R&D Spirits
Ultrasmall Photonic Integrated Circuits Using
Photonic Crystal
Dr. Masaya Notomi
Group Leader, Photonic Nanostructure Research Group
Optical Science Laboratory, NTT Basic Research Laboratories

The unique optical properties of photonic crystal, a material born of nanotechnology, areattracting considerable attention. The Photonic Nanostructure Research Group at NTT Basic
Research Laboratories is researching ways of applying those properties to new forms of opticalinformation processing. Can information-processing devices based on photonic crystal open up
new possibilities for communication networks? We put this question to Group Leader Masaya
Notomi, who also told us about his future vision for photonic devices and networks and the cutting-edge research now taking place in this area.

Successful fabrication of optical switches
after seven years of effort—In what way do you think this research will lead toadvances in information communications?

—Dr. Notomi, could you briefly describe your present
research?

Well, I think it will mean a real breakthrough in
optical communications. Although light is extremely
good at transmitting information at high bit rates,
information processing has not been one of its strong
points. That’s why electronic circuits have traditionally been used in nodes that require information processing. Since I have been researching photonics myentire working career, it seems to me that this has
been an obstacle to higher speeds and lower power
requirements in communication networks. Actually,
the use of light for information processing is not
impossible with present technology, but there areworries that doing so would require excessively large
equipment consuming lots of power and high costs,
resulting in a rather impractical system. On the other
hand, if optical information processing can beachieved on a chip using photonic crystal, the stage is
set for the construction of a low-power low-cost alloptical network providing ultrahigh transmission
speeds. Such a chip would also reduce equipment size
drastically enabling that equipment to be installed
just about anywhere. At the extreme end, we canenvision photonic-crystal chips being incorporated
inside personal computers for use by individual users.

Our group is currently researching photonic crystal,
which is an artificial structure having a periodicallymodulated refractive index (Fig. 1). In this research,
we fabricate a two-dimensional photonic crystal on asilicon-on-insulator substrate using advanced ultrafine processing technology developed by NTT. Our
plan is to use this structure as a basis for developing
new optical materials. Photonic crystal has unique
optical properties. For example, it has a wide photonic band gap in the wavelength region of optical communications that can function as an optical insulator.
The use of this property for creating previously
unachievable devices has, in fact, been researched forabout ten years on a theoretical basis, but it wasn’t
until 2000 that advances in ultrafine processing technology overcame previous implementation difficulties and made it possible to actually fabricate photonic crystal. Our group has worked on the fabrication ofseveral devices since we began full-scale research on
photonic crystal in 1998.

6

NTT Technical Review

R&D Spirits

Photonic crystal behaves likea semiconductor of light.

SOI: silicon on insulator
Color photograph of photonic
crystals on a 4-inch SOI wafer.

Frequency ωω = ck /nFrequency (ωa/2πc)

Dispersion relationshipof conventional materials

Electron micrograph of Si photonic
crystals (hole period ≈ 400 nm).

Dispersion relationshipof photonic crystals
0.8
0.6

Photonic insulator→ Strong confinement

Photonic bandgap

0.4

Slow light or fast light→ Controllable light propagation

0.2
0.0

Wavevector

It behaves like an exotic
never-before-seen opticalmaterial.

X

Wavevector

Z

Fig. 1. Photonic crystals as new artificial optical materials.

Of course, this is still at the conceptual stage, but weare carrying out our research with such possibilities
in mind.
—What technology comes into play when attempting
to achieve devices based on photonic crystal?
First of all, in electrical engineering, functions canbe achieved using the electrical properties of various
types of material. This is not the case in the field of
optics, where refractive index is the only physicalproperty that can be used and it cannot be varied verymuch. In other words, there are no equivalents toelectrical insulators and semiconductors in optics.
This has been a limiting factor in optical technology.
A photonic crystal, however, plays the role of a crystal lattice for light. This means that if a periodic structure can be fabricated with a period of from 200 to
400 nm, which is applicable to optical wavelengths, aphotonic band gap can be achieved that allows nolight to propagate, or in other words, that completely
confines light. The photonic band gap is enabling
designers to control light in ways that were impossible in the past. This, in conjunction with advances inultrafine processing technology, is bringing devices
like photonic ICs and photonic LSIs out of the theoretical domain and into the real world (Fig. 2).

