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PrintedwithpermissionofTheInstituteofPureandAppliedPhysics(IPAP)
JAPANESE JOURNAL OF APPLIED PHYSICSVol.46, No.9A (2007) pp.5655-5673[Part1]
Invited Review Paper
�JSAPInternationalNo.17(January2008)
Organic Electronic Devices Based on Polymeric Material and Tunable Photonic Crystal
Thehistorical background and recent
progress in the development of organic
electronics andphotonic crystals, particu-
larly tunablephotonic crystals realizedby
combiningphotonic crystal structurewith
functionalorganicmolecules,arediscussed.
Thenovelcharacteristicsoforganicelectronic
deviceswithmainly conductingpolymers,
whicharerelatedtotheopticaleffects,and
the tunablephotonic crystals composedof
periodicstructuresofopticalwavelengthorder
combinedwith functionalorganicmaterials
aredemonstrated.
KEYWORDS:organicmaterial, conducting
polymer,photoniccrystal,liquid
crystal
�. IntroductionOrganicelectronics,whichutilizethenovel
electrical andopticalpropertiesoforganic
materials, suchasπ-conjugatedmolecules
andpolymerscalledconductingpolymers,and
photonic crystals (PCs)withperiodic struc-
turesofoptical-wavelengthorder,areconsid-
ered tobekey technologies for sustaining
thehighlyadvanced InformationSocietyof
the21stcentury.Theubiquitouscomputing
forsustainingsuchasocietymustbebased
onportableelectronicsandopto-electronics
devices with light weight, flexibility, low
energyconsumption,lowfabricationcostand
highreliability.
In thispaper,wediscuss thehistorical
backgroundandrecentprogressinthedevel-
opmentoforganic electronics andPCs. In
particular, tunablePCsrealizedbycombining
thePCstructurewithfunctionalorganicmole-
cules,asproposedbyus,willbediscussedin
detail.Inotherwords,inthisarticlewediscuss
organicelectronicdevicesbasedmostlyon
conductingpolymers, restrictingourselves
to fields related to thephotoniceffectand
tunablePCs.
�. Organic ElectronicsWecantracetheoriginoforganicelec-
tronicsback to the fundamentalpioneering
studyoftheelectronicconductingcharacter-
isticsofaromaticmoleculessuchasanthra-
ceneandpentacenebyAkamatu, Inokuchi,
andMatsunaga.1)Sincethen,thesearomatic
molecules cal led organic semiconduc-
torswith conjugatedπ-electronsand their
charge transfercomplexhavebeenstudied
indetail.Ontheotherhand,in1970,studies
ofconductingpolymerswithhighlyextended
π-conjugatedelectron systems in themain
chainoflinearpolymersbecamehighlydevel-
opedafterthediscoveryofsuperconductivity
in (SN)x2)and the insulator–metal transition
inpolyacetylene (CH)x.3)Theseπ-conjugated
polymerscalledconductingpolymers,attracted
greatinterestfrombothscientificandpractical
industrial viewpoints.Subsequently, various
novel applicationsof conductingpolymers
havebeenproposed,whichalsostimulated
fundamentalstudiesofconductingpolymers
andthedevelopmentofnewconductingpoly-
mers.Conductingpolymerscanbeconsidered
topartiallyhavethegraphitestructurewhich
is thefirstcarbonmaterialsused inelectrical
engineeringbyEdison, forexample,as the
filamentofanincandescentlamp.
On theotherhand,basedondetailed
studiesof theuniquedynamical,electrical,
andopticalpropertiesof liquidcrystals (LCs),
LCdisplay(LCD)deviceshavebeenextensively
developed,andnowcathoderaytube(CRT)
displaydeviceshavebeenmostlyreplacedby
LCDs.Thelowenergyconsumptionandlight
weightofLCDsareessentialforportableelec-
tronicdevices.ThissuccessofLCDsutilizing
organicLCsinelectronicstriggeredthedevel-
opmentoforganicelectronicandoptoelec-
tronicsdevices.
Simpleand low-costprocesses for the
fabricationoforganicdevicesarealsoattrac-
tive to engineers. For example, solution
methodscanbeappliedforlarge-areadevices
without theneed for a vacuumsystem. It
shouldalsobenoted that various typesof
organicmaterialswithdesiredelectronicband
schemes canbemolecularlydesignedand
fabricated.
Organicdevices canalsobe fabricated
onvariouskindsofsubstrates, suchas flex-
iblepolymers.That is,mechanicalflexibility is
anotherkeyadvantagesoforganicdevices.
Therearesomanyfieldsinwhichorganic
devicesmayplayimportantroles,butwewill
restrictourselvestoseveralexamplessuchas
organiclight-emittingdiodes(OLEDs),organic
photodetectors(OPDs)andorganicsolarcells
andorganicthinfilmtransistors.
Indeed, theapplicationsoforganic inte-
1 ShimaneInstituteforIndustrialTechnology,Matsue690-0816,Japan2 CollaborationCenter,ShimaneUniversity,Matsue690-0816,Japan3 InnovationCenterforAdvancedScience,OsakaUniverisity,Suita,Osaka565-0871,Japan4 DepartmentofElectricalandElectronicEngineering,NagasakiInstituteofAppliedScience,Nagasaki851-0193,Japan5 DivisionofElectrical,ElectronicandInformationEngineering,GraduateSchoolofEngineering,OsakaUniversity,Suita,Osaka565-0871,Japan(ReceivedMarch7,2007;acceptedMay29,2007;publishedonlineSeptember7,2007) ©2007TheJapanSocietyofAppliedPhysics
Organic Electronic Devices Based on Polymeric Material and Tunable Photonic Crystal—OrganicLED,ConductingPolymerLaser,OrganicSolarCell,OrganicTFT,TunablePhotonicCrystal,TunableLaser,PhotonicLiquidCrystal—KatsumiYoshino1,2,3,4,YutakaOhmori3,AkihikoFujii5,andMasanoriOzaki5
JSAPInternationalNo.17(January2008)�
grateddevices, suchasorganic transistors,
OLEDsandOPDs, indisplaypanelsand inte-
gratedplastic ICs,andorganicsensordevices
havebeengreatlyprogressing.Thepotential
forcommercializationishighfortheseorganic
devicesbecausetheyareseentocompete in
applicationareaswherethemarketcanincur
costs indevelopment. Inportableandwear-
able electronic andoptoelectronicdevices
andcomputerswhichare importantdevices
inubiquitoussystems,organicdeviceswould
playthemostimportantrole.
OLEDs utilizing fluorescent dyes or
conductingpolymers are capableof emis-
sionoverawidevisiblerangewithhigheffi-
ciency,andrequireonlyalowdrivingvoltage.
Recently,OLEDsthathavealonglifetimeand
excellentdurabilityhavebeenrealizedforflat-
paneldisplayapplications. Thereare some
demands for theuseofOLEDsnotonly in
displayapplicationsbutalsoasvarious light
sources.
Therearetwotypesofdevices,LEDsand
lasers, fortransmittingopticalsignalsand /or
energy.LEDshavelowerpowerbutaremuch
lessexpensivethanlaserdevices,andareused
forshortdistancesandmultimodepaths.The
nanosecondtransientelectroluminescenceof
theblueOLEDhasalsobeenreported.OLEDs
canbeexpected tobeapplied toelectro-
opticalconversiondevicesforgeneratinghigh-
speedopticalpulses.OPDsutilizingcopper
phthalocyaninehavebeendemonstrated to
haveahigh-speedresponse.OPDscanalsobe
appliedtooptoelectricalconversiondevicesfor
receivinghigh-speedopticalpulses.
�.� OLEDsOLEDshaveattractedgreat interest for
thin film flat-paneldisplay and solid state
lightingdevice applications.Anadditional
advantageofpolymericorganicLEDs(PLEDs)
utilizing conducting polymer is that they
canbe simply fabricatedbywetprocesses,
includingink-jet,andscreen-printingmethods,
andothersolutionprocessesonvariouskinds
of substrates includingpolymeric substrates
forflexibledevices.4)
Both PLEDs and emissive low-weight-
molecule-basedOLEDhavebeendeveloped
byextensiveutilizationof initially, fluorescent
materialsand, later,phosphorescencemate-
rials.
ForPLEDs,theintroductionofsidechains
to linearconductingpolymerswasground-
breaking, because, with this introduc-
tion, conductingpolymersbecame soluble
and fusible, and their emission intensity
wasenhanceddependingon the lengthof
sidechains.5-7) In this respect, it shouldbe
mentionedthatthesynthesisofpoly(9,9-dial-
kylfluorene)witha largebandgap8)andthe
firstobservationofblueelectroluminescence
(EL) inaPLEDutilizing thismaterial9) stimu-
latedthesynthesisofthistypeofconducting
polymer.That is, conductingpolymerswith
desiredbandgaps couldbedesignedand
preparedtorealizered,greenandblueemis-
sionofPLEDs.10-12)Particularefforthasbeen
expendedforobtainingblue-emissionPLEDs.
