Printed with permission of The Institute of Pure and Applied Physics (IPAP)

JAPANESE JOURNAL OF APPLIED PHYSICS
Vol.46, No.9A (2007) pp.5655-5673[Part1]

Invited Review Paper

Organic Electronic Devices Based on
Polymeric Material andTunable Photonic Crystal— Organic LED, Conducting Polymer Laser, Organic Solar Cell, Organic TFT,
Tunable Photonic Crystal, Tunable Laser, Photonic Liquid Crystal —Katsumi Yoshino1,2,3,4, Yutaka Ohmori3, Akihiko Fujii5, and Masanori Ozaki5

The historical background and recent
progress in the development of organic
electronics and photonic crystals, particularly tunable photonic crystals realized by
combining photonic crystal structure withfunctional organic molecules, are discussed.
The novel characteristics of organic electronic
devices with mainly conducting polymers,
which are related to the optical effects, andthe tunable photonic crystals composed ofperiodic structures of optical wavelength order
combined with functional organic materials
are demonstrated.
KEYWORDS:organic material, conducting
polymer, photonic crystal, liquid
crystal

1. Introduction
Organic electronics, which utilize the novel
electrical and optical properties of organic
materials, such as π-conjugated moleculesand polymers called conducting polymers, andphotonic crystals (PCs) with periodic structures of optical-wavelength order, are considered to be key technologies for sustaining
the highly advanced Information Society of
the 21st century. The ubiquitous computing
for sustaining such a society must be basedon portable electronics and opto-electronics
devices with light weight, flexibility, lowenergy consumption, low fabrication cost andhigh reliability.
In this paper, we discuss the historicalbackground and recent progress in the development of organic electronics and PCs. In

particular, tunable PCs realized by combining
the PC structure with functional organic molecules, as proposed by us, will be discussed indetail. In other words, in this article we discussorganic electronic devices based mostly on
conducting polymers, restricting ourselves
to fields related to the photonic effect andtunable PCs.

2. Organic Electronics
We can trace the origin of organic electronics back to the fundamental pioneering
study of the electronic conducting characteristics of aromatic molecules such as anthracene and pentacene by Akamatu, Inokuchi,
and Matsunaga.1) Since then, these aromatic
molecules called organic semiconductors with conjugated π-electrons and theircharge transfer complex have been studied
in detail. On the other hand, in 1970, studies
of conducting polymers with highly extended
π-conjugated electron systems in the main
chain of linear polymers became highly developed after the discovery of superconductivityin (SN)x2) and the insulator–metal transitionin polyacetylene (CH)x.3) These π-conjugatedpolymers called conducting polymers, attracted
great interest from both scientific and practicalindustrial viewpoints. Subsequently, various
novel applications of conducting polymers
have been proposed, which also stimulated
fundamental studies of conducting polymers
and the development of new conducting polymers. Conducting polymers can be considered
to partially have the graphite structure whichis the first carbon materials used in electricalengineering by Edison, for example, as thefilament of an incandescent lamp.
On the other hand, based on detailed
studies of the unique dynamical, electrical,
and optical properties of liquid crystals (LCs),
LC display (LCD) devices have been extensively
developed, and now cathode ray tube (CRT)
display devices have been mostly replaced by
LCDs. The low energy consumption and lightweight of LCDs are essential for portable electronic devices. This success of LCDs utilizingorganic LCs in electronics triggered the development of organic electronic and optoelectronics devices.
Simple and low-cost processes for the
fabrication of organic devices are also attractive to engineers. For example, solution
methods can be applied for large-area devices
without the need for a vacuum system. It
should also be noted that various types oforganic materials with desired electronic band
schemes can be molecularly designed andfabricated.
Organic devices can also be fabricatedon various kinds of substrates, such as flexible polymers. That is, mechanical flexibility is
another key advantages of organic devices.
There are so many fields in which organic
devices may play important roles, but we will
restrict ourselves to several examples such asorganic light-emitting diodes (OLEDs), organic
photodetectors (OPDs) and organic solar cells
and organic thin film transistors.
Indeed, the applications of organic inte-

1 Shimane Institute for Industrial Technology, Matsue 690-0816, Japan
2 Collaboration Center, Shimane University, Matsue 690-0816, Japan
3 Innovation Center for Advanced Science, Osaka Univerisity, Suita, Osaka 565-0871, Japan
4 Department of Electrical and Electronic Engineering, Nagasaki Institute of Applied Science, Nagasaki 851-0193, Japan
5 Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan(Received March 7, 2007; accepted May 29, 2007; published online September 7, 2007)©2007 The Japan Society of Applied Physics

        JSAP International   No.17 (January 2008)

grated devices, such as organic transistors,
OLEDs and OPDs, in display panels and integrated plastic ICs, and organic sensor devices
have been greatly progressing. The potential
for commercialization is high for these organic
devices because they are seen to compete inapplication areas where the market can incur
costs in development. In portable and wearable electronic and optoelectronic devices
and computers which are important devices
in ubiquitous systems, organic devices would
play the most important role.
OLEDs utilizing fluorescent dyes or
conducting polymers are capable of emission over a wide visible range with high efficiency, and require only a low driving voltage.
Recently, OLEDs that have a long lifetime andexcellent durability have been realized for flatpanel display applications. There are somedemands for the use of OLEDs not only indisplay applications but also as various lightsources.
There are two types of devices, LEDs andlasers, for transmitting optical signals and / orenergy. LEDs have lower power but are muchless expensive than laser devices, and are used
for short distances and multimode paths. The
nanosecond transient electroluminescence ofthe blue OLED has also been reported. OLEDs
can be expected to be applied to electrooptical conversion devices for generating highspeed optical pulses. OPDs utilizing copper
phthalocyanine have been demonstrated tohave a high-speed response. OPDs can also be
applied to optoelectrical conversion devices forreceiving high-speed optical pulses.

tion, conducting polymers became soluble
and fusible, and their emission intensity
was enhanced depending on the length ofside chains.5-7) In this respect, it should be
mentioned that the synthesis of poly(9,9-dialkylfluorene) with a large band gap8) and thefirst observation of blue electroluminescence(EL) in a PLED utilizing this material9) stimulated the synthesis of this type of conducting
polymer. That is, conducting polymers withdesired band gaps could be designed andprepared to realize red, green and blue emission of PLEDs.10-12) Particular effort has been
expended for obtaining blue-emission PLEDs.
It should also be mentioned that the introduction of SP3 carbon or hetero-atoms such as
Si and Sn in the main chain, as shown in Fig. 1,
was found to be effective for increasing the

C4H9
S

CH3
Si
CH3 2

Si
m

C4H9 2x

4 x

PDSiQPPDSiOT

C6H13
Si
CH3
H13C6

C6H13

x

SiHMFPVSix

2.1	 OLEDs
OLEDs have attracted great interest forthin film flat-panel display and solid state
lighting device applications. An additional
advantage of polymeric organic LEDs (PLEDs)
utilizing conducting polymer is that they
can be simply fabricated by wet processes,
including ink-jet, and screen-printing methods,
and other solution processes on various kindsof substrates including polymeric substrates
for flexible devices.4)
Both PLEDs and emissive low-weightmolecule-based OLED have been developedby extensive utilization of initially, fluorescentmaterials and, later, phosphorescence materials.
For PLEDs, the introduction of side chainsto linear conducting polymers was groundbreaking, because, with this introduc-band gap in order to realize blue emission andfor enhancing emission intensity by exciton
confinement.13,14)
The matching of the bottom of the
conduction band and the top of the valenceband of the conducting polymer to the Fermi
energy levels of the cathode and the anode,
respectively, in PLED devices is also important
for achieving carrier injection. In particular, theintroduction of nitrogen atoms to the structure was successful lowering the bottom of
the conduction band to facilitate electron
injection.15,16)
To realize strong emission, it has beenfound to be effective to use phosphorescent materials.17,18) That is, it is effective toenhance EL emission intensity by emissive
recombination of triplet excitons. Here, we

SiPhPPVSi
Sx

SiPhThV

Sn

H13C6

C6H13

x

SnPhFPV
Fig.1

Molecular structures of conducting polymers with heteroatoms such as Si and Sn.

JSAP International   No.17 (January 2008)        Phosphorescent organic LEDs (PHOLEDs),
fabricated using heavy-metal-containing
phosphorescent compounds, exhibit highexternal quantum efficiencies and theirradiative emission comes from the triplet
states. In order to achieve high device performances, host materials with suitable triplet
levels are needed to realize efficient energy
transfer from the host to the dopants. For
example, the green phosphorescent dye ofbis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy) 2Ir(acac)] doped into 3-phenyl-

4-(1’-naphthyl)-5-phenyl-1,2,4-triazole (TAZ)
has been reported to show a peak internal
quantum eficiency of nearly 100 %.21)
Poly(n -vinylcarbazole) (PVCz) is used asa host material for PLEDs because of its hightriplet level for some emitting phosphorescentcomplexes, and it shows high device performance. The starburst molecule, 1,3,5-tris[4(diphenylamino)phenyl]benzene (TDAPB), has
good hole transport characteristics owing to
its highest occupied molecular orbital (HOMO)
levels, and is expected to have triplet energy
levels suitable as the host for the phosphorescent material Ir(ppy)3. TDAPB is reported to bea starburst molecule with excellent hole-transporting characteristics for organic EL devices.22)
In addition, TDAPB is resistant to crystallization because of its steric hindrance; it also
possesses excellent wet-process film-forming

Emission Intensity (arb. units)

(a)

PL

620

1.2 nJ
1.0 nJ

0.7 nJ625 630 635 640
Wavelength (nm)

Fiber
550

600
650
700
Wavelength (nm)Microring
Emission
750

(a)
10
9

D

Photograph of flexible PLED with Ir(piq)3.

Emission Intensity (a.u.)

8

Fig.2

7
6
5
4
3
2

1Å

1
0
615
Fig.3
(a) Typical laser spectraand (b) single-mode emissionof microring structure with
conducting polymer.

        JSAP International   No.17 (January 2008)

645

Pump

500

(b)

properties. Starburst molecules are useful; they
combine the superior characteristics of small
molecules and polymers such as sublimation ofmaterials and wet-process film formation, andare expected to yield high-efficiency devices
fabricated by solution processes.
The phosphorescent dye tris(1-phenyliso­qui­no­line)iridium(III) [Ir(piq)3] has been investigated for red PLEDs. We have used polymeric PVCz and starburst low-molecularweight TDAPB as host materials in order toimprove device performance by combining theadvantages of a polymer and a low molecularweight. In addition, using the red phosphor
results in exciplex emission between the hostand carrier transport material (Fig. 2). We
have employed two types of host materials
for the efficient red PLEDs and have discussed
the energy transfer from the host blend to theEmission Intensity (a. u.)

will only report recent results as examples. A
low-molecular-weight phosphorescent OLED is
usually fabricated by evaporation or moleculardeposition in vacuum. However, for a practical
low cost device, a wet process is desirable.
Here, we will explain one of the trials as an
example.19,20)

620

625
(b)

630

635

640

800

800

600

400

200

0
620

630
640
Wavelength(nm)Emission Intensity(arb.units)

15000

650

10000

5000

0

450

500
550
Wavelength(nm)

600

650

Fig.4
(a) Double-ring lasing and (b) dual-color lasing from microcapillary structurewith conducting polymers.

