which will pave the way
for more sophisticated optical waveguides
and the miniaturization and integration ofrelated devices. This research has a very
ambitious theme.
However, since research cannot proceed
in a single step, topological photonics,
which is widely studied around theworld, is based on structures that can be
fabricated using only semiconductors.
One such structure is the Valley photonic
crystal waveguide. Valley photonic crystals
have time-reversal symmetry, so that somelight travels to the right and some to the left
at the same interface. In a valley photonic
crystal waveguide with Z-shaped steep
bends (Figure 1), inter-valley scattering issuppressed even at the bends, and good
propagation characteristics are obtained.

vol.120 / 2023. JAN.

▶ Please explain how you started
your research and how it has been
progressing.

paper in 2008. In the meantime, Dr. Shun
Takahashi of Kyoto Institute of Technology
was working on a spiral three-dimensional
photonic crystal called a chiral photonic
crystal when he was still a student in
Arakawa and Iwamoto’s lab. Dr. Takahashi
came to discuss the topic at a conferencein 2016 after hearing a presentation
by a student in Prof. Hatsugai’s lab at
Tsukuba University, who is very famous for
topological physics, about the appearanceof topological edge states in a similar
photonic chiral structure. That was thebeginning of our joint research. At about
the same time, I was also doing researchon the use of topological effects on elastic
waves in the field of phononic crystals,
which deals with sound waves. Prof.
Hatsugai and I continue to collaborate
on research, along with my ongoing
discussions with a professor specializing
in phononic crystals. These encountersaccumulated and led me to start researchon topological photonics.
If photonics and materials science areon the vertical axis, topology is like thehorizontal axis. When we look at photonics
from the horizontal axis of topology, there
are concepts that are common to otherfields. For this reason, since we started
our research on topological photonics,
we have expanded our interdisciplinary
collaborations and collaborations withphysics professors. We are trying toadvance photonics with the concept oftopology, and new phenomena realizedin photonics may create new applications
in different fields. I feel that this is anotherinteresting aspect of conducting researchin topology.

I originally planned to become anarchitect. My parents were in theconstruction business, and having watchedthem since I was a child, I somehow
thought that I would be the one to take overthe business. However, I decided to pursue
a career in physics because of a physics
class given by Mr. Hiroshi Kawakatsu when
I was a sophomore in high school. He had
published a book titled “Mr. Kawakatsu’sPhysics Class”. In his classes, he would
start with an experimental subject andhave the students predict the results of theexperiment from a list of options. Then hewould conduct the experiment, and when
the results were obtained, he would later
explain why they were the way they were. It
was that kind of class.
WhenI
entereduniversity, the themes
The RIE-400iP is used for diamondof optical computersand optical information
photonic crystals and InP photonic crystals
processing
werein the topological photonics process.
gainingmomentum,andI
joinedalaboratory in the field▶ You have experience usingof optics, which led me to my current
Samco equipment, what are yourresearch. During my doctoral work, Iimpressions?
studied nonlinear optics of semiconductors
in the Kuroda-Shimura Laboratory. At that
Samco’s equipment has been used bytime, I was allowed to use the equipment in
us for more than 20 years, from the ICPArakawa Laboratory to fabricate samples.
etching system RIE-101iP introduced
I am very glad that I had the experiencein 1999 to the plasma enhanced CVDof designing, fabricating, evaluating, andsystem to be delivered in 2023. The RIEanalyzing the devices by myself. After
400iP, purchased in 2021, has a lowergraduation, I was fortunate enough toelectrode that can be heated to 200°C
be accepted as a research assistant inand is used for diamond photonic crystals
Arakawa Laboratory, where I began my(Figure 2) and InP photonic crystals in theresearch on photonic crystals in earnest.
topological photonics process. The RIEI had been interested in topological
400iP has a more sophisticated control
photonics since it was published in asystem and is easier to understand in2

Samco-Interview▶ How do you spend your holidays?
I am very busy these days, so I don’t getlong weekends off, but I like walking, and I
walk several kilometers on the weekends.
I also like saunas. Sometimes I go to thesauna on a day when I am completelyoff, not even looking at my e-mail mostof the time. Walking and saunas are good
refreshments for me, because I can forget
about my work.
Figure 2:
Diamond photonic crystal
structure

many aspects. In addition, I am satisfied
with the support offered by Samco. They
have been maintaining and taking care of
the RIE-101iP that I have been using since
the days of Prof. Arakawa, and I think they
are quick to respond when it breaks down.
I also like the fact that I can consult withthem immediately about technical issues.
In total, I feel that it is a great thing for us
researchers to have such a manufacturer.
▶ What do you keep in mind in yourdaily research?
One is “Curiosity Driven,” and ourlaboratory places importance on thinking
about what we find interesting. Of course,
there are some research projects that havea specific target and are directed towardthat target, but we believe that just goingalong the rails would not be interesting forthe students. I also encourage students
not to do research based on what I or
someone else tells them to do, but to makeup their own stories and plans and try to
work on them, even if they fail. It’s not thatthey should just go ahead in the dark, butrather that they should give it a try if they
are holding back. It’s a difficult balance.
Another thing I value is the connection
among people. I try to visit students’ rooms
as much as possible and have dinner withthem so that we can discuss various topics
and enjoy thinking. I think it is the bestthing for students to spread their wings
after working in Iwamoto Lab, so although
I am not a behind-the-scenes supporter, I
hope I can help them grow and give thema little boost.
Finally, I would like to talk with students
not in a faculty-student relationship, butin a researcher-researcher relationship.
Of course, if they ask me questions, I willteach them, but I don’t want them to thinkthat what I say is absolute. There are newvol.120 / 2023. JAN.

findings in what students say as well, andI value that. Furthermore, I tell those whoare earning a doctorate, that one only truly
earns their doctorate when they have as
least a deep understanding of their subject
as I do. After all, I am an expert.
▶ Do you have a motto?

▶ Finally, do you have anything you
would you like to say to Samco?
I would like to begin by saying how grateful
we are to the company for supporting ourresearch. I feel that Samco is supporting
Japan’s cutting-edge research with theiradvanced technological capabilities. In
recent years, I feel that in many places
in Japan, there has been an increasing
choice not to have fundamental or basic
technology. While foundry services and thelike are a good thing in some respects, on
the other hand, I fear that locally cultivatedtechnologies may be lost. I hope thatSamco, as an equipment manufacturer
that produces fundamental technologies,
will continue to provide the latest process
technologies. If I may say more, I would liketo see Samco operate that technology inthe form of an open laboratory, a platformthat can be used by any researcher.

I am fond of the saying “every momentcounts” which means that even a small
amount of time should not be wasted. I do
not use this phrase on a regular basis, soI may not call it my motto, but the purpose
is right. Time is important,and I try to spend every
moment carefully. Rest is
Samco is supporting Japan's
also an important use ofcutting-edge research with theirtime, and I try to keep aclear distinction between
advanced technological capabilities.
on and off so that I don’t
just do research all the time.

Clean Room, CCR Building, Komaba II Campus

3Samco-Interview

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