Itshouldalsobementionedthattheintro-
ductionofSP3carbonorhetero-atomssuchas
SiandSninthemainchain,asshowninFig.1,
wasfoundtobeeffective for increasingthe
bandgapinordertorealizeblueemissionand
forenhancingemission intensitybyexciton
confinement.13,14)
The matching of the bottom of the
conductionbandandthetopofthevalence
bandoftheconductingpolymertotheFermi
energy levelsof thecathodeandtheanode,
respectively, inPLEDdevices isalsoimportant
forachievingcarrierinjection.Inparticular,the
introductionofnitrogenatomstothestruc-
turewassuccessful lowering thebottomof
the conductionband to facilitate electron
injection.15,16)
To realize strongemission, ithasbeen
found tobe effective tousephosphores-
centmaterials.17,18)That is, it iseffective to
enhanceEL emission intensityby emissive
recombinationof tripletexcitons.Here,we
SSi
C4H9
C4H9
SiCH3
CH3
H13C6 C6H13
Si
C6H13
CH3
m 2 x42
PDSiQP
x
x
Si
Si
S
x
x
SiPhThV
SiPhPPV
SiHMFPV
H13C6 C6H13
Sn
x
SnPhFPV
PDSiOT
Fig.1 MolecularstructuresofconductingpolymerswithheteroatomssuchasSiandSn.
�JSAPInternationalNo.17(January2008)
4-(1’-naphthyl)-5-phenyl-1,2,4-triazole (TAZ)
hasbeen reported to showapeak internal
quantumeficiencyofnearly100%.21)
Poly(n-vinylcarbazole) (PVCz) isusedas
ahostmaterial forPLEDsbecauseof itshigh
tripletlevelforsomeemittingphosphorescent
complexes,and itshowshighdeviceperfor-
mance.Thestarburstmolecule,1,3,5-tris[4-
(diphenylamino)phenyl]benzene(TDAPB),has
goodholetransportcharacteristicsowingto
itshighestoccupiedmolecularorbital(HOMO)
levels,andisexpectedtohavetripletenergy
levelssuitableasthehostforthephosphores-
centmaterialIr(ppy)3.TDAPBisreportedtobe
astarburstmoleculewithexcellenthole-trans-
portingcharacteristicsfororganicELdevices.22)
Inaddition,TDAPB is resistant tocrystalliza-
tionbecauseof its sterichindrance; it also
possessesexcellentwet-process film-forming
willonlyreportrecentresultsasexamples.A
low-molecular-weightphosphorescentOLEDis
usuallyfabricatedbyevaporationormolecular
depositioninvacuum.However,forapractical
lowcostdevice,awetprocess isdesirable.
Here,wewillexplainoneof thetrialsasan
example.19,20)
PhosphorescentorganicLEDs(PHOLEDs),
fabricated using heavy-metal-containing
phosphorescent compounds, exhibit high
external quantum efficiencies and their
radiative emission comes from the triplet
states. Inordertoachievehighdeviceperfor-
mances,hostmaterialswith suitable triplet
levelsareneeded to realizeefficientenergy
transfer from thehost to thedopants. For
example, thegreenphosphorescentdyeof
bis(2-phenylpyridine)iridium(III) acetylaceto-
nate [(ppy)2Ir(acac)] doped into3-phenyl-
properties.Starburstmoleculesareuseful;they
combinethesuperiorcharacteristicsofsmall
moleculesandpolymerssuchassublimationof
materialsandwet-processfilmformation,and
areexpectedtoyieldhigh-efficiencydevices
fabricatedbysolutionprocesses.
Thephosphorescentdyetris(1-phenyliso-
quinoline)iridium(III) [Ir(piq)3]hasbeen inves-
tigated for redPLEDs.Wehaveusedpoly-
meric PVCz and starburst low-molecular-
weightTDAPBashostmaterials inorder to
improvedeviceperformancebycombiningthe
advantagesofapolymerandalowmolecular
weight. Inaddition,using theredphosphor
results inexciplexemissionbetweenthehost
andcarrier transportmaterial (Fig.2).We
haveemployed two typesofhostmaterials
fortheefficientredPLEDsandhavediscussed
theenergytransferfromthehostblendtothe
Fig.2 PhotographofflexiblePLEDwithIr(piq)3.
(a)
(b)
(a)
(b)
500
6150
1
2
3
4
5
6
7
8
9
10
620 625 630 635 640
550 600 650 700 750 800
Emis
sio
n In
ten
sity
(ar
b. u
nit
s)
Emis
sio
n In
ten
sity
(a.
u.)
Wavelength (nm)
620 625 630 635 640 645Em
issi
on In
tens
ity
(a. u
.)Wavelength (nm)
PL
D
1Å
0.7 nJ
1.2 nJ
1.0 nJ
Fiber
Pump
MicroringEmission
Fig.3 (a)Typicallaserspectraand (b) single-modeemissionof microring structure withconductingpolymer.
Invited Review Paper
JSAPInternationalNo.17(January2008)�
dopantandtheconfinementtripletenergyin
thedopant.23)
Usually, OLED is applied to flat panel
displays.However,therearemanyotherpossi-
bilities,forexample,alightsourceinplaceof
incandescent lampsorfluorescent lampsand
alsoinopticalcommunication.Here,webriefly
mention thatbecauseof the wet-process
fabrication technologyofOLED, it canbe
fabricateddirectlyon the end terminal of
opticalfibers.High-speedopticalcommunica-
tionofvideosignalsutilizing theOLEDand
theOPDwithCu–phthalocyaninehavebeen
successfullydemonstrated.24)
�.� Conducting polymer laserWehavedemonstrated thatelectronic
energyschemesofconductingpolymersbest
fit lasersbecause theyare typical four-elec-
tronic-energy-level systems,withwhichwe
can realizepopulation inversionand lasing
withlowthresholdexcitation.25)
Indeed,evenwithoutoutercavitymirrors,
super-radiationwasobserveduponoptical
excitation inconductingpolymers.26)By the
introductionof fluores-
cent conducting poly-
mers in cavit ies, low
threshold lasingcanbe
realized.26)
We examined the
microring structure of
la ser s by immers ing
opticalfibers insolutions
of conductingpolymer.
Then we were able to
preparemicroringstruc-
turessurroundingoptical
fibers.27) A microring
lasing device was also
prepared by uti l iz ing
themicrodisk structure
on the substrate.28,29)
H o w e v e r, h e r e , w e
res t r ic t ourse lves to
microringfibers.
As shown in Fig.
3(a), thespectralwidth
decreases drast ical ly
aboveacertainthreshold
excitation intensity. It
shouldalsobementioned
that upon decreasing
thediameterofthefiber,
singlemode laseremis-
sion was observed, as
showninFig.3(b).
Weproposedtheformationofmicroring
structures utilizing glass pipes instead of
fibers.30) In thecaseofpipesmicroringscan
be formednotonlyontheoutersurfaceof
thepipebutalsoon its inside surface.The
spectrumshowninFig.4(a)wasobtainedby
opticalexcitationon thedouble-ringdevice
made of poly(2-methoxy-5-dodecyloxy-p-
phenylenevinylene)(MDDOPPV).
BytheanalysisofFig.4(a), theemission
wasconfirmedtooriginatefrombothoutside
andinsiderings.Itshouldalsobestressedthat
byutilizingadifferent conductingpolymer
onthe insidesurfacefromthatontheouter
surface, two-color lasingcanberealized,as
showninFig.4(b).30)
�.� Organic solar cellsTherearetwotypesoforganicsolarcells,
donor–acceptorsolarcellsbasedonelectron
transferbetweendonorsandacceptors,such
as fullereneC60 and theirderivatives, and
dye-sensitizedTiO2cells,theso-calledGraetzel
solarcells.31)
Thefirst typeofsolarcellsresultedfrom
our findingofphotoinducedchargetransfer
betweenconductingpolymersandC60.32-34)
Here,wewilldiscussonlythistypeofdevice.
Asshown inFig.5,photoluminescence
ofconductingpolymer is stronglyquenched
whereas photoconductivity is markedly
enhancedupontheintroductionofC60tothe
conductingpolymer.Thesenovelcharacteris-
ticscanbeexplainedbyphotoinducedcharge
transferbetweentheconductingpolymerand
C60.32-34)
Utilizingvariouscombinationsandstruc-
tures of conducting polymers and fuller-
enes, various typesof solarcellshavebeen
proposed, suchas simple junction layersas
wellasothersobtainedby introducingnew
conceptssuchasphoton-harvestmolecules,
andcondensed interfaces, interpenetrating
networks,andselectivedoping.35-37)
620 630 640 6500
200
400
600
800
Emis
sio
n In
ten
sity
(arb
.un
its)
Wavelength(nm)
450 500 550 600 6500
5000
10000
15000
Wavelength(nm)
Emis
sio
n In
ten
sity
(arb
.un
its)
Fig.4 (a)Double-ringlasingand(b)dual-colorlasingfrommicrocapillarystructurewithconductingpolymers.
-PPV
Concentration of C60 (mol%)
Lum
ines
cen
ce in
ten
sity
(arb
. un
its)
Pho
toco
nd
uct
ivit
y (a
rb. u
nit
s)1.0
0.5
0.0 1
10
100
1000
0 5 10Fig.5 PLquenchingandphotoconduc-tivityenhancementincompositesystemofconductingpolymerandC60.