Vol. 4 No. 1 Jan. 2006

—How is this research progressing at present?
We have already fabricated photonic-crystal waveguides and resonators as well as optical switches thatcombine those devices. Of these, the waveguide is thesimplest of all. With our waveguide, we can slow
down the propagation of light to 1/100 its speed in
vacuum. We expect this feature to enhance light-matter interaction, namely to enable sufficient use ofeven extremely faint light for switching and controlling, and we even envision its application to opticalbuffer memory. We have also achieved a high level ofquality with our resonators, and in 2005, we successfully fabricated an optical switch by combining one
resonator and two waveguides. This was our first genuine device. In addition to these achievements, wehave started work on the fabrication of a spot-size
converter for connecting a photonic-crystal chip with
optical fiber. As you can see, our research is finallybeginning to produce some tangible results, but interms of our ultimate objectives, we sometimes feelthat we are still at the starting gate.
—What form do you think your research will takethree and five years from now?
Our ultimate objective is large-scale integration, or
photonic LSIs. Reaching this goal, however, willneed about ten more years based on current forecasts,
7

R&D Spirits

Why are photonic ICs so difficult?
(1) Poor light confinement(2) Small interaction of light

Photonic crystals provide:(1) Strong light confinement(2) Enhanced interaction

NTT’s research plan and current achievements
Ultrasmall photonic-band-gap waveguides
World’s lowest propagation loss (~2 dB/cm)
Demonstration of slow light (~1/100 c)
1 µm

Wd
air

Ultrasmall
high-Q
resonators

0.7W

World’s highest Q
Qtheory = 7 × 107
Qexp = 1 × 106

Spot-size
converterOFF
Ultralow-power
all-optical digital
switching~10 fJc1-hole c2-hole c3-holeON
λAmode B mode A(signal) (signal)

Fig. 2. Photonic integrated circuits on photonic-crystal platform.

so we see our medium-term goals as focusing on
processes. To be more specific, our goal for the next
three years is to interconnect several devices and perform logic processes that have traditionally been difficult to achieve with light. And since we have yet toconstruct an all-optical information-processing chip,
we would like to do that as well. Within five years, wewould like to be able to deploy some of these devices
in some form of prototype for optical-communication
systems.

Taking the lead at the device process levelamid expanding research in Japan,
the United States, and Europe—Could you tell us about research trends in photonic crystal at both the international and domestic levels?
Research into photonic crystal has become quite
active during the past few years. The first international conference specializing in photonic crystal washeld in 1999 with about 100 researchers attending.
Since then, the number of photonic-crystal
researchers has been increasing in Japan, the UnitedStates, and Europe. Attendance at the 2005 conference was 400–500, which is higher than at the top
conferences on semiconductor lasers.
In terms of research trends, the theory behind pho8

tonic crystal was originally proposed by American
researchers, who are still strong in theory. There areexcellent theorists at MIT and Stanford University
issuing a steady stream of new theories. Europe takes
a more physical approach, while Japan is more
involved in practical research. NTT and a number ofuniversities in Japan are performing research with aneye to photonic ICs, and the devices fabricated by
NTT are on a world-class level. In particular, we haveachieved world record values for waveguide loss andresonator Q-value, which are the most important figures of the performance of photonic-crystal devices.
In the field of photonic crystal, research related todevice fabrication requires appropriate lithographyequipment and process technology, which is one ofNTT’s strong points. Nevertheless, we must strive tokeep ourselves up to date in this area so as not to lose
this lead.
—Are you involved in any joint research with otherresearch institutions?
Not at present, but we have previously been
involved in joint research on several occasions. For
example, we performed joint research on photoniccrystal resonators with the Korea Advanced Instituteof Science and Technology (KAIST). This institutehas produced the most results in photonic-crystal
research in Korea, and their research on light-emitNTT Technical Review

R&D Spirits

ting devices has been making significant progress. At
that time, we were both researching resonators, butthere was really no competition because our researchtarget was not light-emitting devices. The mutual
need for resonators was also an important factor inpursuing this joint research. Collaboration, as
opposed to direct competition, can therefore provide
huge benefits for each partner. Given the right situation, I would definitely want to engage in joint
research again.
—On the other hand, are you now competing withanyone in any areas?
There are indeed other institutions conducting
research on photonic-crystal devices using the samematerials as we are using. We are basically neck andneck with other institutions in terms of waveguideloss, for example. Competition can therefore be
painful at times, but it can also spur us on to achieveeven better results. It is also natural for competitors to
be conscious of each other’s successes, and meetingwith competing researchers at conferences and elsewhere can actually lead to enjoyable conversations.
The field of photonic-crystal research is dominatedby young researchers, so discussions tend to be frankand to the point. The theme itself was born of interdisciplinary studies, and we all feel that we are helping to develop the field of photonic-crystal research.
While knowing that other researchers in this field areboth rivals and colleagues, I always look forward tomeeting with them at academic societies.

a much needed public relations effect for photoniccrystal research.

