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JAPAN NANONET BULLETIN
-- 57th Issue -- November 10, 2005
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Manipulating light with photonic crystals
-- Nanostructures lead to photonic bandgap formation --"
Susumu NODA, Professor, Department of Electronic Science and
Engineering, Graduate School of Engineering, Kyoto University
Young Researchers' Introduction:
"Realization and application of left-handed materials using
ferromagnetic-metal nanocomposite films"
Satoshi TOMITA, Researcher, Precursory Research for Embryonic
Science and Technology (PRESTO), Japan Science and Technology Agency
(JST)
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
Manipulating light with photonic crystals
-- Nanostructures lead to photonic bandgap formation --
(Issued in Japanese: March 9, 2004)
Susumu NODA, Professor, Department of Electronic Science and
Engineering, Graduate School of Engineering, Kyoto University
Optical devices can be miniaturized to as small as one hundred
thousandths of their current size using photonic crystals. Photonic
crystals are a kind of photonic insulators, through which specific
wavelengths of light cannot propagate due to a photonic bandgap. When
two or more substances that have a large difference in their
refractive indices are arranged alternately with a period of a half
wavelength, a band gap structure, through which the light of the
certain wavelength cannot propagate, is formed.
Prof. Noda fabricated the world's first photonic crystals that have a
lattice-like structure with stacked stripes of GaAs. Free-spaces in
the stacked stripes structure form a diamond lattice structure, and
light is reflected due to the difference in refractive index between
air that fills the free-spaces and GaAs. To block light with
wavelengths of 1.4 to 1.6 microns, which are used in optical
communication devices, 200 nm wide stripes have to be stacked with a
period of 700 nm, and the degree of accuracy must be within 30 nm. He
developed alignment equipment that uses optical laser diffraction
patterns when stacking stripes of GaAs. He, then, fabricated photonic
crystals with sharp stripe edges by maintaining the temperature of
wafer fusion which binds the surfaces of the stripes together at about
500 degree C. Light within a photonic bandgap cannot propagate through
a photonic crystal, but when defects are introduced into the crystal
to disturb the periodic structure, light can propagate through the
photonic crystal via the defects. When a pair of crossed stripes is
removed, a sharp bending waveguide is made, and therefore, light can
be bent at a right angle. Point defect cavities act as resonators and
confine light within the cavities. Light can be amplified, or
oscillated, using point defect cavities, and the wavelength of
confined light depends on the size of the point defect cavities. Prof.
Noda showed how to obtain the desired wavelength of light by changing
the sizes of the defects and free-spaces within the stacked stripe
structures.
Making two-dimensional photonic crystals is easier than three-
dimensional photonic crystals. When air holes forming triangular
lattice patterns are made with a period of a half wavelength on a thin
silicon plate with a thickness of 250 nm, a photonic bandgap is formed
due to the periodic structure in the in-plane direction, and light
within the bandgap cannot propagate through the crystals. In the
vertical direction with a non-periodic structure, light is confined
because of total internal reflection, which occurs at the interface
between silicon and air. Introducing a line defect by removing a
single row of air holes forms a linear waveguide. When point defect
holes are introduced by increasing the hole radius, a specific
wavelength of light, corresponding to the radius of the defect hole,
is confined within the defects, and then light is emitted
perpendicular to the surface of the two-dimensional photonic crystal.
Thus, optical branching filters, which separate light with different
wavelengths, which propagate through the waveguides, can be fabricated.
High-performance resonators should confine light over a long period of
time. To increase the Q factor, which is an indication of the
performance level of the resonators, the difference in refractive
indices in the vertical direction must be large if point defect holes
in two-dimensional photonic crystals are used as resonators. Prof.
Noda fabricated point defects using the same method as line defects,
i.e. he removed holes, and increased the Q factor from 450 to 3800. To
obtain higher Q factors, he had to prevent light from leaking out of
the crystal. "When I thought about where the leaks were, I realized
that the edges of the defect holes, which reflect light, were the
places that light leaked. Light leaks up and down due to the strong
reflection as ocean waves hit a quay and the splash shoots up high. So,
buffer materials must be placed on the edges of the defect holes to
prevent light from leaking," says Prof. Noda. When the positions of
the holes adjacent to a defect formed by removing a hole are shifted
to the outside, the phase of the propagating light is shifted by the
periodic disturbance, and the first reflection becomes weak. A 60 nm
shift caused the Q factor to increase to 45,000, which is larger than
that of the current ultra-small optical resonators by two orders of
magnitude. Now, the value has reached almost 1 million, and
theoretically, the value has no upper limit. When he started studying
photonic crystals, others said that he would not achieve anything from
his research. "However, if you think too much about what will happen
next, you will never do anything new. So, you should take the first
step toward something new. It may be harsh at first, but if you make
adjustments as needed, things will get better. If you don't try out
new things, you will never gain anything. All you need to do is to
take the first step," says Prof. Noda.
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/057a.html
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YOUNG RESEARCHERS' INTRODUCTION
Realization and application of left-handed materials using
ferromagnetic-metal nanocomposite films
(Issued in Japanese: May 26, 2005)
Satoshi TOMITA, Researcher, Precursory Research for Embryonic
Science and Technology (PRESTO), Japan Science and Technology Agency
(JST)
The electromagnetic responses of materials are determined by their
electric permittivity and magnetic permeability. Materials, which have
simultaneously negative values of electric permittivity and magnetic
permeability, are called left-handed materials (LHMs), and it has been
theoretically predicted that they will show extraordinary
electromagnetic responses, e.g. inverse Doppler shift and negative
index of refraction. However, LHMs have not yet been found in nature.
In this project, we are trying to produce LHMs using insulating films,
which contain ferromagnetic-metal nanoparticles, named ferromagnetic-
metal nanocomposite films.
In this approach, electron magnetic resonance (EMR) in the
nanoparticles is used to realize negative value of magnetic
permeability in the nanocomposite films. We have already prepared Ni-
polyimide nanocomposite films, in which metallic Ni nanoparticles with
several nanometers in diameter are uniformly embedded in polyimide
matrices. Moreover, EMR in the films has been studied in detail. We
are now planning to carry out microwave transmission experiments on
the composite films under applied magnetic fields.
The results of this study will create a new paradigm for the
electromagnetism of matter and, thus, cause a significant breakthrough
in the science and technology of nanomaterials.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/057b.html
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