�JSAPInternationalNo.17(January2008)
�.� Organic thin film field-effect transistors and others
Organic field-effect transistors (OFETs)
fabricatedwiththeconductingpolymer,poly-
thiophene,onSi substrateswere reported
byKoezukaet al.41)Ontheotherhand,we
reported FETs onpolymer substrates.42) It
shouldalsobementionedthatinourjunction
devicesutilizingpoly(3-alkylthiophene),which
exhibits thermochromicbehavior, strongly
temperature-dependentcharacteristicswere
observed.43)Afterthosereports,manypapers
on the FET werepublished. To apply wet
processes,solubleconductingpolymershave
beenpreferred.Ontheotherhand,toobtain
highcarriermobility, low-molecular-weight
aromaticmoleculeshavebeenpreferredand
used.Therefore,amethodofapplyingwet
processes toaromaticmoleculesaswellas
oligomershasbeensought.
ThehighperformanceofOFETsfabricated
fromanannealedpentacene solutionand
organic transistorswith solution-processed
source /drainelectrodesusingmetalnanopar-
ticleshavebeenreported.
Thethiopheneoligomerswithhighcarrier
mobilityarecandidatesforrealizingall-organic
circuitsonanactivesemiconductor layer.A
highmobilityofgreater than0.1cm2 / (V·s)
canbeobtainedfromoligothiophenetransis-
torsfabricatedbydryprocessing.OLEDswith
oligothiopheneasthehole-transportinglayer
havebeenreported.
Ontheotherhand,solublepoly(3-alkyl-
thiophene) isoneofpasses fromthe liquid
phase to the solidphase.OFETs fabricated
bycasting showhigher field,making them
promisingmaterials for large-areadevices.
Thefield-effectmobilityofpolymertransistors
fabricatedbywetprocessingismorestrongly
dependenton the self-organized structure,
which is influencedbythepolymersolution,
than thatof transistors fabricatedby spin
coating.Variousbipolar-typeOFETsbasedon
compositesofp-typeconductingpolymers
andn-typedyeshavebeenfabricatedbyspin-
coating. It isalsoof interest toexaminethe
OFETshavingcompositesofconductingpoly-
mersanddyeswiththesamebackbone.44)
Organicchromicdevicesarealsoprom-
isingforapplicationsinvariousfields.
We have proposed a color switching
devicebasedonthereversibleinsulator–metal
transitionoccurring in conductingpolymer
upondopingandundoping.45-47) This idea
came touswhenadrastic change inelec-
tricalconductivityowingtoinsulatortransition
upondopingwasdiscovered,45-47)because
wehadexpectedacolorchange fromthat
conductingpolymer intheinsulatingstateto
ametalliccolor inthemetallicstateresulting
from plasma reflection. Indeed, dramatic
change incolorandexcellentcharacteristics
ofcolorswitching inconductingpolymeras
wereexpected.Thecolordependsonthekind
ofconductionpolymer.Forexample,polythio-
pheneswitchesbetweenred inthe insulator
phasetoblueinthemetallicstate.Multicolor
changes canbe realized inpolyanilineand
other conductingpolymers. The switching
speed is faster than thoseof conventional
nematicLC(NLC)devices.The lifeexceeding
105 cycles can be demonstrated.45-47) To
bewidelyused,a longercycle life isneces-
sary, and the use of an ionic liquid has
beenreportedtobeoneof themethodsof
improvement.48)
Bydesigninganappropriatemolecular
structure of the conducting polymer and
It shouldbementioned that,compared
withtheconventionalsolarcellsshowninFig.
6(a),inwhichlightimpingesfromtheindium
tinoxide (ITO)electrode sideonwhich the
conductingpolymer layer is formed, in the
invertedstructureofthedeviceshowninFig.
6(b), theconversionefficiencywas increased
markedly owing the suppression of the
windoweffectof theconductingpolymer.38)
That is,byutilizinga transparentZnOelec-
trodeonwhichaC60layerisformedandirra-
diatingfromthisside,thewindoweffectwas
eliminated,becauseelectron–holeseparation
occursatthe interfacebetweenC60andthe
conductingpolymerjunction.39)
It shouldalsobementioned that in the
interpenetratingnetwork typeof solarcells
ofsimplestructure,byoptimizingthedisper-
sionofC60 inconductingpolymerbyvarious
methods suchasannealing,highefficiency
wasalsoattained,asshowninFig.7.40)
Composite
ITOQuartz
Al
(b)(a)
Quartz
Au
ZnO
EQE
(%)
Wavelength (nm)
(a)
Cu
rren
t (m
A/c
m2 )
Cu
rren
t (m
A/c
m2 )
Voltage (V)
(b)
1:0.25 (no annealing: 1.78%)
1:0.5 (annealing at 100oC: 2.22%)
1:0.75 (annealing at 125oC: 1.79%)
1:1 (annealing at 150oC: 1.49%)
300 400 500 600 700 8000
20
40
60
80
100
–0.5 0.0
–10
10
–10
–4
–2
02
4
6
8
10
–1.0 1.0
–8
–6
0.5
–8
–6
0.5
Fig.6 Schematicdevicestructuresof(a)conventionaltypeand(b)invertedtypewithZnOlayer.
Fig.7 EQEspectraandI–Vcharacteristicsofthin-filmsolarcellswithcompositesystemofconductingpolymerandC60obtainedundervariouspostdepositionannealingconditions.
Invited Review Paper
JSAPInternationalNo.17(January2008)�
devicestructureandsystem,afull-color,reli-
ablelong-lifeopticaldeviceshouldberealized
inthenearfuture.
�. Photonic CrystalRecently, PCswith a three-dimension-
allyperiodicstructureoftheorderofoptical
wavelengthshaveattractedgreat interest,
because in thisnewclassofmaterialnovel
conceptssuchasaphotonicbandgap(PBG)
intheenergyrangeofwhichphotonscannot
existorpropagate.Uponthe introductionof
defects,localizedstatesatwhichphotonscan
localizeare formed in thePBG.That is,PBG
materialsareanewclassoforderedstructures
withaperiodicityofoptical-wavelengthorder,
whichfacilitatesthemanipulationofphotons
inthesamewaythatasemiconductorcontrols
theflowofelectrons.49,50)Thepropagationof
photons insuchPCs is similar to thepropa-
gationofelectrons in semiconductors. The
PBGinaPCplays,forphotons,thesamerole
astheforbiddenenergygapforelectrons in
semiconductors,therebyallowingthemanip-
ulationof light flow.Hence,opticaldevices
thatareanalogoustosemiconductordevices
aremadefeasiblebythisphenomenon.Asa
consequence,PBGdeviceshavethepotential
foraccomplishing foroptical circuitrywhat
semiconductordeviceshavedone forelec-
trical circuitry.Anumberofunusualoptical
propertiesarepredicted inPCs. Inparticular,
thestudyofstimulatedemission in thePBG
isoneof themostattractivesubjects,since,
in thePBG, spontaneousemission is inhib-
itedand low-threshold lasersbasedonPCs
areexpected.49,51-54) InordertorealizePCs,a
largenumberofintensivestudiesonmicrofab-
ricationbasedonsemiconductorprocessing
technology55-57)andself-assemblyconstruc-
tionof nanoscale spheres58,59) havebeen
carriedout.
Silicaopal isatypeofnaturallyoccurring
PCthatconsistsofwell-orderedthree-dimen-
sional (3-D)arraysofSiO2spheres,thathave
diametersintherangeofvisiblewavelengths.
Indeed,theiridescentcolorationofsuchopals
iscausedbythediffractionoflightfromthese
regulararraysof silicaparticlesofmonodis-
perseddiameterofoptical-wavelengthorder.
�.� Fabrication of �-D PC: synthetic opal and inverse opal
Synthetic opals are prepared by the
following process. Monodispersed silica
colloidal spheresaresynthesized inaqueous
solutionsbywell-knownprocedures.Ordered
colloidalcrystalshavingvariousPBGscanbe
preparedby sedimentationof the suspen-
sionofmonodispersedSiO2spheresofvarious
diameters. These crystals are annealed at
100–120 ºCand sinteredat600–700 ºC to
induceasmalldegreeofinterparticlesintering,
whichmechanically stabilizes the structure.
Thesemechanically robust,porousopalsare
cut intoplates. Thin filmof syntheticopal
canalsobeformedbysedimentationofSiO2
spheresinasandwichcellmadeoftwoglass
plateswithaseparationofseveraltohundreds
µm. Porous opal prepared by this proce-
durehasafcccrystal latticestructure,which
is sometimes faultedwithhexagonal-close-
packedstackingarrangements.Polymeropals
arealsoprepareddirectlyutilizingpolymer
spheresofnanosize indiameter (Fig.8).As
evident in thescanningelectronmicroscope
(SEM) image, in thisopal, regular stacksof
silicasphereswereconfirmed.Figure9shows
thetransmissionandreflectionspectraofthe
syntheticopalspreparedby thismethod.As
clearly shown in this figure,clear reflection
peaksand transmissiondipswereobserved
dependingonthesizeofthespheres,which
indicatestheregularperiodicarrayofparticles.
Thepeakandthedipcorrespondtotheband
gapofthe3-DPCofopal.