2.2	 Conducting polymer laser
We have demonstrated that electronicenergy schemes of conducting polymers best
fit lasers because they are typical four-electronic-energy-level systems, with which we
can realize population inversion and lasing
with low threshold excitation.25)
Indeed, even without outer cavity mirrors,
super-radiation was observed upon opticalexcitation in conducting polymers.26) By the

optical excitation on the double-ring device
made of poly(2-methoxy-5-dodecyloxy- p phenylenevinylene) (MDDOPPV).
By the analysis of Fig. 4(a), the emission
was confirmed to originate from both outside
and inside rings. It should also be stressed thatby utilizing a different conducting polymer
on the inside surface from that on the outersurface, two-color lasing can be realized, as
shown in Fig. 4(b).30)

2.3	 Organic solar cells
There are two types of organic solar cells,
donor–acceptor solar cells based on electrontransfer between donors and acceptors, suchas fullerene C60 and their derivatives, anddye-sensitized TiO2 cells, the so-called Graetzel
solar cells.31)
The first type of solar cells resulted fromour finding of photoinduced charge transfer
between conducting polymers and C60.32-34)
Here, we will discuss only this type of device.
As shown in Fig. 5, photoluminescenceof conducting polymer is strongly quenchedwhereas photoconductivity is markedly
enhanced upon the introduction of C60 to the
conducting polymer. These novel characteristics can be explained by photoinduced chargetransfer between the conducting polymer andC60.32-34)
Utilizing various combinations and structures of conducting polymers and fullerenes, various types of solar cells have been
proposed, such as simple junction layers aswell as others obtained by introducing newconcepts such as photon-harvest molecules,
and condensed interfaces, interpenetrating
networks, and selective doping.35-37)

shown in Fig. 3(b).
We proposed the formation of microring
structures utilizing glass pipes instead offibers.30) In the case of pipes microrings canbe formed not only on the outer surface ofthe pipe but also on its inside surface. The
spectrum shown in Fig. 4(a) was obtained by

1000

Luminescence intensity (arb. units)

dopant and the confinement triplet energy inthe dopant.23)
Usually, OLED is applied to flat panel
displays. However, there are many other possibilities, for example, a light source in place ofincandescent lamps or fluorescent lamps andalso in optical communication. Here, we briefly
mention that because of the wet-process
fabrication technology of OLED, it can be
fabricated directly on the end terminal of
optical fibers. High-speed optical communication of video signals utilizing the OLED andthe OPD with Cu–phthalocyanine have beensuccessfully demonstrated.24)

introduction of fluorescent conducting polymers in cavities, lowthreshold lasing can berealized.26)
We examined themicroring structure oflasers by immersing
optical fibers in solutionsof conducting polymer.
Then we were able toprepare microring structures surrounding opticalfibers. 27) A microringlasing device was also
prepared by utilizing
the microdisk structureon the substrate. 28,29)
H o w e v e r, h e r e , w e
re s t r i c t o u r s e l v e s t omicroring fibers.
As shown in Fig.
3(a), the spectral widthde cre as e s dr as tic all y
above a certain thresholdexcitation intensity. Itshould also be mentionedthat upon decreasing
the diameter of the fiber,
single mode laser emission was observed, as

-PPV
1.0
100

0.5
10

0.0

0

5

Concentration of C60 (mol%)

10

Photoconductivity (arb. units)

Emission Intensity(arb.units)

Invited Review Paper

1
Fig.5
PL quenching and photoconductivity enhancement in composite systemof conducting polymer and C60.

JSAP International   No.17 (January 2008)        Au

AlComposite
ZnO
QuartzQuartz

ITO

(a)

(b)

Fig.6
Schematic device structures of (a) conventional type and (b) inverted typewith ZnO layer.

2.4	 Organic thin film field-effecttransistors and others

It should be mentioned that, comparedwith the conventional solar cells shown in Fig.
6(a), in which light impinges from the indium
tin oxide (ITO) electrode side on which theconducting polymer layer is formed, in the
inverted structure of the device shown in Fig.
6(b), the conversion efficiency was increased
markedly owing the suppression of thewindow effect of the conducting polymer.38)
That is, by utilizing a transparent ZnO electrode on which a C60 layer is formed and irradiating from this side, the window effect waseliminated, because electron–hole separation
occurs at the interface between C60 and the
conducting polymer junction.39)
It should also be mentioned that in theinterpenetrating network type of solar cellsof simple structure, by optimizing the dispersion of C60 in conducting polymer by various
methods such as annealing, high efficiency
was also attained, as shown in Fig. 7.40)

Organic field-effect transistors (OFETs)
fabricated with the conducting polymer, polythiophene, on Si substrates were reported
by Koezuka et al.41) On the other hand, we
reported FETs on polymer substrates. 42) Itshould also be mentioned that in our junction
devices utilizing poly(3-alkylthiophene), which
exhibits thermochromic behavior, strongly
temperature-dependent characteristics wereobserved.43) After those reports, many papers
on the FET were published. To apply wet
processes, soluble conducting polymers havebeen preferred. On the other hand, to obtainhigh carrier mobility, low-molecular-weightaromatic molecules have been preferred andused. Therefore, a method of applying wet
processes to aromatic molecules as well as
oligomers has been sought.
The high performance of OFETs fabricatedfrom an annealed pentacene solution andorganic transistors with solution-processed

1:0.25 (no annealing: 1.78%)
1:0.5 (annealing at 100oC: 2.22%)
1:0.75 (annealing at 125oC: 1.79%)(annealing at 150oC: 1.49%)100

EQE (%)

80

(a)

(b)

60
–1.0

40

Current (mA/cm2)

1:1

–0.5

Voltage (V)

10
8
6
4
2
0
0.0
–2

0.5

1.0

–4

20

–6

0
300

–10

–8
400

500

600

700

800

Wavelength (nm)Fig.7
EQE spectra and I–V characteristics of thin-film solar cells with composite system of conducting polymer and C60
obtained under various postdeposition annealing conditions.

        JSAP International   No.17 (January 2008)

source / drain electrodes using metal nanoparticles have been reported.
The thiophene oligomers with high carrier
mobility are candidates for realizing all-organic
circuits on an active semiconductor layer. A
high mobility of greater than 0.1 cm2 / (V·s)
can be obtained from oligothiophene transistors fabricated by dry processing. OLEDs witholigothiophene as the hole-transporting layer
have been reported.
On the other hand, soluble poly(3-alkylthiophene) is one of passes from the liquid
phase to the solid phase. OFETs fabricatedby casting show higher field, making thempromising materials for large-area devices.
The field-effect mobility of polymer transistors
fabricated by wet processing is more strongly
dependent on the self-organized structure,
which is influenced by the polymer solution,
than that of transistors fabricated by spin
coating. Various bipolar-type OFETs based oncomposites of p-type conducting polymers
and n-type dyes have been fabricated by spincoating. It is also of interest to examine the
OFETs having composites of conducting polymers and dyes with the same backbone.44)
Organic chromic devices are also promising for applications in various fields.
We have proposed a color switching
device based on the reversible insulator–metaltransition occurring in conducting polymer
upon doping and undoping.45-47) This idea
came to us when a drastic change in electrical conductivity owing to insulator transition
upon doping was discovered,45-47) because
we had expected a color change from that
conducting polymer in the insulating state toa metallic color in the metallic state resulting
from plasma reflection. Indeed, dramaticchange in color and excellent characteristics
of color switching in conducting polymer as
were expected. The color depends on the kindof conduction polymer. For example, polythiophene switches between red in the insulator
phase to blue in the metallic state. Multicolor
changes can be realized in polyaniline andother conducting polymers. The switching
speed is faster than those of conventional
nematic LC (NLC) devices. The life exceeding
10 5 cycles can be demonstrated. 45-47) To
be widely used, a longer cycle life is necessary, and the use of an ionic liquid has
been reported to be one of the methods ofimprovement.48)
By designing an appropriate molecular
structure of the conducting polymer andInvited Review Paper

device structure and system, a full-color, reliable long-life optical device should be realized
in the near future.

3. Photonic Crystal

3.1	 Fabrication of 3-D PC: synthetic
opal and inverse opal
Synthetic opals are prepared by thefollowing process. Monodispersed silica
colloidal spheres are synthesized in aqueous

Fig.8

SEM image of plastic opal and inverse opal.

clearly shown in this figure, clear reflection
peaks and transmission dips were observed
depending on the size of the spheres, whichindicates the regular periodic array of particles.
The peak and the dip correspond to the band
gap of the 3-D PC of opal.
The opal prepared by this procedure
contains an interconnecting structure of tetrahedral and octahedral voids. These voids are
fully interconnected by channels throughhexagonal close-packed layers. Inverse opals
can be prepared by filling the voids withsecondary materials such as liquid photopolymer and consequently removing silica
particles by HF etching. Carbon inverse opals
with 3-D periodicity can be prepared by
infiltrating phenol resin and subsequently
removing silica using HF, followed by annealing
for carbonization. Carbon can also be directly
infiltrated in voids by chemical vapor eposition(CVD). For example, propylene (C3H6) is chosen
as the precursor gas in order to increase thereaction yield in the CVD process.60)
As clearly shown in Fig. 8(b), regularTransmittance (a.u.)solutions by well-known procedures. Ordered
colloidal crystals having various PBGs can beprepared by sedimentation of the suspension of monodispersed SiO2 spheres of various
diameters. These crystals are annealed at
100–120 ºC and sintered at 600–700 ºC to
induce a small degree of interparticle sintering,which mechanically stabilizes the structure.
These mechanically robust, porous opals are
cut into plates. Thin film of synthetic opal
can also be formed by sedimentation of SiO2
spheres in a sandwich cell made of two glass
plates with a separation of several to hundredsµm. Porous opal prepared by this procedure has a fcc crystal lattice structure, which
is sometimes faulted with hexagonal-closepacked stacking arrangements. Polymer opals
are also prepared directly utilizing polymer
spheres of nanosize in diameter (Fig. 8). As
evident in the scanning electron microscope(SEM) image, in this opal, regular stacks ofsilica spheres were confirmed. Figure 9 shows
the transmission and reflection spectra of thesynthetic opals prepared by this method. As

Reflectance (a.u.)