Opening the door to the world of optics
through the properties of nanostructures— Dr. Notomi, how was your technical “backbone”
formed?
To begin with, I was a member of the superconductivity research laboratory at my university, but among
the many students there, I was the only one researching the charge-density wave state instead of superconductivity, which is another low-temperaturephysics theme. In any case, it was a field entirely
unrelated to light. This was a time of “superconductivity fever” in the world of research, and while I wasadvised by my instructors not to diverge from superconductivity themes, I did not feel the need to follow
others and I simply formed my own path. Then, on
entering NTT, I was again expected to research superconductivity, but as that was something that I had chosen not to pursue in university, I again held fast anddid not change my mind. In the end, I expressed thedesire to research new physical properties using
nanostructures, and I was consequently assigned to
NTT Optoelectronics Laboratories. This assignment
surprised me at first as I was thinking more along thelines of electronic devices, but researching artificial
nanostructures turned out to be just what I wanted andI adjusted to my new environment quickly.
—What research themes have you tackled to date?

—What other international activities have you been
involved with?
I have had the pleasure of serving on committees
associated with various international conferencessuch as IEEE LEOS, Optical Fiber Communication(OFC) of the Optical Society of America (OSA),
Integrated Photonic Research (IPR), CLEO PacificRim, and SPIE (International Society for OpticalEngineering). I would often be asked to serve on suchcommittees when there were plans to set up a section
on photonic crystal within general academic societies
related to optics. I have also served on the editorial
boards of academic journals dealing especially withphotonic crystal research. Recently, I have frequently
been a guest speaker at international conferences,perhaps five or six times a year, which has been making me quite busy during the conference season. But
I feel that speaking like this at such forums provides
Vol. 4 No. 1 Jan. 2006

The first theme that I worked on at NTT was semiconductor quantum wires and dots. The idea here wasto fabricate one-dimensional or even zero-dimensional semiconductor structures using fine processing
technology. Since our goal was to confine electrons—
at dimensions even smaller than the wavelength oflight—in artificial structures, our problem was toachieve feature dimensions on the order of 10 nm.
This turned out to be a huge challenge, but I came towrite books and papers on the subject and evenobtained a degree in relation to it. But to be quite honest, I eventually realized that continuing that research
theme in its present form was going to be an extremely difficult undertaking.
Then, around that time, a professor from Linköping
University in Sweden came to NTT as a guest
researcher. He suggested that I perform measurements on our samples at Linköping University
9

R&D Spirits

because he thought that I might be able to obtain
interesting data. Thinking that this was a good opportunity that I should not pass up, I took up his offer andended up spending a year there from 1996 to 1997 as
a visiting researcher. During that time, while measuring samples and writing papers, I thought about avariety of things. In fact, I had lots of time to think,which was very fortunate. I also had a chance toobserve world research trends, and it was at that timethat I first took notice of research on photonic crystal.
Compared with quantum wires, the feature dimensions of photonic crystal (200–400 nm) are one orderof magnitude larger. These were dimensions that I
thought I could work with. Then, upon further study,
I found that there were many things that had yet to betheoretically explained about photonic crystal since it
was a new field, and my interest in photonic crystal
deepened all the more. But what attracted me the mostwas that I was in a position to work on the most difficult problem in photonics—the confinement of light.
With my future direction found, I started research on
photonic crystal soon after returning to Japan.
At first, I proceeded alone, but as I began to talk tomore and more of the people around me, the numberof researchers working with me began to increase.
Then, in 1999, when whole groups at NTT Optoelectronics Laboratories were moved to the present NTT
Basic Research Laboratories, photonic crystal
became the central research theme of my group. After
that, I also became involved in management duties inparallel with my own research.
—What do you aim for in yourresearch activities?
While there are various theories and technologies on mymind, what I have come to aim
for on the whole is how to collaborate with many researchers
to achieve results that could not
be achieved by a single person.
This style dominates my research
today and makes my work feeleven more worthwhile. Of
course, arriving at this point in
my research career took sometime, but the environment that I
need has gradually taken form. In
just these last two years, I feelthat research itself is starting toprogress rapidly, and I think that10things are going to become very interesting from hereon.