The opal prepared by this procedure
containsaninterconnectingstructureoftetra-
hedralandoctahedralvoids.Thesevoidsare
fully interconnectedby channels through
hexagonalclose-packed layers. Inverseopals
canbepreparedby filling the voids with
secondarymaterials such as liquidphoto-
polymer and consequently removing silica
particlesbyHFetching.Carboninverseopals
with 3-D periodicity can be prepared by
infiltratingphenol resin and subsequently
removingsilicausingHF,followedbyannealing
forcarbonization.Carboncanalsobedirectly
infiltratedinvoidsbychemicalvaporeposition
(CVD).Forexample,propylene(C3H6)ischosen
astheprecursorgas inorderto increasethe
reactionyieldintheCVDprocess.60)
As clearly shown inFig. 8(b), regular
Wavelength (nm)
Refl
ecta
nce
(a.
u.)
Tran
smit
tan
ce (
a.u
.)
5000
1180nm 300nm 550nm 1µm
1000 1500 2000 2500
Fig.8 SEMimageofplasticopalandinverseopal.
Fig.9 Transmissionandreflectionspectraofsyntheticopalformedfromsilicaparticleswithvariousdiameters.
�0JSAPInternationalNo.17(January2008)
stepwise changes at thephase transition
points,which isconsistentwith thedirectly
measuredrefractive indexofvariousphases.
It shouldalsobementionedsimilarstepwise
changeofthestopbandwasalsoobservedin
Pcsofsmectic-LC-infiltratedopals,asshown
inFig.10.That is, theseresults indicatethe
possibilityofthetemperaturetuningofPCsof
LC-infiltratedopals.
PhasetransitionbehaviorofLCwasalso
confirmedtochangewhenitwasinfiltratedto
nanovoidsintheopalfromdielectricmeasure-
ments.Thedispersionfrequencyofthedielec-
tricconstantofNLCinthenanosizevoids in
theopal ishigher infrequencybyaboutone
orderofmagnitude than that inaconven-
tionalsandwichcell,which isconsistentwith
thefasterresponsespeedoftheelectrooptic
effectintheopalinfiltratedwithLC.Thismay
originate fromthe strong interactionofLC
moleculeswiththeinnersurfaceofthenano-
scalevoidsintheopal,resultinginthestrong
recoverforce.Itshouldbementionedthatthis
tenabilityinLC-infiltratedopalisreversible.
c)Thermochromismofconductingpoly-
mersinfiltratedintoopal
We proposed the novel temperature
tuningmethodoftheopticalstopbandusing
the change in the refractive index associ-
atedwiththethermochromismofconducting
polymer infiltrated into synthetic opals.
Figure11showsthetransmissionspectraofa
syntheticopalinfiltratedwithpoly(3-octadecy-
lthiophene)(PAT-18)asafunctionoftempera-
ture.Asisevidentfromthisfigure,thewave-
arraysofvoidswereobservedininverseopal.
Thesizeofvoidsdependonthediameterof
silicaspheres.
�.� Tunability of optical properties in opal-based PCs
Wehave combined experimental and
theoreticalstudiestodeviseandevaluatenew
typesofPCs: tunablePCs (TPCs)basedon
syntheticopals.TheTPCsconsistofperiodic
particlearrayswhichprovideeitheroptical
orelectrical tunability for thepropertiesof
thePC,particularly thewidthandposition
ofthePBG.Thetunabilitycanberealizedby
changing theperiodicity, crystal structure,
or refractive index, forexample.Oneof the
methodsof changing the refractive index
ofPCs,syntheticopalswithSiO2spheres, is
to infiltratevariousactivematerials, suchas
metals, semiconductors,organicmolecules
andpolymers, into thenano-scale voidsof
theopals.58)TPCsareexpected tocombine
theadvantageouspropertiesofconventional
PCswiththetunabilityoftherefractiveindices
ofsuchactivematerialsasconductingpoly-
mers,photoresponsivematerials,andmeso-
genicmaterials.Modificationsof theperiod-
icityandcrystalstructurecanalsoberealized
byapplyingmechanicalstresstoplasticopals
fabricatedbythesedimentationofnanoscale
polymerspheres.Variousnewfunctionalities
areexpectedintheseTPCsmadeofinfiltrated
opalsandinverseopals.
3.2.1 Temperature tuningTemperature tuningof theopticalprop-
ertiesofPCshasbeenachievedbyvarious
approaches. We have demonstrated the
temperature tuningof thestopbandof the
reflectionspectruma)byheat treatmentat
temperatures higher than
900 ºC,b)upon refractive
indexchangeat thephase
transition temperature in
LC-infiltratedopal, and c)
duetothethermochromism
ofconductingpolymersinfil-
tratedinasyntheticopal.
a)HeattreatmentofSiO2
opal
Thestopbandposition
ofSiO2opal canbe tuned
via theeffectofheat treat-
ment.Uponheattreatment
at temperatures higher
than900 ºC, reflectionspectrachangewith
changingtreatmentperiodTh.Braggdiffrac-
tionpeaks at each incident angle shift to
shorterwavelengthswithincreasingTh,which
canbeinterpretedintermsofthedecreaseof
periodicity.That is,thelatticeconstantevalu-
ated from theanalysis ofdiffractionpeak
decreasesmonotonicallywith increasingheat
treatmentperiodTh,whichwereconsistent
withtheelectronmicroscopeobservationand
canbeinterpretedtobeduetotheprogress
of sintering.Fromtheseresults, ithasbeen
confirmedthattheeffectivelatticeperiodicity
ofPCscanbetunedatanyvalue inawide
rangebyheattreatment.61) Itshouldalsobe
notedthattheporesizeofopalscanalsobe
controlledbyheat treatment,whichshould
alsoinfluencetheinfiltrationofforeignmate-
rialanditscharacteristics.62)Itshouldbenoted
thatthistypeoftuningispermanent.
b)LC-infiltratedopal
The s top band of
opals infiltratedwithLCs,
suchasNLCandsmectic
LC,showsamarkedshift
withchanging tempera-
ture. It should also be
mentionedthatthewave-
lengthof the stopband
shifts stepwise at the
phase transition points
betweenvariousphases,
whichcanbe interpreted
intermsofthechangein
refractiveindex.Asshown
inFig.3, the refractive
indexofNLC (ZLI-1132)
evaluatedbytheanalysis
of the stopband shows
100
80
60
40
20
0
Tran
smis
sio
n (
%)
800750700650600550500Wavelength (nm)
RT 70°C 90°C 100°C 120°C 140°C
730
725
720
715
Peak
wav
elen
gth
(n
m)
1401006020Temperature (°C)
Fig.11 TransmissionspectraofopalthinfilminfiltratedwithPAT-18asafunctionoftemperature.Theinsetshowstemperaturedependenceofpeakwavelengthoftrans-missionspectra.
Temperature (°C)
Refr
acti
ve in
dex
Nematic
201.525
1.530
1.535
1.540
30 40 50 60 70 80 90
Iso
Fig.10 Refractiveindexofnematicliquidcrystalevaluatedbystopbandanalysis.
Invited Review Paper
JSAPInternationalNo.17(January2008)��
lengthof the stopbandshifts considerably
with increasingtemperature.Forexample,at
thewavelengthof725nm,thetransmittance
canbevariedfrom1.6%(roomtemperature,
RT)to47%(140ºC)bychangingthetemper-
ature. Itshouldbenotedthattheseshiftsof
the transmission spectrawith temperature
wereconfirmedtobereversible.Thechangein
therefractiveindexofPAT-18evaluatedfrom
thechange inthestopbandwith increasing
temperaturecoincideswiththatofthedirectly
estimatedvalue.That is,theblueshiftofthe
peakwavelengthof the stopbandcanbe
attributed to thedecrease in the refractive
indexofPAT-18which isduetothe increase
of thebandgapofPAT-18,with increasing
temperature.
3.2.2 Mechanical tuning in plastic opalTheperiodicityand filling factorof the
opalsandinfiltratedopalscanbecontrolledby
applyingmechanicalstress.Inthecaseofsilica
opal,uponapplyingpressure,wecanobserve
achangeintheperiodicity,whichresultsinthe
changeof theopticalpropertiessuchas the
stopbandposition in thetransmissionspec-
trum.
Inthecaseofpolymeropals,particularly
opalsmadeof elastomer spheres,we can
reversibly change theopticalpropertiesby
applyingmechanicalstress.AsshowninFig.
12, thereflectionpeakof theplasticopal is
confirmedtoshiftdrasticallyupontheapplica-
tionofuniaxialmechanicalstressperpendicular
tothelightbeam.Thiseffectcanbeexplained
asachange in theperiodicity in thedirec-
tionofthe lightbeam.That is, thereflection
peakexhibited largeredshiftupontheappli-
cationofpressure,owingto the increase in
theperiodicity inthedirectionof lightbeam.
Ontheotherhand,thereflectionpeakexhib-
itedblueshiftasaresultofstretchingdueto
thedecreaseof theperiodicity in thedirec-
tionofthe lightbeam.Theseresultssuggest
thepossibilityofmechanicaltuningofthePC.
Mechanical tenabilitywasalsodemonstrated
inelastomeric-polymer inverseopalprepared
bythereplicamethod.
3.2.3 Voltage tuningVoltagetuningcanberealizedinsynthetic
PAT-infiltratedopalusedasoneof theelec-
trodes inanelectrochemicalcell.Uponelec-
trochemicaldopingoftheconductingpolymer
infiltratedintotheopal,theshiftofthereflec-
tionpeakwasobserved.By analyzing the
reflectionpeakasfunctionoftheelectricfield,
theobservedresultscanbeexplainedinterms
of thechange in therefractive indexof the
conductingpolymerinfiltratedintoopalsupon
electrochemicaldopingofBF4–ions.Thisresult
indicates thepossibilityofvoltagetuningof
PCs.