Recently, PCs with a three-dimensionally periodic structure of the order of optical
wavelengths have attracted great interest,
because in this new class of material novel
concepts such as a photonic band gap (PBG)
in the energy range of which photons cannot
exist or propagate. Upon the introduction of
defects, localized states at which photons canlocalize are formed in the PBG. That is, PBG
materials are a new class of ordered structureswith a periodicity of optical-wavelength order,
which facilitates the manipulation of photons
in the same way that a semiconductor controls
the flow of electrons.49,50) The propagation ofphotons in such PCs is similar to the propagation of electrons in semiconductors. The
PBG in a PC plays, for photons, the same roleas the forbidden energy gap for electrons insemiconductors, thereby allowing the manipulation of light flow. Hence, optical devices
that are analogous to semiconductor devices
are made feasible by this phenomenon. As aconsequence, PBG devices have the potential
for accomplishing for optical circuitry what
semiconductor devices have done for electrical circuitry. A number of unusual opticalproperties are predicted in PCs. In particular,
the study of stimulated emission in the PBG
is one of the most attractive subjects, since,in the PBG, spontaneous emission is inhibited and low-threshold lasers based on PCs
are expected.49,51-54) In order to realize PCs, alarge number of intensive studies on microfabrication based on semiconductor processing
technology55-57) and self-assembly construction of nanoscale spheres 58,59) have been
carried out.
Silica opal is a type of naturally occurring
PC that consists of well-ordered three-dimensional (3-D) arrays of SiO2 spheres, that havediameters in the range of visible wavelengths.
Indeed, the iridescent coloration of such opals
is caused by the diffraction of light from theseregular arrays of silica particles of monodispersed diameter of optical-wavelength order.

1

0

180nm

300nm

550nm

500

1000

1500

1µm

2000

Wavelength (nm)

2500
Fig.9
Transmission and reflection
spectra of synthetic opal formed fromsilica particles with various diameters.

JSAP International   No.17 (January 2008)        Refractive index

1.540
Nematic
1.535
1.530

Iso

temperatures higher than
900 ºC, b) upon refractive
index change at the phasetransition temperature in
LC-infiltrated opal, and c)due to the thermochromismof conducting polymers infiltrated in a synthetic opal.

stepwise changes at the phase transition
points, which is consistent with the directly
measured refractive index of various phases.
It should also be mentioned similar stepwisechange of the stop band was also observed in
Pcs of smectic-LC-infiltrated opals, as shownin Fig. 10. That is, these results indicate thepossibility of the temperature tuning of PCs of
LC-infiltrated opals.
Phase transition behavior of LC was also
confirmed to change when it was infiltrated to
nanovoids in the opal from dielectric measurements. The dispersion frequency of the dielectric constant of NLC in the nanosize voids inthe opal is higher in frequency by about oneorder of magnitude than that in a conventional sandwich cell, which is consistent withthe faster response speed of the electrooptic
effect in the opal infiltrated with LC. This may
originate from the strong interaction of LC
molecules with the inner surface of the nanoscale voids in the opal, resulting in the strong
recover force. It should be mentioned that thistenability in LC-infiltrated opal is reversible.

a) Heat treatment of SiO2
opal
30
40
50
60
70
80
90
The stop band position
Temperature (°C)
of SiO2 opal can be tuned
Fig.10 Refractive index of nematic liquid crystal evaluated by stop band analysis.
via the effect of heat treatment. Upon heat treatmenta t t emp er a t ure s higher
arrays of voids were observed in inverse opal.
than 900 ºC, reflection spectra change with
The size of voids depend on the diameter ofchanging treatment period Th. Bragg diffracsilica spheres.
tion peaks at each incident angle shift toshorter wavelengths with increasing Th, which
can be interpreted in terms of the decrease of
3.2	 Tunability of optical properties
periodicity. That is, the lattice constant evaluin opal-based PCs
We have combined experimental andated from the analysis of diffraction peaktheoretical studies to devise and evaluate new
decreases monotonically with increasing heattypes of PCs: tunable PCs (TPCs) based ontreatment period Th, which were consistentsynthetic opals. The TPCs consist of periodic
with the electron microscope observation andc) Thermochromism of conducting polyparticle arrays which provide either optical
can be interpreted to be due to the progress
mers infiltrated into opal
or electrical tunability for the properties ofof sintering. From these results, it has been
We proposed the novel temperature
the PC, particularly the width and position
confirmed that the effective lattice periodicity
tuning method of the optical stop band usingof the PBG. The tunability can be realized byof PCs can be tuned at any value in a wide
the change in the refractive index associchanging the periodicity, crystal structure,
range by heat treatment.61) It should also be
ated with the thermochromism of conducting
or refractive index, for example. One of thenoted that the pore size of opals can also be
polymer infiltrated into synthetic opals.
methods of changing the refractive index
controlled by heat treatment, which should
Figure 11 shows the transmission spectra of aof PCs, synthetic opals with SiO2 spheres, isalso influence the infiltration of foreign matesynthetic opal infiltrated with poly(3-octadecyto infiltrate various active materials, such as
rial and its characteristics.62) It should be noted
lthiophene) (PAT-18) as a function of temperametals, semiconductors, organic moleculesthat this type of tuning is permanent.
ture. As is evident from this figure, the waveand polymers, into the nano-scale voids ofthe opals.58) TPCs are expected to combineb) LC-infiltrated opal
730
RT
the advantageous properties of conventional
The stop band of
70°C
725
PCs with the tunability of the refractive indicesopals infiltrated with LCs,
90°C
100
720
100°C
of such active materials as conducting polysuch as NLC and smectic
120°C
715
mers, photoresponsive materials, and mesoLC, shows a marked shift
140°C
20
60
100 14080
genic materials. Modifications of the periodwith changing temperaTemperature (°C)
icity and crystal structure can also be realized
ture. It should also beby applying mechanical stress to plastic opals
mentioned that the wave60
fabricated by the sedimentation of nanoscale
length of the stop bandpolymer spheres. Various new functionalities
shifts stepwise at the40
are expected in these TPCs made of infiltrated
phase transition points
opals and inverse opals.
between various phases,
20
which can be interpreted
in terms of the change in
3.2.1 Temperature tuning0
Temperature tuning of the optical proprefractive index. As shown500 550 600 650 700 750 800
erties of PCs has been achieved by variousin Fig. 3, the refractive
Wavelength (nm)approaches. We have demonstrated theindex of NLC (ZLI-1132)
temperature tuning of the stop band of theevaluated by the analysis
Fig.11 Transmission spectra of opal thin film infiltrated with PAT-18 as a function oftemperature. The inset shows temperature dependence of peak wavelength of transreflection spectrum a) by heat treatment atof the stop band shows
mission spectra.

10        JSAP International   No.17 (January 2008)

Peak wavelength (nm)Transmission (%)

1.525
20

Invited Review Paper

1.0

3.5
3.0
2.5
2.0

650

700

750

800

Reflection spectra as a function of applied mechanical stress.

length of the stop band shifts considerablywith increasing temperature. For example, atthe wavelength of 725 nm, the transmittancecan be varied from 1.6 % (room temperature,RT) to 47 % (140 ºC) by changing the temperature. It should be noted that these shifts ofthe transmission spectra with temperature
were confirmed to be reversible. The change inthe refractive index of PAT-18 evaluated fromthe change in the stop band with increasing
temperature coincides with that of the directly
estimated value. That is, the blue shift of thepeak wavelength of the stop band can be
attributed to the decrease in the refractive
index of PAT-18 which is due to the increaseof the band gap of PAT-18, with increasing
temperature.

3.2.2 Mechanical tuning in plastic opal
The periodicity and filling factor of theopals and infiltrated opals can be controlled byapplying mechanical stress. In the case of silica
opal, upon applying pressure, we can observea change in the periodicity, which results in thechange of the optical properties such as thestop band position in the transmission spectrum.
In the case of polymer opals, particularly

Peak Wavelength (nm)600

Wavelength (nm)Fig.12

4.0

680
720
Wavelength (nm)

740

0.5

0.0
550

0V
140V
170V
200V
270V

4.5
Reflectance (a.u.)

Strain
0
0.11
0.23
0.38

5.0

1.64
1.62

730

1.60

720

1.58
1.56

710

1.54

700
690

760

nLC

Normalized reflection intensity (arb. units)

Fig.13 (a) Reflection spectra of
LC-infiltrated polymer inverse
opal as a function of applied
voltage. (b) Peak reflection wavelength and effective refractive
index of LC in voids as a functionof voltage.

1.52
0

opals made of elastomer spheres, we canreversibly change the optical properties byapplying mechanical stress. As shown in Fig.
12, the reflection peak of the plastic opal isconfirmed to shift drastically upon the application of uniaxial mechanical stress perpendicular
to the light beam. This effect can be explained
as a change in the periodicity in the direction of the light beam. That is, the reflection
peak exhibited large redshift upon the application of pressure, owing to the increase in
the periodicity in the direction of light beam.
On the other hand, the reflection peak exhibited blue shift as a result of stretching due tothe decrease of the periodicity in the direction of the light beam. These results suggest
the possibility of mechanical tuning of the PC.
Mechanical tenability was also demonstrated
in elastomeric-polymer inverse opal preparedby the replica method.

3.2.3 Voltage tuning
Voltage tuning can be realized in synthetic
PAT-infiltrated opal used as one of the electrodes in an electrochemical cell. Upon electrochemical doping of the conducting polymer
infiltrated into the opal, the shift of the reflection peak was observed. By analyzing the

200
100
Applied Voltage (V)

1.50
300

reflection peak as function of the electric field,
the observed results can be explained in termsof the change in the refractive index of the
conducting polymer infiltrated into opals uponelectrochemical doping of BF4– ions. This result
indicates the possibility of voltage tuning ofPCs.
On the other hand, LCs have a highoptical anisotropy and are sensitive to external
stress such as an electric field. Because of such
optical anisotropy and field sensitivity, TPC has
been proposed to be made of opal or inverse
opal infiltrated with LC. In the LC-infiltrated
opal thin film made of SiO2 spheres, the stopband shift upon voltage application was
confirmed. This is interpreted to originate fromthe refractive index change due to the molecular reorientation caused by the voltage application.
Figure 13(a) shows reflection spectra of
the polymer inverse opal infiltrated with 5CB
as a function of the amplitude of applied rectangular voltage (f=1 kHz). The light was irradiated perpendicularly to the replica film, i.e.,in the [111] direction. As is evident from thisfigure, the reflection peak shifts to shorter
wavelengths with increasing voltage. Thishas been interpreted to be due to the refrac-

JSAP International   No.17 (January 2008)        11

1,000

excitation intensity
mJ/pulse

800

Emission Intensity (arb. units)

excitation intensity
mJ/pulse

17
5.9

6.1

600

excitation intensity
mJ/pulse

21
7.1

400
200
0
500

0.35
0.98

400

2.9
1.5

3.5
1.8

0.25
0.13

1.4

300
200
100
0

0.52
0.024

50
40
30
20
10
0

430

440

450

460

590

600

610 700 710 720 730 740

Wavelength (nm)Fig.14Emission spectra of MDDO-PPV as a function of excitation intensity in green opal infiltrated with THF.

polarization directionof standing wave

photon energy

LC molecules

out-of-phasestanding wave
photonic bandgap (PBG)/aain-phasestanding wavekout-of-phasestanding wave
Fig.15in-phasestanding wave

Schematic explanation of the appearance of photonic band gap in spiral periodic structure.

