According to our Special Topics analysis of Photonic
Crystals, the work of Dr. Masaya Notomi ranks at #4 by
total cites, based on 49 papers cited a total of 2,332
times. Four of these papers appear on the lists of the
most-cited papers in this topic over the past decade and
over the past two years. In
Essential Science IndicatorsSMfrom
Thomson
Reuters, Dr. Notomi's record includes 70 papers cited a
total of 2,648 times between January 1, 1998 and October
31, 2008.
Dr. Notomi is a Distinguished Technical Member of NTT Basic Research
Laboratories in Japan, where he leads the Photonic Nanostructure Research
Group. He has also been a Guest Associate Professor at the Tokyo Institute
of Technology.
In this interview, he talks
with ScienceWatch.com about his highly cited
work.
Would you tell us a bit about your
educational background and research experiences?
When I was a graduate student in the department of applied physics at the
University of Tokyo, I studied exotic condensed electron states in
chemically synthesized one-dimensional inorganic conductors. When I joined
NTT Laboratories in 1988, I started a project of low-dimensional electron
systems artificially fabricated by nanofabrication lithographic
technologies, namely semiconductor quantum wires and dots, particularly for
novel optical devices or components.
Over several years of the study, we had demonstrated various interesting
optical properties of quantum wires and dots. I was satisfied with the
results, but I started to feel that it is not so easy to drastically change
the performance of optical devices using those lithographically patterned
quantum wires or dots. It requires a large area of 10-nm-order quantum
wires/dots with very good homogeneity, which was still not easy even with
the state-of-the-art nanofabrication technologies. That was around 1997.
Then, I switched my subject to photonic crystals.
What first attracted you to work in photonic
crystals?
The concept of photonic crystals, which was introduced in the 1980s, is
essentially the photonic analogue of band electrons in solids. A dielectric
structure whose refractive index is periodically modulated can exhibit a
variety of novel optical properties due to its band nature. Photonic
crystals were first realized in radio frequencies, and in the 1990s several
groups had already started to fabricate photonic crystals in optical
frequencies, which needed nanofabrication technologies.
"I believe that photonics is
superior to electronics in ultrahigh speed
information processing with low energy
consumption, but photonics has some
drawbacks, which are deeply related to
fundamental aspects of
photons."
My first impression was that it seemed rather easier to fabricate them in
comparison with quantum wires/dots because the required dimension is one
order of magnitude larger, typically 200-400 nm. Though later I found that
this simple-minded impression was not very correct, I was somehow attracted
to them because they seemed to have potential to drastically change the
optical properties more directly than quantum wires/dots, which change the
optical properties only via small light-matter interaction. At that time, I
had a strong temptation to overcome the weak points of photonics, and I
felt that photonic crystals could do much in this context.
Your most-cited original paper in our analysis is
the 2000 Physical Review B article, "Theory of light
propagation in strongly modulated photonic crystals: Refractionlike
behavior in the vicinity of the photonic band gap." Would you walk our
readers through this paper—its goals, findings, and
significance?
Photonic crystals are a very simple concept, and many existing materials
and phenomena, such as diffraction grating and x-ray diffraction in
crystals, can be, in principle, categorized as photonic crystals or
phenomena in them. Some people criticized that photonic crystals were just
another interpretation of old things. However, two- or three-dimensional
dielectric periodic structures with a large refractive index contrast had
not existed or been considered before the photonic crystal research, and I
expected that such strongly modulated photonic crystals should lead to
totally new optical phenomena.
At that time (1998-1999), various exotic light propagation phenomena were
found in photonic crystals. Superprism, which I was coauthoring, is one
such example. The propagation angle of light can be steered in a very
exotic way in photonic crystals at certain conditions, which found various
interesting applications. However, I noticed that such phenomena do not
essentially require a large index contrast. In fact, these phenomena are
closely related to light propagation in diffraction gratings.
Thus, I next started to look for a distinguishing phenomenon that only
occurs in strongly modulated photonic crystals. Soon after, I found that
light propagation in strongly modulated photonic crystals can be
fundamentally different from diffraction gratings when the frequency is
close to the photonic band gap edges. In such situations, photonic crystals
behave as if they were homogeneous dielectric materials having artificial
refractive index.
The most amazing part of this finding is that this artificial index can be
theoretically negative. In those days, refractive index had been believed
to be always positive. So, next I tried to figure out what exotic phenomena
may arise when the index happens to be negative. It was surprising for me
that various interesting phenomena, such as flat lens, open cavity, image
transfer, etc., were easily derived by just assuming negative index for
conventional geometrical optics. I was especially mesmerized by the
flat-lens effect by negative refraction because this lens is so
fundamentally different from conventional lens. It does not follow the
well-known Newton's formula, and can produce a three-dimensional image. In
this particular paper, I demonstrated these phenomena with analytical
simple theories and numerical simulations.