On the other hand, LCs have a high
opticalanisotropyandaresensitivetoexternal
stresssuchasanelectricfield.Becauseofsuch
opticalanisotropyandfieldsensitivity,TPChas
beenproposedtobemadeofopalorinverse
opal infiltratedwithLC. In theLC-infiltrated
opalthinfilmmadeofSiO2spheres,thestop
band shift upon voltage application was
confirmed.Thisisinterpretedtooriginatefrom
therefractiveindexchangeduetothemolec-
ularreorientationcausedbythevoltageappli-
cation.
Figure13(a)showsreflectionspectraof
thepolymer inverseopal infiltratedwith5CB
asafunctionoftheamplitudeofappliedrect-
angularvoltage(f=1kHz).The lightwas irra-
diatedperpendicularlytothereplicafilm, i.e.,
inthe[111]direction.As isevidentfromthis
figure, the reflectionpeak shifts to shorter
wavelengthswith increasing voltage. This
hasbeeninterpretedtobeduetotherefrac-
1.0
0.5
0.0
No
rmal
ized
refl
ecti
on
inte
nsi
ty (
arb
. un
its)
800750700650600550
Wavelength (nm)
Strain 0 0.11 0.23 0.38
Fig.12 Reflectionspectraasafunctionofappliedmechanicalstress.
Refl
ecta
nce
(a.
u.)
Peak
Wav
elen
gth
(n
m)
nLC
300200
680
0V140V170V200V270V
720 760
1000690 1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
700
710
720
730
740
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Wavelength (nm)
Applied Voltage (V)
Fig.13 (a)ReflectionspectraofLC-infiltratedpolymer inverseopal as a function of appliedvoltage.(b)Peakreflectionwave-lengthandeffective refractiveindexofLCinvoidsasafunctionofvoltage.
��JSAPInternationalNo.17(January2008)
tive indexchangecausedby themolecular
reorientationalongtheappliedelectric field.
In the initialstate, therefractive indexofLC
filled intonanosizevoids in theopal should
bemacroscopicallyaveragedand isequiva-
lenttothatintheisotropicphase,becauseof
the3-Dsymmetryof thearrangementand
shapeofthevoidswithoutelectricfield.Under
appliedelectric field,however, theLCmole-
culesalignalongthefieldparalleltothedirec-
tionoflightpropagation,andtheratioofthe
moleculesaligningparallel tothedirectionof
lightpropagationslightlyincreases.Asaresult,
thecomponentoftherefractive indexnofor
ordinary light increases ineachvoid,andthe
averaged refractive indexof LCdecreases.
Consequently, the reflectionpeak shifts to
shorterwavelengths.Asimilarbehavior, that
is,theshiftofthepeakpositionofthereflec-
tionspectrumuponapplyingvoltage,hasalso
beenobservedinsilicaopalinfiltratedwithLC.
Figure13(b) showsthevoltagedepen-
denceofthereflectionpeakwavelengthλLCof
thepolymerinverseopal infiltratedwith5CB.
ThetotalshiftofλLCupontheapplicationof
300 V isabout35nm,which ismuch larger
thanthatofthesilicaopalinfiltratedwith5CB.
Thisshouldbeattributedtothelargevolume
fractionofthevoidsfilledwithLC,therefrac-
tiveindexofwhichchangesupontheapplica-
tionofvoltagecomparedwiththatinthesilica
opal.
TheorientationofdirectorsofLCschanges
upon applying magnetic field. Therefore,
magneticfieldtuningofreflectionandtrans-
missioninopalsinfiltratedwithLCscanbereal-
ized.
In theabove-describedelectric fieldand
magnetic field tuningofPCcharacteristics
inLC-infiltratedopals,anyLCs,suchasNLC,
smecticLC,andcholestericLC,canbeused.
Onthecontrary,opalsinfiltratedwithLCscan
beusedasopticallydetectablemagneticfield
sensors.
3.2.4 Optical tuningTheopticalpropertiesofmaterials that
havebeeninfiltratedintosyntheticopalsand
that also constituteopal replicas canalso
becontrolledby light irradiation.Wehave
demonstrated that theopticalpropertyof
PCs infiltratedwithphotochromicmolecules
orpolymersandpolymerscontainingphoto-
chromicmoetiessuchasazobenzene in the
sidechainscanbecontrolledby light irradia-
tion.Inthiscase,wecaneitherpermanentlyor
out-of-phasestanding wave
out-of-phasestanding wave
photonic bandgap (PBG)
polarization directionof standing wave
LC molecules
in-phasestanding wave
in-phasestanding wave
k/a
a
ph
oto
n e
ner
gy
Fig.15 Schematicexplanationoftheappearanceofphotonicbandgapinspiralperiodicstructure.
1,000
800
600
400
200
0
0
50
40
30
20
10
500
400
300
200
100
0430 440 450 460 590 600 610 700 710 720 730 740
Wavelength (nm)
Emis
sion
Inte
nsit
y (a
rb. u
nits
)excitation intensity
mJ/pulse6.1
0.350.98
0.520.024
2.91.5
3.51.8
1.40.250.13
excitation intensitymJ/pulse
17 5.9
excitation intensitymJ/pulse
21 7.1
Fig.14 EmissionspectraofMDDO-PPVasafunctionofexcitationintensityingreenopalinfiltratedwithTHF.
Invited Review Paper
JSAPInternationalNo.17(January2008)��
explainedbytheamplifiedspontaneousemis-
sion(ASE)andthelatterbymultimodelasing
(ML)influencedbytheopticalfeedbackdueto
theperiodicstructureoftheopalmatrix.
�. Photonic Crystal based on Self-Organized Helix Structure of Chiral Liquid Crystals
LCs includinga chiralmoleculehavea
self-organizedhelical structure that canbe
regardedasaone-dimensional (1-D)periodic
structure and shows characteristic optical
properties.63) In such systemswithahelix,
lightpropagating along thehelical axis is
selectivelyreflected,dependingonthepolar-
izationstates, if thewavelengthof the light
matchestheopticalpitchofthehelicalstruc-
ture, this is called selective reflection. The
wavelengthregion inwhichthe lightcannot
propagateisthestopband,andisconsidered
the1-Dpseudo-bandgap.Lasingattheband
edgehasbeenreportedinthecholestericLC
(CLC),64,65)chiralsmecticLC,66-68)andpolym-
erizedcholestericLC(PCLC).69,70)These laser
actions in the1-Dhelical structureofchiral
LCsare interpretedtobebasedontheband
edgeofthe1-Dphotonicbandgapinwhich
thephotongroupvelocityissuppressed.71)
�.� Photonic band gap and band edge lasing in CLC
In thehelicalperiodic structureof the
CLCs,lightpropagatingalongthehelicalaxisis
selectivelyreflected,dependingonthepolar-
izationstates. If thewavelengthof the light
matchestotheopticalperiodicityofthehelical
structure, this is called selective reflection.
Inthiscase, therearetwotypesofcircularly
polarizedstandingwaveswith
zerogroupvelocityattheedges
ofthestopband,asshownin
Fig.15.Here,therodsindicate
themolecular longaxesofthe
CLCmoleculesandthearrows
showthepolarizationdirection
ofthestandingwaves.Forone
standingwave,thepolarization
directionofthe light isparallel
tothemolecularlongaxisand,
if we dope a laser dye, the
polarizationbecomesparallel
to the transitionmomentof
the doped dye. This light is
subjectedto theextraordinary
refractive indexof theLCand
has lowerenergywithrespect
toatravelingwave,whichcorrespondstothe
longeredgeofthestopband.Moreover,this
circularlypolarizedstandingwaveeffectively
interactswiththe lasermediumandwecan
expect the lowthreshold laseractionat the
longerwavelengthedgeofthebandgap.
4.1.1 Electrical tunability of lasing wave-length in dye-doped ferroelectric LC
ChiralsmecticLCswithatiltedstructure
showa ferroelectricity,andarecalled ferro-
electric LCs (FLCs). Theyarepromising for
electrooptic applicationsbecauseofa fast
responsetotheelectric field.72)TheFLCalso
hasahelical structureand shows selective
reflectionduetothe1-Dperiodicstructurein
almostthesamemannerastheCLC.73)
Figure16 shows emission spectra of
dye-dopedFLCasafunctionofpumpenergy.
For lowpumpenergy (1.76 µJ /pulse), the
spectrum isdominatedbyabroad sponta-
neousemissionandadip isobserved in the
broad spectrum. The dip originates from
the selective reflectionband resulting from
thehelixof FLC.As theexcitationenergy
increases,theemissionintensity isenhanced.
Atahighexcitationenergy (10.4 µJ /pulse),
lasingbecomesevidentasasharppeakatthe
lowerenergyedgeofthedip.Thefullwidthat
halfmaximum(FWHM)oftheemissionpeakis
lessthan0.5nm.Thelaserlightemittedfrom
theFLCiscircularlypolarizedandthesenseof
thepolarization is right-handed,whichcoin-
cideswiththehelical senseof theFLCused
here.Thisstronglysupportstheideathatthe
laseractioninthedye-dopedFLCisbasedon
theband-edgeeffectintheperiodicstructure
oftheFLChelix.
transientlycontrolthePCby light irradiation.