12        JSAP International   No.17 (January 2008)

tive index change caused by the molecularreorientation along the applied electric field.
In the initial state, the refractive index of LC
filled into nanosize voids in the opal should
be macroscopically averaged and is equivalent to that in the isotropic phase, because of
the 3-D symmetry of the arrangement andshape of the voids without electric field. Underapplied electric field, however, the LC molecules align along the field parallel to the direction of light propagation, and the ratio of themolecules aligning parallel to the direction oflight propagation slightly increases. As a result,the component of the refractive index no for
ordinary light increases in each void, and theaveraged refractive index of LC decreases.
Consequently, the reflection peak shifts toshorter wavelengths. A similar behavior, thatis, the shift of the peak position of the reflection spectrum upon applying voltage, has also
been observed in silica opal infiltrated with LC.
Figure 13(b) shows the voltage dependence of the reflection peak wavelength λ LC of
the polymer inverse opal infiltrated with 5CB.
The total shift of λ LC upon the application of
300 V is about 35 nm, which is much larger
than that of the silica opal infiltrated with 5CB.
This should be attributed to the large volume
fraction of the voids filled with LC, the refractive index of which changes upon the application of voltage compared with that in the silica
opal.
The orientation of directors of LCs changes
upon applying magnetic field. Therefore,
magnetic field tuning of reflection and transmission in opals infiltrated with LCs can be realized.
In the above-described electric field andmagnetic field tuning of PC characteristics
in LC-infiltrated opals, any LCs, such as NLC,
smectic LC, and cholesteric LC, can be used.
On the contrary, opals infiltrated with LCs canbe used as optically detectable magnetic fieldsensors.

3.2.4 Optical tuning
The optical properties of materials thathave been infiltrated into synthetic opals andthat also constitute opal replicas can also
be controlled by light irradiation. We havedemonstrated that the optical property of
PCs infiltrated with photochromic molecules
or polymers and polymers containing photochromic moeties such as azobenzene in theside chains can be controlled by light irradiation. In this case, we can either permanently orInvited Review Papertransiently control the PC by light irradiation.
Materials that exhibit photoinduced phasetransition are also interesting for infiltration
into synthetic opals.

explained by the amplified spontaneous emission (ASE) and the latter by multimode lasing(ML) influenced by the optical feedback due tothe periodic structure of the opal matrix.

3.2.5 Solvent effects

4. Photonic Crystal based on
Self-Organized Helix Structureof Chiral Liquid Crystals

3.3	 Enhancement of spectral narrowing and lasing by infiltration intoopal
We have reported the observation of
the inhibited spontaneous emission of anorganic dye, rhodamine 6G, infiltrated in apolymer replica of synthetic opal as a PC. The
morphology-dependent resonances, superimposed on the broadband emission of rhodamine 6G owing to spherical wavelength-sized
microcavity enhancement of dye emission,
have been observed.
The spectral narrowing of photoluminescence (PL) and the evolution of sharp emission lines upon optical excitation have also
been observed in opals made of SiO2 spheres
infiltrated with conducting polymers such aspoly(2-methoxy-5-dodecyloxy- p -phenylenevinylene) (MDDOPPV) and also fluorescentdyes such as rhodamine 6G, NK-3483, andcoumarin 120. The lasing characteristics were
found to be dependent on the combination of
the dyes and conducting polymers as well as
the periodicity of the opal and also the refractive index of the solvent. Lasing was observed
when fluorescent dyes exhibiting green PL,
red PL, and purple PL were infiltrated in green
opal, red opal, and purple opal, respectively.
These results suggest that the periodicity ofthe opal plays an important role in lasing.
Figure 14 shows the emission spectra of
the purple, green and red opals infiltrated with
THF solution of coumarin 120, MDDO-PPV,and NK-3483, respectively, as a function of
excitation intensity. With increasing excitation intensity, the PL peak became markedly
enhanced in intensity and the spectral width
became much narrower. In addition, sharp
new emission lines appear. The former can be

LCs including a chiral molecule have aself-organized helical structure that can be
regarded as a one-dimensional (1-D) periodic
structure and shows characteristic opticalproperties.63) In such systems with a helix,
light propagating along the helical axis isselectively reflected, depending on the polarization states, if the wavelength of the lightmatches the optical pitch of the helical structure, this is called selective reflection. The
wavelength region in which the light cannot
propagate is the stop band, and is consideredthe 1-D pseudo-band gap. Lasing at the bandedge has been reported in the cholesteric LC(CLC),64,65) chiral smectic LC,66-68) and polymerized cholesteric LC (PCLC).69,70) These laser
actions in the 1-D helical structure of chiralLCs are interpreted to be based on the bandedge of the 1-D photonic band gap in which
the photon group velocity is suppressed.71)

4.1	 Photonic band gap and bandedge lasing in CLC

4.1.1 Electrical tunability of lasing wavelength in dye-doped ferroelectric LC
Chiral smectic LCs with a tilted structure
show a ferroelectricity, and are called ferroelectric LCs (FLCs). They are promising forelectrooptic applications because of a fast
response to the electric field.72) The FLC also
has a helical structure and shows selective
reflection due to the 1-D periodic structure inalmost the same manner as the CLC.73)
Figure 16 shows emission spectra ofdye-doped FLC as a function of pump energy.
For low pump energy (1.76 µJ / pulse), thespectrum is dominated by a broad spontaneous emission and a dip is observed in the
broad spectrum. The dip originates fromthe selective reflection band resulting fromthe helix of FLC. As the excitation energy
increases, the emission intensity is enhanced.
At a high excitation energy (10.4 µJ / pulse),
lasing becomes evident as a sharp peak at thelower energy edge of the dip. The full width at
half maximum (FWHM) of the emission peak isless than 0.5 nm. The laser light emitted fromthe FLC is circularly polarized and the sense of
the polarization is right-handed, which coincides with the helical sense of the FLC used
here. This strongly supports the idea that thelaser action in the dye-doped FLC is based on
the band-edge effect in the periodic structureof the FLC helix.

In the helical periodic structure of the
CLCs, light propagating along the helical axis isselectively reflected, depending on the polarization states. If the wavelength of the lightmatches to the optical periodicity of the helicalstructure, this is called selective reflection.
In this case, there are two types of circularly
polarized standing waves withzero group velocity at the edgesof the stop band, as shown in
Fig. 15. Here, the rods indicate
20
Pump Energy
the molecular long axes of the3000
CLC molecules and the arrows
10.4 J/pulse
show the polarization direction
15of the standing waves. For one
2000
standing wave, the polarization10
direction of the light is parallelto the molecular long axis and,1000
1.76 J/pulse
5
if we dope a laser dye, thepolarization becomes parallel00
to the transition moment of
400
450
500
550
600
the doped dye. This light isWavelength (nm)subjected to the extraordinaryrefractive index of the LC and
Fig.16 Emission spectra of dye-doped FLC as a function of pump pulse
energy.
has lower energy with respect

Emission Intensity (arb.units)

To clarify the possibility of tuning thestop band of opals used as PCs, the shift ofthe stop band and the diffraction peak werestudied as a function of the refractive index of
the solvent. The reflection peak shifts drastically upon changing the solvent. The wavelength of the reflection peak increases withincreasing refractive index of the solvent.
These findings suggest the possibility of tuning
the PBG in the infiltrated PCs.

to a traveling wave, which corresponds to thelonger edge of the stop band. Moreover, thiscircularly polarized standing wave effectively
interacts with the laser medium and we can
expect the low threshold laser action at thelonger wavelength edge of the band gap.

JSAP International   No.17 (January 2008)        13

1.0applied electric field:
0
2.0
2.5

3.0

3.5 kV/cm

0.8
0.6
0.4
0.2
0
460

470

480

490

500

Wavelength (nm)The helix of FLC can be easily deformed
by applying electric field and its response isfast because of the strong interaction between
the spontaneous polarization and electric field.
FLC has a spontaneous polarization Ps normal
to the molecules and parallel to the smectic
layers. When the electric field is applied in thelayer, for lower field, Ps tends to point along
the field direction and FLC molecules start toreorient toward the direction normal to the,resulting in the deformation of the helix. In theequilibrium state, the deformation of the helix
might cause the elongation of its periodicity.
Above the threshold field, all FLC molecules
orient towards the same direction and thehelix is unwound. The fact that the periodicityof the helical structure of the dye-doped FLC
can be controlled by applying an electric field
prompts us to expect the possibility of electric field tuning of the laser emission wavelength. Figure 17 shows the lasing spectra of
the dye-doped FLC at high excitation energy

Fig.18 Laser emission frombent film of polymerized CLC.

(24 µJ / pulse) as a function of the applied
electric field. It should be noted that lasing
wavelength shifts greatly towards longer
wavelengths with increasing field, whichcorresponds to the shift of the selective reflection band. In spite of a weak field (3.5 kV / cm),wide tuning of the lasing wavelength wasachieved.
Lasing in the FLC mentioned above has
been performed in the cell configuration inwhich the helical axis is perpendicular to thesubstrates and laser light is emitted perpendicularly to the cell surface. In this configuration, the pump beam is absorbed in thevicinity of the interface between the LC andthe substrate, and doped dye in the bulk is
not effectively excited. We have designed aplanar cell configuration of dye-doped FLC
for lasing, in which the helix axis is parallelto the substrates, and demonstrated optically pumped lasing in a waveguide.68) Also inthis waveguide LC laser, the emission wave-

excitation beamzylaser emission

E=0

E

PVA
ITO
glass substrate

LC molecule
Fig.19 (a) Schematic explanation of the cell structure fortunable waveguide laser uponholographic excitation.
(b ) Voltage dependence o flasing wavelength in dye-doped
NLC waveguide holographically
excited.

14        JSAP International   No.17 (January 2008)x

laser emission

length can be controlled over a wide range byapplying electric field.