Advancing photonic-crystal research with aneye to new possibilities—Where do you think your research is headed fromhere on?
For one thing, we want to move toward photonic
integrated circuits as I mentioned earlier. In parallelwith this effort, I also wish to research related themes,
especially the physical properties of photonic crystal
not found in conventional materials and the other fascinating physical phenomena that it exhibits besides
the photonic band gap.
For example, while the refractive index of materials
in the natural world is always positive, light canundergo negative refraction in photonic crystal (Fig.
3). That is to say, light passing through an interface
like glass will normally become diffused, but on passing through a photonic-crystal interface, it can undergo the opposite effect and become focused. Thismeans that a flat panel can act as a lens! In this case,imaging is also peculiar. A normal lens makes faraway things appear closer and nearby things farther,but photonic-crystal imaging does just the opposite—
it makes faraway things farther and nearby things
nearer. In the past, this negative refraction of light was
discussed in theory from a mathematical perspective,

Negative refraction

Pointsourcen

NegativePC–nImaging
Negative
PC

Air
PC

PC: photonic crystal

Fig. 3. Negative refraction in photonic crystals.

NTT Technical Review

R&D Spirits

but as no demonstrations were forthcoming, it wasn’t
actually given much attention. This changed in 2000
when I wrote a paper describing the discovery anddemonstrations of negative refraction using photonic
crystal [1]. The paper generated keen interest in negative refraction as a new principle that held many possibilities. I myself find the possibility of three-dimensional photography based on negative refraction very
interesting. Technically speaking, this might be moredifficult than photonic ICs, and there is a study being
performed on whether it would be appropriate as an
NTT research theme. At any rate, I would love to pursue this research if and when appropriate.
—What is your ultimate goal as a researcher?
I wish to propose new theories, make things basedon those theories, and demonstrate their operation
with the hope that they will come to be accepted andused by the outside world. I would also like to workon cultivating photonic-crystal research—which is
now enjoying a sudden rise in interest—into a major
field worthy of its own full-fledged textbooks.
—How do you yourself find working at NTT Laboratories?
I would be hard pressed to find anything better. On
entering the company, I first thought in a somewhat
simplistic manner that the wonderful facilities provided by NTT Laboratories would make it a goodplace to undertake basic research. But it didn’t takeme long to realize that the wonderful people who
worked there were of even greater importance. Here,
there are specialists in a wide variety of fields. If
there’s something that I don’t understand, no doubt
there will be someone who is quite familiar with thesubject and can help me out. NTT Laboratories is likea fountain of knowledge. It is also a place where organizational boundaries are relatively easy to cross to
exchange information and know-how. For example, I
have been able to conduct joint projects withresearchers in NTT Photonics Laboratories and Network Innovation Laboratories. Another attractive
point is that NTT Laboratories provides systematicconnections between basic research, platform
research, and application research and even withcommercialization on the business side of things.
Although I had a choice to stay on at university andcontinue with basic research, I now feel that I wasvery fortunate to have entered NTT. I am working in
a superb research environment rare even by world
Vol. 4 No. 1 Jan. 2006

standards, and I would like to produce results thatmake the best of what NTT Laboratories has to offer.
—Dr. Notomi, could you leave us with a message foryoung researchers?
Well, I must first say that young researchers appear
to be a bit conservative these days. This is perhaps
particularly true in the field of optics. The reason forthis may be a feeling that the future direction of thisfield has already been decided and that there is no use
in trying to develop new themes. In research anddevelopment, however, it is hard to obtain goodresults without challenging yourself and taking on acertain amount of risk. When I was starting out on
photonic-crystal research, it was often referred to as adead-end theme. But a strong feeling of convictionthat I was on the right path and a step-by-step
approach eventually led to great things. I would
therefore emphasize to young researchers that it isvery important to undertake a research theme that you
are convinced has merits and to continue with it without getting pessimistic in the face of setbacks. Be
positive in your research and do not jump to conclusions about its possibilities. Challenge yourself toachieve great things!
Reference[1]

M. Notomi, “Theory of Light Propagation in Strongly-modulated
Photonic Crystals: Refraction-like Behavior in the Vicinity of the
Photonic Band Gap,” Phys. Rev. B62, 10696, 2000.

Interviewee profile■ Career highlights
Masaya Notomi received the B.E., M.E., and Dr.
Eng. degrees in applied physics from the University ofTokyo, Tokyo in 1986, 1988, and 1997, respectively.
In 1988, he joined NTT Optoelectronics Laboratories,
Atsugi. Since then, his research interest has been tocontrol the optical properties of materials and devices
by using artificial nanostructures, and he has engaged
in research on semiconductor quantum wires/dots andphotonic crystal structures. He is currently a Distinguished Technical Member of NTT Basic ResearchLaboratories, Atsugi and the leader of the photonic
nanostructure research group. From 1996-1997, he
was with Linköping University, Sweden as a visiting
researcher. Since 2002, he has been a guest associate
professor of Tokyo Institute of Technology. He is amember of the Japan Society of Applied Physics andthe American Physical Society.

11