Materials thatexhibitphotoinducedphase
transitionarealso interesting for infiltration
intosyntheticopals.
3.2.5 Solvent effectsTo clarify thepossibility of tuning the
stopbandofopalsusedasPCs, theshiftof
thestopbandandthediffractionpeakwere
studiedasafunctionoftherefractiveindexof
thesolvent.Thereflectionpeakshiftsdrasti-
callyuponchanging thesolvent.Thewave-
lengthof thereflectionpeak increaseswith
increasing refractive indexof the solvent.
Thesefindingssuggestthepossibilityoftuning
thePBGintheinfiltratedPCs.
�.� Enhancement of spectral nar-rowing and lasing by infiltration into opal
Wehave reported the observation of
the inhibited spontaneous emissionof an
organicdye, rhodamine6G, infiltrated ina
polymerreplicaofsyntheticopalasaPC.The
morphology-dependentresonances,superim-
posedonthebroadbandemissionofrhoda-
mine6Gowingtosphericalwavelength-sized
microcavityenhancementofdyeemission,
havebeenobserved.
Thespectralnarrowingofphotolumines-
cence (PL)and theevolutionofsharpemis-
sion linesuponopticalexcitationhavealso
beenobservedinopalsmadeofSiO2spheres
infiltratedwithconductingpolymerssuchas
poly(2-methoxy-5-dodecyloxy-p-phenylen-
evinylene) (MDDOPPV)andalso fluorescent
dyes suchas rhodamine6G,NK-3483,and
coumarin120.Thelasingcharacteristicswere
foundtobedependentonthecombinationof
thedyesandconductingpolymersaswellas
theperiodicityoftheopalandalsotherefrac-
tiveindexofthesolvent.Lasingwasobserved
when fluorescentdyesexhibitinggreenPL,
redPL,andpurplePLwereinfiltratedingreen
opal, redopal,andpurpleopal, respectively.
Theseresults suggest that theperiodicityof
theopalplaysanimportantroleinlasing.
Figure14showstheemissionspectraof
thepurple,greenandredopalsinfiltratedwith
THF solutionof coumarin120,MDDO-PPV,
andNK-3483, respectively,asa functionof
excitation intensity.With increasingexcita-
tion intensity, thePLpeakbecamemarkedly
enhanced in intensityandthespectralwidth
becamemuchnarrower. Inaddition, sharp
newemissionlinesappear.Theformercanbe
20
15
10
5
0600550500450400
3000
2000
1000
0
Pump Energy
10.4 J/pulse
1.76 J/pulse
Wavelength (nm)
Emis
sio
n In
ten
sity
(ar
b.u
nit
s)
Fig.16 Emissionspectraofdye-dopedFLCasafunctionofpumppulseenergy.
��JSAPInternationalNo.17(January2008)
lengthcanbecontrolledoverawiderangeby
applyingelectricfield.
4.1.2 Laser action in photopolymerized CLCOpticallypumped laseractionhasbeen
observedinadye-dopedflexiblefreestanding
filmofphotopolymerizedCLC(PCLC). Inthe
PCLCfilm,theself-organizedhelicalstructure
actsasa1-DPC.Atahighexcitationintensity
abovethethreshold, laseraction isobserved
attheedgeofthe1-Dphotonicbandofthe
PCLChelical structure.ThisPCLC film laser
possessesanexcellentmechanical flexibility,
and the laser action is alsoobserved in a
bentfilmofPCLC,asshowninFig.18.This
impliesthatthe1-Dperiodicstructureneeded
forthelaseractionismaintainedeveninthe
deformed film.Using such flexibilityof the
PCLCfilm,afocusingeffectof laseremission
isdemonstratedinacircularlydeformedfilm.
Moreover,thehelicalpitchofthePCLChasno
temperaturedependence, incontrast tothat
ThehelixofFLCcanbeeasilydeformed
byapplyingelectric fieldand its response is
fastbecauseofthestronginteractionbetween
thespontaneouspolarizationandelectricfield.
FLChasaspontaneouspolarizationPsnormal
to themoleculesandparallel to thesmectic
layers.Whentheelectricfieldisappliedinthe
layer, for lowerfield,Ps tendstopointalong
thefielddirectionandFLCmoleculesstartto
reorient towardthedirectionnormal to the,
resultinginthedeformationofthehelix.Inthe
equilibriumstate,thedeformationofthehelix
mightcausetheelongationof itsperiodicity.
Abovethe threshold field,allFLCmolecules
orient towards the samedirectionand the
helixisunwound.Thefactthattheperiodicity
ofthehelicalstructureofthedye-dopedFLC
canbecontrolledbyapplyinganelectricfield
promptsus toexpect thepossibilityofelec-
tric field tuningof the laseremissionwave-
length.Figure17showsthelasingspectraof
thedye-dopedFLCathighexcitationenergy
1.2
1.0
0.8
0.6
0.4
0.2
0500490480470460
Wavelength (nm)
No
rmal
ized
Em
issi
on
Inte
nsi
ty (
a.u
.)
3.5 kV/cm3.02.52.00applied electric field:
Fig.17 Normalizedemiss ion spectraat high excitationenergyindye-dopedFLCasafunctionofappliedelectricfield.
Fig.18 LaseremissionfrombentfilmofpolymerizedCLC.
excitation beamz
y x
laser emissionlaser emission
LC molecule
E=0 E
(a) (b)
glass substrate
PVAITO
Lasi
ng
Wav
elen
gth
(n
m)
Applied Voltage (V)0
600
610
620
0.5 1.0 1.5Fig.19 (a) Schematicexplana-tion of the cell structure fortunablewaveguide laseruponholographicexcitation.(b) Voltage dependence oflasingwavelengthindye-dopedNLCwaveguideholographicallyexcited.
(24 µJ /pulse) as a functionof the applied
electric field. It shouldbenoted that lasing
wavelength shifts greatly towards longer
wavelengths with increasing field, which
correspondstotheshiftoftheselectivereflec-
tionband.Inspiteofaweakfield(3.5kV /cm),
wide tuningof the lasingwavelengthwas
achieved.
Lasing in theFLCmentionedabovehas
beenperformed in thecell configuration in
whichthehelicalaxis isperpendiculartothe
substratesand laser light isemittedperpen-
dicularly to thecell surface. In thisconfigu-
ration, thepumpbeam is absorbed in the
vicinityof the interfacebetweentheLCand
thesubstrate,anddopeddye in thebulk is
noteffectivelyexcited.Wehavedesigneda
planarcell configurationofdye-dopedFLC
for lasing, inwhich thehelixaxis isparallel
to the substrates, anddemonstratedopti-
callypumpedlasinginawaveguide.68)Alsoin
thiswaveguideLC laser, theemissionwave-
Invited Review Paper
JSAPInternationalNo.17(January2008)��
Fig.21 (a)Transmissionspectrumofdye-dopeddoublePCLCcompositefilmwithtwistdefect.(b)EmissionspectrumofdoublePCLCcompositefilmatabovethethresholdpumppulseenergy(200nJ/pulse).
withatwistdefect.
The PCLC film with the twist defect
waspreparedas follows.81)Photo-polymer-
izableCLCmonomerwasspin-coated from
atoluenesolutionontoaglasssubstrateon
whichapolyimide(AL-1254)wascoatedand
rubbedinonedirection. Inordertoobtaina
uniformplanaralignment, the coatedCLC
wasannealedata temperature justbelow
theclearingpoint.TheCLCmoleculesonthe
substratealign theirdirectorsparallel to the
glassplate,that is, thehelicalaxis isperpen-
diculartotheglasssubstrate.UVlightirradia-
tionwasperformedusingaXelamptoinduce
photopolymerizationof theUV-curableCLC
monomer.TwoPCLCfilmswereputtogether
asthedirectorsofLCmoleculesatthe inter-
facebetween these films to formacertain
angleϕ . Inotherwords, there isadiscontin-
uousphasejumpoftheazimuthalangleofthe
helicalstructuresbetweenthesePCLCfilmsat
the interface,anditactsasatwistdefect in
thehelicoidalperiodicstructure,asshownin
Fig.20.
Figure21(a)showsthetransmissionspec-
trumofthedye-dopeddouble-PCLCcomposite
ofunpolymerizedcholestericLC.Thismeans
thattheoperationwavelengthoflaseraction
isthermallystable,whichisagreatadvantage
fordeviceapplication.