4.1.2 Laser action in photopolymerized CLC
Optically pumped laser action has been
observed in a dye-doped flexible freestanding
film of photopolymerized CLC (PCLC). In the
PCLC film, the self-organized helical structureacts as a 1-D PC. At a high excitation intensity
above the threshold, laser action is observedat the edge of the 1-D photonic band of the
PCLC helical structure. This PCLC film laser
possesses an excellent mechanical flexibility,
and the laser action is also observed in abent film of PCLC, as shown in Fig. 18. Thisimplies that the 1-D periodic structure needed
for the laser action is maintained even in the
deformed film. Using such flexibility of the
PCLC film, a focusing effect of laser emission
is demonstrated in a circularly deformed film.
Moreover, the helical pitch of the PCLC has notemperature dependence, in contrast to that

Lasing Wavelength (nm)Normalized Emission Intensity (a.u.)

1.2

Fig.17 Normalized
e mi s s i o n s p e c t r a
at high excitationenergy in dye-doped
FLC as a function of
applied electric field.

620

610

600
0

(a)

0.5
1.0
1.5
Applied Voltage (V)(b)

Invited Review Paper

The distributed feedback (DFB) laser action
can be achieved with a transient grating using
interference fringes induced by two excitation
laser beams (holographic excitation). In thisgeometry, the lasing wavelength λ DFB uponholographic excitation can be expressed by

λ DFB = neff λ ex / m sin θ ,where neff is the effective refractive index
of the active medium, λ ex is the wavelength of
excitation beams, m is the order of diffraction,and θ is the half-angle between two excitation beams. According to this equation, thelasing wavelength can be tuned by changing
θ and / or neff. Therefore, if neff can be electrically controlled, an electrical tuning of laser
emission upon holographic excitation can be
expected. Based on this concept, we propose
an electrical tuning method of the lasing
wavelength using a dye-doped NLC as anactive laser medium as schematically shown in
Fig. 19(a). If LC having extraordinary and ordinary refractive indices, ne and no, is used as
the active material for the laser medium, theeffective refractive index n eff can be electrically controlled due to the field-induced reorientation of LC molecules. Therefore, whena dye-doped NLC waveguide is holographically excited, the lasing wavelength should betunable by changing the applied electric field
across the LC layer.
Figure 19(b) shows the voltage dependence of the lasing wavelength of thedye-doped NLC waveguide. Above V =0.8V,
the lasing peak for the transverse magnetic

4.2	 Photon localization in chiralLCs with defect and its tunablility
The localization of the light utilizing thedefect mode caused by imperfections in the
periodic structure is promising for potential
applications such as low thresholdlasers and micro-waveguides.74-78)

4.2.1 Twist-defect-mode lasingin CLC
Laser actions reported so
far in chiral LCs are observed at
the edge wavelength of the stopband and are associated with thegroup velocity anomaly at thephotonic band edge. On the other
hand, low threshold laser action
based on the photon localization at the defect in the periodic
structure can also be expected.
The introduction of a defect into
the periodic helical structure of
the CLCs has been theoreticallystudied.79,80) In particular, Kopp
and Genack have predicted theexistence of a single circularly
polarized localized mode in thetwist defect of CLCs.80) We experimentally demonstrated the defectmode in the 1-D photonic band
gap of the CLC film having a twist
defect for the first time. The laser
action based on the twist defectmode (TDM) was also observed in
dye-doped PCLC composite film

80
(a)

70
Transmittance (%)

4.1.3 Electrically tunable lasing in nematic
waveguide under holographic excitation(TM)-guided mode (closed circle) showeda continuous red-shift with increasing theapplied voltage. The threshold voltage for the
peak shift is associated with the Frederiks transition of the NLC. The electrical tuning of laser
action could be performed reversibly.

60
50
40

24
22
20
18
16
580 600 620 640
Wavelength (nm)

30
20
10

Emission Intensity (arb. units)of unpolymerized cholesteric LC. This meansthat the operation wavelength of laser actionis thermally stable, which is a great advantage
for device application.

Transmittance (%)

with a twist defect.
The PCLC film with the twist defectwas prepared as follows.81) Photo-polymerizable CLC monomer was spin-coated froma toluene solution onto a glass substrate on
which a polyimide (AL-1254) was coated andrubbed in one direction. In order to obtain auniform planar alignment, the coated CLC
was annealed at a temperature just below
the clearing point. The CLC molecules on thesubstrate align their directors parallel to theglass plate, that is, the helical axis is perpendicular to the glass substrate. UV light irradiation was performed using a Xe lamp to induce
photopolymerization of the UV-curable CLC
monomer. Two PCLC films were put together
as the directors of LC molecules at the interface between these films to form a certain
angle ϕ . In other words, there is a discontinuous phase jump of the azimuthal angle of thehelical structures between these PCLC films at
the interface, and it acts as a twist defect in
the helicoidal periodic structure, as shown in
Fig. 20.
Figure 21(a) shows the transmission spectrum of the dye-doped double-PCLC composite

Fig.20 S c h e m a t i c e x p l a nation of the double PCLC
composite film having a twist
defect, which is a discontinuity of the director rotation
around the helix axis.

(b)
6000

4000

2000

0
500

550
600
650
Wavelength (nm)700

Fig.21 (a) Transmission spectrum of dye-doped double PCLC composite
film with twist defect. (b) Emission spectrum of double PCLC composite
film at above the threshold pump pulse energy (200 nJ / pulse).

JSAP International   No.17 (January 2008)        15film containing a discontinuous defect interface. A stop band, the 1-D photonic band gap,
is confirmed in the spectral range from 580 to
640 nm. It should be noted that a sharp peak
appears at 611 nm within the photonic bandgap, which might be related to the defectmode induced by the introduction of the twist
defect interface.
F i gur e 21( b ) s h o w s t h e e m i s s i o nspectrum of the dye-doped double-PCLC
composite film with the defect interface atthe pump energy of 200 nJ / pulse. At a highexcitation energy (200 nJ / pulse), laser action
appears at 611 nm, which is within the band
gap and coincides with the TDM wavelength.
With increasing excitation energy, anothersharp emission peak appears at 638 nm which,corresponds to the edge wavelength of thestop band. This emission peak might be associated with the band-edge lasing that appearsin PCLC without any defect. Consequently,
defect-mode lasing occurs at a lower pumping
energy compared with that of band-edge
lasing.

4.2.2 Transient defect mode induced by partial deformation of helix
The TDM based on the composite film of
two PCLCs has been achieved. However, its
wavelength cannot be tuned by applying an
external field such as an electric field or light.
We have proposed a new type of defect modein the helix, that can be dynamically tuned byapplying an external field.82) Figure 22 showsa schematic explanation of a photonic defectin CLC. If the periodicity (pitch) of the helix ispartially changed, that is, the pitch is partiallysqueezed or expanded, these irregularities in
the periodic structure should act as defects

and cause light localization. As a method of
inducing partial change in the helix pitch, we
propose that the local modification of helicaltwisting power (HTP) induced by irradiation
with focused Gaussian laser light. Opticalcontrol of HTP can be realized by using thephotochemical effects of the doped azobenzene, nonlinear optical effects, or simply byheating. Photoinduced reversible control ofthe HTP of CLC has been demonstrated in
CLC containing photochromic azobenzene,
and applications to reflection display devices,
optical shutters, and optical memory, forexample, have been studied.83,84) By trans–cis
photoisomerization of the doped azobenzene,
the HTP of the host CLC changes, so thatphotoinduced control of HTP can be realized.
Figure 23 shows the calculated transmission spectra for the right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) light passing throughthe CLC with a chiral-defect-induced partial
helix deformation. The calculation was carriedout assuming that the HTP is modulated as
HTP(z )=HTP 0[1+α exp(-2z 2 / w 2)], where thez-axis is parallel to the helix axis and 2w is thediameter of the light beam, α is the modulation strength of HTP, and HTP0 is the initial
HTP. The helix sense of CLC is right-handed.
The pitch of the helix is 350 nm, and ordinary and extraordinary refractive indices of
LC are 1.5 and 1.7, respectively. The thickness of the CLC is 5 µm. A transmission peakbased on the defect mode was observed in
the stop band for the RCP light. On the other
hand, when the incident light was LCP, no
defect mode was observed. In other words,
the defect mode due to the photon localization in our system can be realized only for thecircularly polarized light with the same handedness as the helix, which is similar to theresult for the TDM. The position of the defectmode depends on α and w. With decreasing α ,a transmission peak due to the defect mode
shifts towards the longer-wavelength edge.
On the contrary, with increasing α , the peakshifts towards the shorter-wavelength edge.
The increase and decrease in α correspondto the squeezing and expansion of the helix
pitch. Therefore, the tuning of defect modes
could be achieved by partial squeezing andexpansion of the helix.

4.2.3 Chiral defect fabricated by direct laser
writing technique
We have proposed a novel approach
to introducing chiral defects (local modulation of the helix pitch) into the helix structure of CLC.85,86) A schematic explanationof the fabrication procedure is shown in Fig.
24(a). A 100 fs pulse of a Ti:sapphire laser
at the wavelength of 800 nm and repetition
rate of 80 MHz was focused on the sample
cell through an objective lens with numerical aperture (NA) 1.4. A right-handed PCLC
material doped with 1 wt % of DCM dyealigned homogeneously in a cell with a gap
of 6–7 µm. Direct laser writing was performed
with a confocal laser scanning microscope.
The laser was scanned over an area of 146.2×
146.2 µm2, with a scan-line resolution of 2048
lines per scan area. First, the laser light was
tightly focused near the substrate surface
in the CLC cell. Two-photon polymerization
occurred at the laser focal point and a locallypolymerized PCLC thin film was obtained on
the substrate surface. The sample was thenflipped over and laser writing was performed

LCP

local modulation of HPT

defectTransmittance

1.0
0.8

RCP

0.6
0.4
0.2
0.0

Fig.22 Schematic explanation of the optically induced chiral defect based on thepartial deformation of the helix pitch.

500

600
Wavelength (nm)700

Fig.23 Transmission spectra of RCP and LCP light passing through the CLC
with deformation-induced chiral defect (α = -0.2, w=300 nm).

16        JSAP International   No.17 (January 2008)

helix axis
polymerized CLC

Emission Int. (a.u.)

unpolymerized CLC

scanfemto second laser

again near the opposite surface of the cell. As
a result, a hybrid structure was fabricated, inwhich an unpolymerized CLC region remained
between two PCLC films on the cell surface.
Figure 24(b) shows the transmission
spectra for right-handed circularly polarized
light of the fabricated CLC defect structure.
A single-defect mode is observed within theselective reflection band of the CLC. The
theoretical transmission spectrum was calculated using Berreman’s 4×4 matrix, and good
agreement with the experimental result was
obtained. Figure 19(c) shows the emission
spectrum of the CLC single-defect structure
at high pumping energy, along with the corresponding transmission spectrum. Single-modelaser action is observed at 628 nm, whichcorresponds to the defect-mode wavelength.
The lasing threshold for the defect mode
structure is 16.7 mJ / cm2, which is less thanhalf the threshold in CLC without the defectstructure. The reduction of the lasing thresholdin the defect structure is evidence of a high-Q
cavity formed by the defect.