It was interesting that negative index was found in metallic metamaterials
by Dr. Smith and his colleagues at UCSD at almost the same time. The
physics of negative index is fundamentally different, but the timing was
exactly matched synchronicitically. Since negative index brought in
fundamentally new ways to study conventional optics, many people excitedly
joined this field. After our publications, a vast number of publications
came up in the field of negative index materials.
A couple of your papers discuss slow light
phenomena. What exactly are these and why are they important?
As one of the most important aspects of photonic crystals, photonic
crystals can artificially control the light velocity. The light velocity is
usually determined by the material’s refractive index, which cannot
be varied very much, but it can be drastically altered by the photonic band
structure engineering. Prior to our research, it had been demonstrated that
exotic material dispersion in ultracold atoms driven by a laser can
drastically reduce the light velocity. This achievement was great, but I
wanted to realize "slow light in a chip" using a dielectric structure at
room temperature.
In 2001, we achieved a photonic crystal waveguide with substantially small
loss. Theoretical calculations showed that our waveguide can show
significantly slow light states. Soon after, we succeeded in measuring the
group velocity of light in our photonic crystal waveguides, and
demonstrated that the light velocity is reduced down to c/90. This work was
the first demonstration of slow light in dielectric structures, and later
on various kinds of dielectric slow light media were reported.
"...slow light has become one of the
hottest topics in today's photonics
research."
We also published several papers relating to slow light, and recently we
achieved c/50000 using an ultrahigh-Q micro-cavity in a photonic crystal
(This work was selected as Scientific American magazine's Top 50
in 2007). "Slow light in a chip" is important for two reasons. First, it
can potentially lead to an optical buffer memory, which is a fundamentally
difficult and important component for future photonic circuits. Second,
light-matter interaction is strongly enhanced in slow-light media because
of long interaction time and spatial compression of light. Thus, it can
lead to ultralow-power all-optical devices in a chip. For these reasons,
slow light has become one of the hottest topics in today's photonics
research.
Is there any aspect of your work that you are
particularly excited about?
Yet another important aspect of photonic crystals is their ability in
strongly confining light. It is well known that it is intrinsically
difficult to confine light in a small space, which severely limits the
potential of photonics technologies. A photonic crystal having a band gap
is essentially a photonic insulator that does not exist in nature and can
block the light propagation and penetration without absorption. This leads
to strong confinement of light in a wavelength-scale volume. Currently,
many researchers in this field are working towards ultrasmall and
ultrahigh-Q cavities in photonic crystals. We reported a wavelength-sized
cavity that can store light more than one nanosecond. Such performance is
hardly available in systems other than photonic crystals. We are currently
studying what we can do with such small and high Q cavities. As I
explained, we have applied them to slow light. As another example, we apply
them to all-optical switches and memories and have achieved extremely small
consumption energy operation.
I have always been looking for breakthroughs in today's photonics
technologies. I believe that photonics is superior to electronics in
ultrahigh speed information processing with low energy consumption, but
photonics has some drawbacks, which are deeply related to fundamental
aspects of photons. Electrons can be easily confined, accelerated,
decelerated, and stopped by applying voltage. However, photons cannot
because of intrinsically small interactions. This makes information
processing purely by photons difficult. As I explained, photonic crystals
can alter the nature of lightwaves in media. Ultrasmall cavities and slow
light states may enable us to overcome these intrinsic weak points of
photonics. People are also constantly finding novel optical phenomena in
photonic crystals such as negative refraction, which will add new
functionality in photonics, as well.
What are your hopes for this field for the
future?
My ultimate hope is to control (or design) all the optical properties of
materials totally by artificial ways. Photonic crystals are definitely very
effective in this respect, but there are other fields relating this issue,
such as metamaterials, quantum nanostructures. Although the controllability
of intrinsic optical properties in existing materials is rather limited, we
may be able to expand the possibility of optics to unthinkably large extent
by combining those new fields in optics.
What would you like the "take-away lesson" about
your research to be?
In the beginning process of my research on negative refraction, I was
motivated by a strong urge to clarify the fundamental newness in photonic
crystals. Especially when the research field is in an embryonic stage, many
things are mixed up with old things. It is very important to try to figure
out what the newness is. In my case, this approach almost automatically led
me to find the negative refraction phenomenon.
Dr. Masaya Notomi
Photonic Nanostructure Research Group
NTT Basic Research Laboratories
Atsugi, JAPAN
Notomi M, "Theory of light propagation in strongly
modulated photonic crystals: Refractionlike behavior in the
vicinity of the photonic band gap," Phys. Rev. B
62(16): 10696-705, 15 October 2000. Source:
Essential Science Indicators from
Thomson
Reuters.