4.1.3 Electrically tunable lasing in nematic waveguide under holographic excitation
Thedistributedfeedback(DFB)laseraction
canbeachievedwithatransientgratingusing
interferencefringesinducedbytwoexcitation
laserbeams (holographicexcitation). In this
geometry, the lasingwavelengthλDFBupon
holographicexcitationcanbeexpressedby
λDFB=neffλex /m sinθ,
whereneff istheeffectiverefractiveindex
oftheactivemedium,λexisthewavelengthof
excitationbeams,mistheorderofdiffraction,
andθ is thehalf-anglebetweentwoexcita-
tionbeams.Accordingto thisequation, the
lasingwavelengthcanbetunedbychanging
θ and /orneff.Therefore,ifneffcanbeelectri-
callycontrolled,anelectrical tuningof laser
emissionuponholographicexcitationcanbe
expected.Basedonthisconcept,wepropose
an electrical tuningmethodof the lasing
wavelengthusingadye-dopedNLCas an
activelasermediumasschematicallyshownin
Fig.19(a).IfLChavingextraordinaryandordi-
naryrefractive indices,neandno, isusedas
theactivematerial forthe lasermedium,the
effective refractive indexneff canbeelectri-
callycontrolledduetothefield-inducedreori-
entationofLCmolecules.Therefore,when
adye-dopedNLCwaveguide isholographi-
callyexcited,thelasingwavelengthshouldbe
tunablebychangingtheappliedelectricfield
acrosstheLClayer.
Figure19(b) showsthevoltagedepen-
dence of the lasing wavelength of the
dye-dopedNLCwaveguide.AboveV=0.8V,
the lasingpeak for the transversemagnetic
Fig.20 Schematic expla-nationof thedoublePCLCcompositefilmhavingatwistdefect,which isadisconti-nuityofthedirectorrotationaroundthehelixaxis.
6000
4000
2000
0700650600550500
Emis
sio
n In
ten
sity
(ar
b. u
nit
s)
80
70
60
50
40
30
20
10
Tran
smit
tan
ce (
%)
Wavelength (nm)
(a)
(b)
24
22
20
18
16640620600580
Tran
smit
tan
ce (
%)
Wavelength (nm)
(TM)-guidedmode (closed circle) showed
a continuous red-shiftwith increasing the
appliedvoltage.Thethresholdvoltageforthe
peakshiftisassociatedwiththeFrederikstran-
sitionoftheNLC.Theelectricaltuningoflaser
actioncouldbeperformedreversibly.
�.� Photon localization in chiral LCs with defect and its tunablility
The localizationof the lightutilizingthe
defectmodecausedby imperfections in the
periodic structure ispromising forpotential
applicationssuchaslowthreshold
lasersandmicro-waveguides.74-78)
4.2.1 Twist-defect-mode lasing in CLC
Laser actions reported so
far inchiralLCsareobservedat
theedgewavelengthofthestop
bandandareassociatedwiththe
group velocity anomaly at the
photonicbandedge.Ontheother
hand, lowthreshold laseraction
based on the photon localiza-
tionat thedefect intheperiodic
structure canalsobeexpected.
The introductionofadefect into
theperiodichelical structureof
theCLCshasbeen theoretically
studied.79,80) Inparticular,Kopp
andGenackhavepredicted the
existence of a single circularly
polarized localizedmode in the
twistdefectofCLCs.80)Weexper-
imentallydemonstratedthedefect
mode in the1-Dphotonicband
gapoftheCLCfilmhavingatwist
defectforthefirsttime.Thelaser
actionbasedonthetwistdefect
mode(TDM)wasalsoobservedin
dye-dopedPCLCcomposite film
��JSAPInternationalNo.17(January2008)
filmcontainingadiscontinuousdefect inter-
face.Astopband,the1-Dphotonicbandgap,
isconfirmedinthespectralrangefrom580to
640nm.Itshouldbenotedthatasharppeak
appearsat611nmwithinthephotonicband
gap,whichmightbe related to thedefect
modeinducedbytheintroductionofthetwist
defectinterface.
Figure 21(b) shows the emiss ion
spectrum of the dye-doped double-PCLC
composite filmwith thedefect interfaceat
thepumpenergyof200nJ /pulse.Atahigh
excitationenergy (200nJ /pulse), laseraction
appearsat611nm,which iswithintheband
gapandcoincideswiththeTDMwavelength.
With increasingexcitationenergy, another
sharpemissionpeakappearsat638nmwhich,
corresponds to theedgewavelengthof the
stopband.Thisemissionpeakmightbeasso-
ciatedwiththeband-edgelasingthatappears
inPCLCwithoutanydefect.Consequently,
defect-modelasingoccursatalowerpumping
energy comparedwith thatofband-edge
lasing.
4.2.2 Transient defect mode induced by par-tial deformation of helix
TheTDMbasedonthecompositefilmof
twoPCLCshasbeenachieved.However, its
wavelengthcannotbetunedbyapplyingan
externalfieldsuchasanelectricfieldorlight.
Wehaveproposedanewtypeofdefectmode
inthehelix,thatcanbedynamicallytunedby
applyinganexternalfield.82)Figure22shows
aschematicexplanationofaphotonicdefect
inCLC.Iftheperiodicity(pitch)ofthehelixis
partiallychanged,thatis,thepitchispartially
squeezedorexpanded,these irregularities in
theperiodic structureshouldactasdefects
andcause light localization.Asamethodof
inducingpartialchangeinthehelixpitch,we
proposethatthe localmodificationofhelical
twistingpower (HTP) inducedby irradiation
with focusedGaussian laser light.Optical
controlofHTPcanberealizedbyusing the
photochemicaleffectsof thedopedazoben-
zene,nonlinearopticaleffects,or simplyby
heating.Photoinduced reversiblecontrolof
theHTPofCLChasbeendemonstrated in
CLCcontainingphotochromicazobenzene,
andapplicationstoreflectiondisplaydevices,
optical shutters, and opticalmemory, for
example,havebeenstudied.83,84)Bytrans–cis
photoisomerizationofthedopedazobenzene,
theHTPof thehostCLCchanges, so that
photoinducedcontrolofHTPcanberealized.
Figure23 shows the calculated trans-
mission spectra for the right-handedcircu-
larlypolarized (RCP)and left-handedcircu-
larlypolarized (LCP) lightpassing through
theCLCwithachiral-defect-inducedpartial
helixdeformation.Thecalculationwascarried
outassuming that theHTP ismodulatedas
HTP(z)=HTP0[1+α exp(-2z2 /w2)],where the
z-axisisparalleltothehelixaxisand2wisthe
diameterofthelightbeam,α isthemodula-
tionstrengthofHTP,andHTP0 is the initial
HTP.ThehelixsenseofCLC is right-handed.
Thepitchof thehelix is350 nm,andordi-
naryandextraordinary refractive indicesof
LCare1.5and1.7, respectively.The thick-
nessoftheCLCis5µm.Atransmissionpeak
basedon thedefectmodewasobserved in
thestopbandfortheRCPlight.Ontheother
hand,when the incident lightwasLCP,no
defectmodewasobserved. Inotherwords,
thedefectmodeduetothephotonlocaliza-
tioninoursystemcanberealizedonlyforthe
circularlypolarizedlightwiththesamehand-
ednessas thehelix,which is similar to the
resultfortheTDM.Thepositionofthedefect
modedependsonαandw.Withdecreasingα,
a transmissionpeakduetothedefectmode
shifts towards the longer-wavelengthedge.
Onthecontrary,with increasingα , thepeak
shifts towards theshorter-wavelengthedge.
The increaseanddecrease inα correspond
tothesqueezingandexpansionof thehelix
pitch.Therefore, thetuningofdefectmodes
couldbeachievedbypartial squeezingand
expansionofthehelix.
4.2.3 Chiral defect fabricated by direct laser writing technique
We have proposed a novel approach
to introducingchiraldefects (localmodula-
tionof thehelixpitch) into thehelix struc-
tureofCLC.85,86)A schematic explanation
ofthefabricationprocedure isshowninFig.
24(a).A100 fspulseofaTi:sapphire laser
at thewavelengthof800nmandrepetition
rateof80MHzwas focusedon thesample
cell throughanobjective lenswithnumer-
icalaperture (NA)1.4.Aright-handedPCLC
material doped with1 wt % of DCM dye
alignedhomogeneously inacellwithagap
of6–7µm.Directlaserwritingwasperformed
withaconfocal laser scanningmicroscope.
Thelaserwasscannedoveranareaof146.2×
146.2µm2,withascan-lineresolutionof2048
linesperscanarea.First, the laser lightwas
tightly focusednear the substrate surface
in theCLCcell.Two-photonpolymerization
occurredatthelaserfocalpointandalocally
polymerizedPCLCthinfilmwasobtainedon
thesubstratesurface.Thesamplewas then
flippedoverandlaserwritingwasperformed
Fig.22 Schematicexplanationof theoptically inducedchiraldefectbasedonthepartialdeformationofthehelixpitch.
Fig.23 TransmissionspectraofRCPandLCPlightpassingthroughtheCLCwithdeformation-inducedchiraldefect(α=-0.2,w=300nm).
defect
local modulation of HPT
1.0
0.8
0.6
0.4
0.2
0.0
Tran
smit
tan
ce
700600500Wavelength (nm)
RCP
LCP
Invited Review Paper
JSAPInternationalNo.17(January2008)��
Fig.24 (a)Schematicexplanationoffabri-cationprocedureofPCLCwithchiraldefectbasedon localphotopolymerizationusingscanningconfocalmicroscope.(b)Transmis-sionand (c)emission spectraofCLCwithchiraldefect.
Wavelength (nm)
Emis
sio
n In
t. (
a.u
.)
600 700 800
pump energy:16 nJ/pulse
1.0
0.5
020
10
0
Tran
smit
tan
ce (
a.u
.)