Fig.24 (a) Schematic explanation of fabrication procedure of PCLC with chiral defectbased on local photopolymerization using
scanning confocal microscope. (b) Transmission and (c) emission spectra of CLC withchiral defect.

1.0
(b)
0.5

0
20

10
0

(c)
pump energy:
16 nJ/pulse

600
700
800
Wavelength (nm)structure is a problem that must be resolved.
Not only 3-D PCs but also 1-D PCs are attractive subjects. Although, the 1-D PC does nothave a complete PBG, there are numerous
applications using the extraordinary dispersionof the photon and localized photonic state ina defect layer. So far, intensive studies on 1-D
PC applications have been reported: air-bridge
microcavities,94) photonic band-edge lasers,95)
nonlinear optical diodes,96) and the enhancement of optical nonlinearity.77,97,98) Recently we
introduced a LC layer in a dielectric multilayer
structure as a defect in 1-D PC,99) in which thewavelength of defect modes was controlled byapplying electric field because of a change inthe optical length of the defect layer caused by
the field-induced molecular reorientation of LC.

5.1	 Tunable-defect-mode lasing inperiodic structure containing LC layer
as defect
We have introduced a LC layer in a 1-D PC
as a defect, where the wavelength of defectmodes was controlled by applying electric field
because of a change in the optical length of thedefect layer caused by the field-induced molecular reorientation of LC.100) We also proposeda wavelength-tunable laser based on an electrically controllable defect mode in a 1-D dielectric periodic structure containing a dye-doped
LC as a defect layer.101,102) Figure 25 shows
the 1-D PC with a LC defect. A dielectric multilayer consisting of stack of alternating SiO2
and TiO2 layers deposited on an ITO-coatedglass substrate is used as the 1-D PC. In order

ITO electrode
Deielectric multilayer(SiO2, TiO2)

Voltagesource5. Tunable-Defect-Mode Lasing in Periodic Structure Containing LC Layer as DefectLCs have high optical anisotropy and are
sensitive to external stress such as an electricfield. On the basis of such optical anisotropy
and field sensitivity, a TPC has been proposedin opal or inverse opal infiltrated with LC.87-91)
Although opal and inverse opal are simple andinexpensive means of realizing 3-D PCs by selforganization of colloidal particles, 92,93) the
introduction of defects into the 3-D periodic

Transmittance (a.u.)

Invited Review Paper

LC defect layerzxyPolarized lightFig.25

Schematic explanation of 1-D PC containing liquid crystal as defect.

JSAP International   No.17 (January 2008)        17

0V
1.2 V

80

Lasing Wavelength (nm)Transmittance (%)

100

60
40
20
0

500

600
700
Wavelength (nm)800

Fig.26 Transmission spectra of 1-D PC with LC defect as a function of applied
voltage.

610
600
590
0

0.5

Transmittance (%)

18        JSAP International   No.17 (January 2008)

1.0

1.5

2.0

2.5

Applied Voltage (V)Fig.27 Voltage dependence of defect-mode lasing wavelength in the 1-D
PC with dye-doped NLC defect.

tric field, the long molecular axis of the LC isaligned parallel to the substrates (y-axis).
In order to investigate the characteristicsof the defect mode, the transmission spectrum of the linearly polarized light propagating along the z-axis wasmeasured from the opposite side of the cell using a100
charge-coupled device (CCD)80
multichannel spectrometer.
60
Figure 26 shows the voltage
dependence of transmission
40
1-D PC
spectra for incident light
20
CLC
(a)
polarized along the y -axis,
0which corresponds to the80
rubbing direction and the60
initial orientation directionof the LC molecules in the40
defect layer, as shown in
20
Fig.
25. A rectangular wave(b)0
voltage of 1kHz was applied
500
600
700
800
900
between ITO layers to
Wavelength (nm)100change the molecular alignment of the LC in the defect
1-D
HPC
80
layer. The solid and dashedCLC
60
lines correspond to spectra
at 0 and 1.2 V, respectively.
40
The peaks of the defect20
modes shift to shorter(c)wavelengths upon applying
0660 670680
690 700
voltage. This peak shift origiWavelength (nm)nates from the decrease in
the optical length of theFig.28 (a) Theoretical transmission spectra of 1-D PC without any defect (solidline) and CLC without PC structure (dashed line). (b) Theoretical transmission
defect layer caused by thespectra of 1-D PC containing CLC as a defect. (c) Magnified transmission spectra
field-induced reorientacorresponding to (b).

Transmittance (%)

to introduce the defect layer, a dye-doped
NLC (Merck E47) was sandwiched between
substrates with dielectric multilayers using 2µm
spacers. The refractive index anisotropy Δn of
E47 is 0.209 at RT. In the absence of an elec-

620

tion of the LC molecules. Consequently, we
confirmed that the wavelength of the defectmode in a 1-D PC with the LC layer as a defectcan be controlled by applying voltage.
On the basis of the above-describeddefect-mode tuning, the lasing wavelength can be controlled by applying electric field.101,102) Figure 27 shows the voltage
dependence of the lasing peak wavelength
in a 1-D PC with DCM-doped LC. The lasing
peak shifts toward shorter wavelengths withincreasing voltage, in the same manner as
the defect mode shift shown in Fig. 26. The
wavelength shift of the lasing peak is about
25 nm, even upon applying low voltage. As
is evident from Fig. 27, a threshold voltage
exists for the peak shift, and the lasing wavelength shifts towards shorter wavelengths
above the threshold of 1.1 V. This is associated with Frederiks transition of the LC in thedefect layer.

5.2	 Double periodic structure: Helix
defect in 1-D PC
We have also introduced a CLC layer in a
1-D PC as a defect.103-105) Figure 28(a) shows
the theoretical transmission spectrum of a
10-pair multilayer without a CLC defect (solidline), and a simple CLC without a PC structure (dashed line). The PBG of the CLC wasobserved between 605 and 680 nm, which
is within the PBG of the multilayer. Figure28(b) shows the calculated transmission spectrum of a 1-D HPC with a CLC defect. Many
peaks appear at regular intervals in the PBG

Refractive Index

2.5
2.0
1.5
1.0
800
600
400
200
0

2

4
6
8
Position (µm)

10

(a)

of the HPC. These peaks are related to thedefect modes resulting from the introductionof the CLC defect. However, additional peaks
were observed, as indicated by arrows, which
disrupted the regular interval between thedefect-mode peaks at both band edges of the
CLC.
The transmission spectra in Fig. 28(b),magnified around the longer edge of the PBG
of the CLC, are shown in Fig. 28(c). Four main
peaks due to the defect modes appear atregular intervals (661, 673, 687, and 699 nm),although the peak at 687 nm splits. This splitting is attributed to the optical anisotropy of
the CLC. Therefore, two kinds of defect modes
corresponding to left- and right-handed circularly polarized light could exist outside the PBG
of the CLC. On the other hand, one additional
peak was observed at 678.6 nm, which corresponds to the band-edge wavelength of the
CLC. By detailed consideration of the polarization states of transmitted light, the additional
peak was clearly distinguished from the otherdefect mode peaks. Such a peak was notobserved in a 1-D PC with a uniform defect,
such as an isotropic medium or nematicLCs.100,106) Namely, this peak is a defect mode
peculiar to the helix defect in the 1-D PC, andis associated with photon localization originating from the band-edge effect of the CLC
helix. Note that this defect mode peak is very
sharp and the FWHM of this peak is 0.05 nm,
which is more than four times smaller thanthat of other defect-mode peaks (0.23 nm).
From the peak width, the Q -factor of the

12

2.5
2.0
1.5
1.0
80
60
40
20
0

0

2

4
6
8
Position (µm)

(b)

10

12
Fig.29 C a l c u l a t e d e l e c t r i c f i e l dstrength in the cell of (a) 1-D PC
containing CLC defect and (b) simple
CLC.

additional mode at the band edge of the CLC
contributions of both the band-edge effect ofwas estimated to be 14000, which was much
CLC and the defect-mode effect of the outerhigher than those of the other defect modes.
periodic structure, light is strongly localized in
In order to clarify the appearance of thethe double-periodic structure.
high-Q defect mode in the double-periodic
We have investigated the laser action
structure, we have performed a theoreticalin a 1-D HPC with a CLC defect. At a highestimation of the electric field distribution
pumping energy of 18 nJ / pulse, as shown inin the three types of 1-D periodic structures
Fig. 30(a), only one sharp lasing peak appears
described above, by a finite difference time
at 643.5 nm. The calculated transmission
domain (FDTD) method. Figure 29 shows the
spectrum of this system is also shown in Fig.
calculated electric field distributions and refrac30(b). Upon comparing Figs. 30(a) and 30(b),tive indices in two types of periodic structures.
the lasing peak is seen to coincide with the
The wavelength of the incident light to thewavelength of the peculiar defect mode. Noteperiodic structures corresponds
to the high-Q defect mode wavelength. It should be noted thatlight is strongly localized in the
400(a)double-periodic structure and the
maximum electric field intensity
300
is more than 15 times as much as
200
that of a simple CLC. Light is localized at the center of the CLC layer
100
in the double periodic structure0
shown in Fig. 29(a) and its field(b)pattern is similar to that in the80
CLC shown in Fig. 29(b), which
1-D PC60
CLC
indicates that light in the doubleperiodic structure is confined by40
the band-edge effect of CLC.
20
Additionally, light confinement is0
effectively enhanced by the outer630 640 650 660 670
periodic structure because theWavelength (nm)wavelength of light is within theFig.30 (a) Emission spectrum of 1-D PC-containing dye-doped CLC defectPBG of the outer periodic struclayer. (b) Theoretical transmission spectra corresponding to the emission
ture. Namely, because of thespectrum.

Transmittance (%) Emission Int. (arb.units)

0Electric Field Strength (a.u.)Electric Field Strength (a.u.)

Refractive Index

Invited Review Paper

JSAP International   No.17 (January 2008)        19that the laser action was a single mode basedon one additional mode, although many
modes exist because of the high Q-factor. The
threshold of laser action in the 1-D HPC with a
CLC defect was lower than that in simple CLC
without a 1-D PC.103) This result is attributed
to the strong optical confinement due to thehigh Q-factor of the additional mode. Similar
results have been confirmed in the 1-D PC
containing FLC as a defect.107)

6. Tunable Two-Dimensional
Photonic Crystal
Various types of two-dimensional (2-D)
PCs have been studied.108-111) In 2-D PCs, the1 ) H. Akamatu, H. Inokuchi, and Y. Matsunaga:
Nature 173 (1954) 168.
2 ) R. L. Greene, G. B. Street, and L. J. Suter: Phys.
Rev. Lett. 35 (1975) 577.
3 ) C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J.
Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, andA. G. MacDiarmid: Phys. Rev. Lett. 39 (1977)
1098.
4 ) T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu, and J.
C. Strum: Appl. Phys. Lett. 72 (1998) 519.
5 ) R. Sugimoto, S. Takeda, H. B. Gu, and K. Yoshino: Chem. Express 1 (1986) 635.
6 ) K. Yoshino, S. Nakajima, D. H. Park, and R. Sugimoto: Jpn. J. Appl. Phys. 27 (1988) L716.
7 ) K. Yoshino, Y. Manda, K. Sawada, M. Onoda,
and R. Sugimoto: Solid State Commun. 69(1989) 143.
8 ) M. Fukuda, K. Sawada, and K. Yoshino: Jpn. J.
Appl. Phys. 28 (1989) L1433.
9 ) Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino:
Jpn. J. Appl. Phys. 30 (1991) L1941.
10 ) Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino:
Jpn. J. Appl. Phys. 30 (1991) L1938.
11 ) M. Hamaguchi and K. Yoshino: Appl. Phys. Lett.
69 (1996) 143.
12 ) M. Hamaguchi and K. Yoshino: Appl. Phys. Lett.
67 (1995) 3381.
13 ) Y. Yoshida, Y. Nishihara, R. Ootake, A. Fujii, M.
Ozaki, K. Yoshino, H. K. Kim, N. S. Baek, and S.
K. Choi: J. Appl. Phys. 90 (2001) 6061.
14 ) Y. Yoshida, Y. Nishihara, A. Fujii, M. Ozaki, K.
Yoshino, H. K. Kim, N. S. Baek, and S. K. Choi: J.
Appl. Phys. 95 (2004) 4193.

20        JSAP International   No.17 (January 2008)

PBG and defect states also play importantroles. We are interested in the characteristicsof photons in waveguides in 2-D PCs.
With a Y-shaped waveguide in LC-infiltrated 2-D PCs, switching of the light propagation direction and its temporal behavior havebeen theoretically clarified. Details are now
under study.

7. Summary
In this article, the historical backgroundand recent progress in organic electronics andphotonic crystals have been discussed. Novel
characteristics of organic electronic devices
mainly utilizing conducting polymers and related

15 ) A. Fujii, R. Ootake, T. Fujisawa, M. Ozaki, Y.
Ohmori, T. Laga, H.-F. Lu, H. S. O. Chan, S.-C.
Ng, and K. Yoshino: Appl. Phys. Lett. 77 (2000)
660.
16 ) S. C. Ng, H. F. Lu, H. S. O. Chan, A. Fujii, T. Laga, and K. Yoshino: Macromolecules 34 (2001)
6895.
17 ) D. F. O’Brien, M. A. Baldo, M. E. Thompson, and
S. R. Forrest: Appl. Phys. Lett. 74 (1999) 442.
18 ) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E.
Thompson, and S. R. Forrest: Appl. Phys. Lett.
75 (1999) 4.
19 ) Y. Hino, H. Kajii, and Y. Ohmori: Jpn. J. Appl.
Phys. 43 (2004) 2315.
20 ) Y. Hino, H. Kajii, and Y. Ohmori: IEICE Trans.
Electron. E87-C (2004) 2053.
21 ) C. Adachi, M. A. Baldo, M. E. Thompson, and S.
R. Forrest: J. Appl. Phys. 90 (2001) 5048.
22 ) Y. Hino, H. Kajii, and Y. Ohmori: Org. Electron. 5(2004) 265.
23 ) Y. Hino, H. Kajii, and Y. Ohmori: Jpn. J. Appl.
Phys. 44 (2005) 2790.
24 ) Y. Ohmori, H. Kajii, M. Kaneko, K. Yoshino, M.
Ozaki, A. Fujii, M. Hikita, and H. Takenaka: IEEE J.
Sel. Top. Quantum Electron. 10 (2004) 70.
25 ) S. V. Frolov, M. Shkunov, Z. V. Vardeny, and K.
Yoshino: Phys. Rev. B 56 (1997) R4363.
26 ) S. V. Frolov, M. Ozaki, W. Gellermann, Z. V. Vardeny, and K. Yoshino: Jpn. J. Appl. Phys. 35(1996) L1371.
27 ) S. V. Frolov, Z. V. Vardeny, and K. Yoshino: Appl.
Phys. Lett. 72 (1998) 1802.
28 ) S. V. Frolov, A. Fujii, D. Chinn, K. Yoshino, and Z.
V. Vardeny: Appl. Phys. Lett. 72 (1998) 2811.

with optical effects and tunable photonic crystals composed of a periodic structure of opticalwavelength order combined with functionalorganic materials have been demonstrated.
These new devices exhibit tremendous
novel characteristics, with which various
new applications will be developed. We areconvinced that these organic electronic devices
and photonic crystals will become the key technology in sustaining the 21st century.
It should also be noted that these devices
are also important from environmental viewpoints.

29 ) S. V. Frolov, A. Fujii, D. Chinn, M. Hirohata, R.
Hidayat, M. Teraguchi, T. Masuda, K. Yoshino,
and Z. V. Vardeny: Adv. Mater. 10 (1998) 869.
30 ) Y. Yoshida, T. Nishimura, A. Fujii, M. Ozaki, andK. Yoshino: Appl. Phys. Lett. 86 (2005) 141903.
31 ) B. O’Regan and M. Gratzel: Nature 353 (1991)
737.
32 ) S. Morita, A. A. Zakhidov, and K. Yoshino: Solid
State Commun. 82 (1992) 249.
33 ) K. Yoshino, S. Morita, and A. A. Zakhidov:
Chem. Express 7 (1992) 817.
34 ) K. Yoshino, X. H. Yin, S. Morita, T. Kawai, and
A. A. Zakhidov: Solid State Commun. 85 (1993)85.
35 ) S. Morita, A. A. Zakhidov, and K. Yoshino: Jpn. J.
Appl. Phys. 32 (1993) L873.
36 ) K. Yoshino, K. Tada, A. Fujii, E. M. Conwell, and
A. A. Zakhidov: IEEE Trans. Electron Devices 44(1997) 1315.
37 ) K. Yoshino, S. Lee, A. Fujii, H. Nakayama, W.
Schneider, A. Naka, and M. Ishikawa: Adv. Mater. 11 (1999) 1382.
38 ) T. Umeda, T. Shirakawa, A. Fujii, and K. Yoshino:
Jpn. J. Appl. Phys. 42 (2003) L1475.
39 ) T. Shirakawa, T. Umeda, Y. Hashimoto, A. Fujii,
and K. Yoshino: J. Phys. D 37 (2004) 847.
40) T. Umeda, H. Noda, T. Shibata, A. Fujii, K. Yoshino, and M. Ozaki: Jpn. J. Appl. Phys. 45 (2006)
5241.
41 ) H. Koezuka, K. Hyodo, and A. G. MacDiarmid: J.
Appl. Phys. 58 (1985) 1279.
42) K. Kaneto, S. Takeda, and K. Yoshino: Jpn. J.
Appl. Phys. 24 (1985) L553.

Invited Review Paper

43) S. Takeda, K. Kaneto, and K. Yoshino: Chem.
Express 1 (1986) 383.
44) H. Kajii, H. Okuya, A. Sakakibara, and Y. Ohmori: Jpn. J. Appl. Phys. 44 (2005) L1567.
45 ) K. Yoshino, K. Kaneto, and Y. Inuishi: Jpn. J. Appl. Phys. 22 (1983) L157.
46) K. Kaneto, K. Yoshino, and Y. Inuishi: Jpn. J. Appl. Phys. 22 (1983) L412.
47) K. Kaneto, H. Agawa, and K. Yoshino: J. Appl.
Phys. 61 (1987) 1197.
48) W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R.
Mattes, J. Ding, G. M. Spinks, J. Mazurkiewics,
D. Zhou, G. G. Wallance, D. R. MacFariane, S.
A. Forsyth, and M. Forsyth: Science 297 (2002)
983.
49) E. Yablonovitch: Phys. Rev. Lett. 58 (1987) 2059.
50 ) S. John: Phys. Rev. Lett. 58 (1987) 2486.
51 ) J. D. Joannopoulos, R. D. Meade, and J. N.
Winn: Photonic Crystals: Molding the Flowof Light (Princeton Univ. Press, Princeton, NJ,
1995).
52 ) J. Martorell and N. M. Lawandy: Phys. Rev. Lett.
65 (1990) 1877.
53 ) K. Yoshino, S. B. Lee, S. Tatsuhara, Y. Kawagishi,
M. Ozaki, and A. A. Zakhidov: Appl. Phys. Lett.
73 (1998) 3506.
54 ) K. Yoshino, S. Tatsuhara, Y. Kawagishi, M.
Ozaki, A. A. Zakhidov, and Z. V. Vardeny: Appl.
Phys. Lett. 74 (1999) 2590.
55 ) J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R.
Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, and E. P. Ippen:
Nature 390 (1997) 143.
56 ) O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D.
O’Brien, P. D. Dapkus, and I. Kim: Science 284(1999) 1819.
57 ) S. Noda, K. Tomoda, N. Yamamoto, and A.
Chutinan: Science 289 (2000) 604.
58 ) K. Yoshino, K. Tada, M. Ozaki, A. A. Zakhidov,
and R. H. Baughman: Jpn. J. Appl. Phys. 36(1997) L714.
59 ) Y. A. Vlasov, K. Luterova, I. Pelant, B. Honerlage,
and V. N. Astratov: Appl. Phys. Lett. 71 (1997)
1616.
60 ) A. A. Zakhidov, R. H. Baughman, Z. Iqbal, C.
Cui, I. Khayrullin, S. O. Dantas, J. Marti, and V. G.
Ralchenko: Science 282 (1998) 290.
61 ) K. Yoshino: unpublished.
62 ) K. Yoshino, R. Ozaki, J. Matsumoto, M. Ojima,
S. Hiwatashi, Y. Matsuhisa, and M. Ozaki: IEEE
Trans. Dielectr. Electr. Insul. 13 (2006) 678.
63 ) S. Chandrasekhar: Liquid Crystals (Cambridge
University Press, Cambridge, U.K., 1992).
64) V. I. Kopp, B. Fan, H. K. Vithana, and A. Z.
Genack: Opt. Lett. 23 (1998) 1707.

65 ) K. Funamoto, M. Ozaki, and K. Yoshino: Jpn. J.
Appl. Phys. 42 (2003) L1523.
66 ) M. Ozaki, M. Kasano, D. Ganzke, W. Haase,
and K. Yoshino: Adv. Mater. 14 (2002) 306.
67 ) M. Ozaki, M. Kasano, D. Ganzke, W. Haase,
and K. Yoshino: Adv. Mater. 15 (2003) 974.
68 ) M. Kasano, M. Ozaki, D. Ganzke, W. Haase, andK. Yoshino: Appl. Phys. Lett. 82 (2003) 4026.
69 ) H. Finkelmann, S. T. Kim, A. Munoz, P. PalffyMuhoray, and B. Taheri: Adv. Mater. 13 (2001)
1069.
70 ) T. Matsui, R. Ozaki, K. Funamoto, M. Ozaki, andK. Yoshino: Appl. Phys. Lett. 81 (2002) 3741.
71 ) J. P. Dowling, M. Scalora, M. J. Bloemer, and C.
M. Bowden: J. Appl. Phys. 75 (1994) 1896.
72 ) J. W. Goodby, R. Blinc, N. A. Clark, S. T. Lagerwall, M. A. Osipov, S. A. Pikin, T. Sakurai, K. Yoshino, and B. Zeks: Ferroelectric Liquid Crystals(Gordon and Breach Science, Philadelphia, PA,
1991).
73 ) K. Hori: Mol. Cryst. Liq. Cryst. 82 (1982) L13.
74 ) J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R.
Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, and E. P. Ippen:
Nature 390 (1997) 143.
75 ) O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D.
O’Brien, P. D. Dapkus, and I. Kim: Science 284(1999) 1819.
76 ) S. Noda, K. Tomoda, N. Yamamoto, and A.
Chutinan: Science 289 (2000) 604.
77 ) T. Hattori, N. Tsurumachi, and H. Nakatsuka: J.
Opt. Soc. Am. B 14 (1997) 348.
78 ) R. Ozaki, T. Matsui, M. Ozaki, and K. Yoshino:
Jpn. J. Appl. Phys. 41 (2002) L1482.
79 ) Y. C. Yang, C. S. Kee, J. E. Kim, H. Y. Park, J. C.
Lee, and Y. J. Jeon: Phys. Rev. E 60 (1999) 6852.
80 ) V. I. Kopp and A. Z. Genack: Phys. Rev. Lett. 89(2002) 033901.
81 ) M. Ozaki, R. Ozaki, T. Matsui, and K. Yoshino:
Jpn. J. Appl. Phys. 42 (2003) L472.
82 ) T. Matsui, M. Ozaki, and K. Yoshino: Phys. Rev.
E 69 (2004) 061715.
83 ) E. Sackmann: J. Am. Chem. Soc. 93 (1971)
7088.
84) H. K. Lee, K. Doi, H. Harada, O. Tsutsumi, A.
Kanazawa, T. Shiono, and T. Ikeda: J. Phys.
Chem. B 104 (2000) 7023.
85 ) H. Yoshida, C. H. Lee, Y. Matsuhisa, A. Fujii, andM. Ozaki: to be published in Adv. Mater.
86 ) H. Yoshida, C. H. Lee, A. Fujii, and M. Ozaki:
Appl. Phys. Lett. 89 (2006) 231913.
87 ) K. Yoshino, S. Satoh, Y. Shimoda, Y. Kawagishi,
K. Nakayama, and M. Ozaki: Jpn. J. Appl. Phys.
38 (1999) L961.

88 ) K. Yoshino, Y. Shimoda, Y. Kawagishi, K. Nakayama, and M. Ozaki: Appl. Phys. Lett. 75(1999) 932.
89 ) K. Busch and S. John: Phys. Rev. Lett. 83 (1999)
967.
90 ) Y. Shimoda, M. Ozaki, and K. Yoshino: Appl.
Phys. Lett. 79 (2001) 3627.
91 ) M. Ozaki, Y. Shimoda, M. Kasano, and K. Yoshino: Adv. Mater. 14 (2002) 514.
92 ) K. Yoshino, K. Tada, M. Ozaki, A. A. Zakhidov,
and R. H. Baughman: Jpn. J. Appl. Phys. 36(1997) L714.
93 ) Y. A. Vlasov, K. Luterova, I. Pelant, B. Honerlage,
and V. N. Astratov: Appl. Phys. Lett. 71 (1997)
1616.
94) J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R.
Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, and E. P. Ippen:
Nature 390 (1997) 143.
95 ) J. P. Dowling, M. Scalora, M. J. Bloemer, and C.
M. Bowden: J. Appl. Phys. 75 (1994) 1896.
96 ) M. D. Tocci, M. J. Bloemer, M. Scalora, J. P.
Dowling, and C. M. Bowden: Appl. Phys. Lett.
66 (1995) 2324.
97 ) K. Yoshino: J. Soc. Elect. Mat. Eng. 9 (2000) 101.
98 ) Y. Dumeige, P. Vidakovic, S. Sauvage, I. Sgnes,
and J. A. Levenson: Appl. Phys. Lett. 78 (2001)
3021.
99 ) R. Ozaki, T. Matsui, M. Ozaki, and K. Yoshino:
Jpn. J. Appl. Phys. 41 (2002) L1482.
100) R. Ozaki, T. Matsui, M. Ozaki, and K. Yoshino:
Jpn. J. Appl. Phys. 41 (2002) L1482.
101) R. Ozaki, T. Matsui, M. Ozaki, and K. Yoshino:Appl. Phys. Lett. 82 (2003) 3593.
102) R. Ozaki, Y. Matsuhisa, M. Ozaki, and K. Yoshino: Appl. Phys. Lett. 84 (2004) 1844.
103) Y. Matsuhisa, R. Ozaki, M. Ozaki, and K. Yoshino: Jpn. J. Appl. Phys. 44 (2005) L629.
104) Y. Matsuhisa, R. Ozaki, K. Yoshino, and M. Ozaki: Appl. Phys. Lett. 89 (2006) 101109.
105) Y. Matsuhisa, R. Ozaki, K. Yoshino, and M. Ozaki: Thin Solid Films 509 (2006) 189.
10 ) R. Ozaki, T. Sanda, H. Yoshida, Y. Matsuhisa,
M. Ozaki, and K. Yoshino: Jpn. J. Appl. Phys. 45(2006) L493.
107) Y. Matsuhisa, W. Haase, A. Fujii, and M. Ozaki:
Appl. Phys. Lett. 89 (2006) 201112.
108) H. Takeda and K. Yoshino: J. Appl. Phys. 92(2002) 5658.
109) H. Takeda and K. Yoshino: Phys. Rev. B 67 (2003)
073106.
110) H. Takeda and K. Yoshino: Phys. Rev. E 67 (2003)
056607.
111) H. Takeda and K. Yoshino: Phys. Rev. E 70 (2004)
026601.

JSAP International   No.17 (January 2008)        21Invited Review Paper

Katsumi Yoshino was bornin Shimane Japan in 1941. He
graduated in 1964 from Department of Electrical Engineering, Faculty of Engineering,Osaka University, where he received Ph. D. degree. In 1969
he became a research associate
at the Department of Electrical Engineering, Osaka University, and in 1972, he waspromoted to an Assistant Professor in 1972, to an Associate Professor in 1978 and to Full Professor in 1988.
From 1974 to 1975 he was a visiting scientist at the
Hahn-Meitner Institut für Kernforschung in Berlin. From
2005, he has been a Professor of Emeritus of Osaka
University. He has been also a professor of Shimane University and Nagasaki Institute of Applied Science and an
Advisor of Shimane Institute for Industrial Technology.
From 2007 he has been an Director General of Shimane
Institute for Industrial Technology.
He was the vice president of the Institute of Electrical Engineers of Japan and the president of the Japanese Liquid Crystal Society. He received the Applied
Physics Award (1984), Osaka Scientific Award (1990),
Book of Year Award (1997) and Outstanding Achievement Award from The Institute of Electrical Engineers ofJapan (1998), Fellow from The Institute of Electronics,
Information and Communication Engineers (2001), TheBest Paper Award from the Society of Electrical Material Engineering (2001), the Best Paper Award from The
Japanese Liquid Crystal Society (2002), Japanese Liquid
Crystal Society Award (2003), Fellow from IEEE (2004),
The Best Paper Award from Japanese Institute of Electrical Engineer (2004) and The Contribution Award forPolymer Science from the Society of Polymer Science,
Japan.

22        JSAP International   No.17 (January 2008)

Yutaka Ohmori graduatedin 1972 from Department ofElectrical Engineering, Facultyof Engineering, Osaka University, Osaka, Japan, where hereceived the Doctor of Engineering degree. In 1977, hejoined Nippon Telegraph Telephone Public Corporation (nowNTT Corporation), where he worked mainly researchon optical semiconductor devices. In 1989, he became
an associate professor in Electrical Engineering, Facultyof Engineering, Osaka University. In 2000, he becamea professor in Collaborative Research Center for Advanced Science and Technology, Electronic Materials andSystems Engineering, Osaka University, and now he is a
professor of Center for Advanced Science and Innovation (CASI), where he worked on optical and electricaldevices utilizing organic materials including conducting
polymers. Professor Ohmori is a fellow of the Instituteof Electrical and Electronics Engineers, Inc. (IEEE), and amember of the Japan Society of Applied Physics (JSAP),
the Institute of Electronics, Information and Communication Engineers (IEICE), American Physical Society (APS),
The International Society for Optical Engineering (SPIE),
and the Materials Research Society (MRS).

Masanori Ozaki received his
B.E., M.E., and D.E. degrees
from Osaka University in 1983,
1995, and 1988, respectively.
He joined Department of Electronic Engineering in Osaka
University as a research associate in 1988, and was promoted
to an Associate Professor in1994 and to a Professor in 2005. He has been engaged
in the research on physical properties and applicationsof organic functional materials, particularly liquid crystals
and conjugated polymers. He stayed in Physics Department at University of Utah from 1994 to 1995 as a visiting scientist and studied spectroscopy of conjugatedpolymers. He obtained Best Paper Award of Japan Liquid Crystal Society. His current research interests are nano-structured organic materials and their application toelectronic and photonic devices.
Akihiko Fujii was born in Osaka, Japan in 1969. He received
his B.E., M.E., and D.E. degrees
from Osaka University in 1993,
1995, and 1997, respectively.
After one-year JSPS fellowship, he joined Department ofElectronic Engineering in Osaka
University as a research associate in 1998, and was promoted to an Associate Professor in Division of Electrical, Electronic and Information
Engineering, Graduate School of Engineering, Osaka
University in 2006.
He has been engaged in research on optical andelectrical devices utilizing organic materials including dye
molecules and conducting polymers.
He is a member of the Japan Society of Applied
Physics, the institute of Electrical Engineers of Japan, theInstitute of Electronics, Information and Communication
Engineers, and the Society of Polymer Science, Japan.