(b)
(c)
scan
femto second laser
unpolymerized CLC
polymerized CLChelix axis
againneartheoppositesurfaceofthecell.As
aresult,ahybridstructurewasfabricated, in
whichanunpolymerizedCLCregionremained
betweentwoPCLCfilmsonthecellsurface.
Figure 24(b) shows the transmission
spectra for right-handedcircularlypolarized
lightof thefabricatedCLCdefectstructure.
Asingle-defectmode isobservedwithinthe
selective reflectionbandof theCLC. The
theoretical transmissionspectrumwascalcu-
latedusingBerreman’s4×4matrix,andgood
agreementwith theexperimental resultwas
obtained.Figure19(c) shows theemission
spectrumof theCLCsingle-defectstructure
athighpumpingenergy,alongwiththecorre-
spondingtransmissionspectrum.Single-mode
laser action is observedat628 nm,which
correspondstothedefect-modewavelength.
The lasing threshold for thedefectmode
structure is16.7mJ /cm2,which is less than
half thethreshold inCLCwithoutthedefect
structure.Thereductionofthelasingthreshold
inthedefectstructureisevidenceofahigh-Q
cavityformedbythedefect.
�. Tunable-Defect-Mode Las-ing in Periodic Structure Con-taining LC Layer as Defect
LCshavehighopticalanisotropyandare
sensitivetoexternalstresssuchasanelectric
field.Onthebasisofsuchopticalanisotropy
andfieldsensitivity,aTPChasbeenproposed
inopalorinverseopalinfiltratedwithLC.87-91)
Althoughopalandinverseopalaresimpleand
inexpensivemeansofrealizing3-DPCsbyself-
organizationof colloidalparticles,92,93) the
introductionofdefects intothe3-Dperiodic
structure isaproblemthatmustberesolved.
Notonly3-DPCsbutalso1-DPCsareattrac-
tivesubjects.Although,the1-DPCdoesnot
havea completePBG, therearenumerous
applicationsusingtheextraordinarydispersion
ofthephotonandlocalizedphotonicstatein
adefectlayer.Sofar, intensivestudieson1-D
PCapplicationshavebeenreported:air-bridge
microcavities,94)photonicband-edgelasers,95)
nonlinearopticaldiodes,96)andtheenhance-
mentofopticalnonlinearity.77,97,98)Recentlywe
introducedaLClayerinadielectricmultilayer
structureasadefectin1-DPC,99)inwhichthe
wavelengthofdefectmodeswascontrolledby
applyingelectricfieldbecauseofachangein
theopticallengthofthedefectlayercausedby
thefield-inducedmolecularreorientationofLC.
�.� Tunable-defect-mode lasing in periodic structure containing LC layer as defect
WehaveintroducedaLClayerina1-DPC
asadefect,wherethewavelengthofdefect
modeswascontrolledbyapplyingelectricfield
becauseofachangeintheopticallengthofthe
defectlayercausedbythefield-inducedmolec-
ularreorientationofLC.100)Wealsoproposed
awavelength-tunablelaserbasedonanelectri-
callycontrollabledefectmodeina1-Ddielec-
tricperiodicstructurecontainingadye-doped
LCasadefect layer.101,102)Figure25 shows
the1-DPCwithaLCdefect.Adielectricmulti-
layerconsistingof stackofalternatingSiO2
andTiO2 layersdepositedonan ITO-coated
glasssubstrateisusedasthe1-DPC.Inorder
Fig.25 Schematicexplanationof1-DPCcontainingliquidcrystalasdefect.
LC defect layer
Deielectric multilayer(SiO2, TiO2)
yx
z
ITO electrode
Voltagesource
Polarized light
��JSAPInternationalNo.17(January2008)
to introduce thedefect layer,adye-doped
NLC (MerckE47)wassandwichedbetween
substrateswithdielectricmultilayersusing2µm
spacers.TherefractiveindexanisotropyΔnof
E47is0.209atRT.Intheabsenceofanelec-
Fig.26 Transmissionspectraof1-DPCwithLCdefectasafunctionofappliedvoltage.
Fig.27 Voltagedependenceofdefect-modelasingwavelengthinthe1-DPCwithdye-dopedNLCdefect.
Fig.28 (a)Theoreticaltransmissionspectraof1-DPCwithoutanydefect(solidline)andCLCwithoutPCstructure(dashedline). (b)Theoretical transmissionspectraof1-DPCcontainingCLCasadefect.(c)Magnifiedtransmissionspectracorrespondingto(b).
tionof theLCmolecules.Consequently,we
confirmedthatthewavelengthofthedefect
modeina1-DPCwiththeLClayerasadefect
canbecontrolledbyapplyingvoltage.
On the basis of the above-described
defect-mode tuning, the lasing wave-
lengthcanbe controlledbyapplyingelec-
tricfield.101,102)Figure27showsthevoltage
dependenceof the lasingpeakwavelength
ina1-DPCwithDCM-dopedLC.The lasing
peakshifts towardshorterwavelengthswith
increasing voltage, in the samemanneras
thedefectmodeshiftshowninFig.26.The
wavelengthshiftof the lasingpeak isabout
25nm,evenuponapplying lowvoltage.As
isevident fromFig.27,a thresholdvoltage
existsforthepeakshift,andthelasingwave-
length shifts towards shorterwavelengths
above the thresholdof1.1V.This isassoci-
atedwithFrederikstransitionoftheLCinthe
defectlayer.
�.� Double periodic structure: Helix defect in �-D PC
WehavealsointroducedaCLClayerina
1-DPCasadefect.103-105)Figure28(a)shows
the theoretical transmission spectrumofa
10-pairmultilayerwithoutaCLCdefect(solid
line),andasimpleCLCwithoutaPCstruc-
ture (dashed line).ThePBGof theCLCwas
observedbetween605and680 nm,which
iswithin thePBGof themultilayer.Figure
28(b)showsthecalculatedtransmissionspec-
trumofa1-DHPCwithaCLCdefect.Many
peaksappearatregular intervals in thePBG
100
80
60
40
20
0800700600500
0 V1.2 V
Tran
smit
tan
ce (
%)
Wavelength (nm)
Lasi
ng W
avel
engt
h (n
m)
Applied Voltage (V)
0
590
600
610
620
0.5 1.0 1.5 2.0 2.5
Wavelength (nm)
Wavelength (nm)
500
660 670 680 690 700
0
20
40
60
80
0
20
40
60
80
100
0
20
40
60
80
100
600
1-D PCCLC
1-D HPCCLC
700 800 900
(a)
(b)
(c)Tran
smit
tanc
e (%
)Tr
ansm
itta
nce
(%)
tricfield,thelongmolecularaxisoftheLCis
alignedparalleltothesubstrates(y-axis).
Inorderto investigatethecharacteristics
of thedefectmode, the transmissionspec-
trumof the linearlypolarized lightpropa-
gatingalongthez-axiswas
measured from theoppo-
sitesideof thecellusinga
charge-coupleddevice(CCD)
multichannel spectrometer.
Figure26showsthevoltage
dependenceoftransmission
spectra for incident light
polarizedalong they-axis,
which corresponds to the
rubbingdirection and the
initialorientationdirection
of theLCmolecules in the
defect layer, as shown in
Fig.25.Arectangularwave
voltageof1kHzwasapplied
be tween I TO laye r s to
changethemolecularalign-
mentoftheLCinthedefect
layer.Thesolidanddashed
linescorrespond tospectra
at0and1.2V, respectively.
The peaks of the defect
modes sh i f t to shor ter
wavelengthsuponapplying
voltage.Thispeakshiftorigi-
nates fromthedecrease in
the optical length of the
defect layer causedby the
f ield-induced reorienta-
Invited Review Paper
JSAPInternationalNo.17(January2008)��
Fig.29 Calcula ted e lec t r i c f ie ldstrength in the cell of (a) 1-D PCcontainingCLCdefectand(b) simpleCLC.
Fig.30 (a)Emissionspectrumof1-DPC-containingdye-dopedCLCdefectlayer.(b)Theoreticaltransmissionspectracorrespondingtotheemissionspectrum.
Position (µm)
(a)
Position (µm)
Elec
tric
Fie
ld S
tren
gth
(a.u
.)Re
frac
tive
Inde
x
(b)
00
200
400
600
8001.0
1.5
2.0
2.5
2 4 6 8 10 12 Elec
tric
Fie
ld S
tren
gth
(a.u
.)Re
frac
tive
Inde
x0
0
20
40
60
801.0
1.5
2.0
2.5
2 4 6 8 10 12
Wavelength (nm)630 640 650 660 670
0
20
40
60
80
0
100
200
300
400
1-D PCCLC
(a)
(b)
Tran
smit
tanc
e (%
)Em
issi
on In
t. (a
rb.u
nits
)
of theHPC.Thesepeaksare related to the
defectmodesresultingfromtheintroduction
oftheCLCdefect.However,additionalpeaks
wereobserved,asindicatedbyarrows,which
disrupted the regular intervalbetween the
defect-modepeaksatbothbandedgesofthe
CLC.
The transmission spectra inFig.28(b),
magnifiedaroundthelongeredgeofthePBG
oftheCLC,areshowninFig.28(c).Fourmain
peaksdue to thedefectmodesappear at
regularintervals(661,673,687,and699nm),
althoughthepeakat687nmsplits.Thissplit-
ting isattributedtotheopticalanisotropyof
theCLC.Therefore,twokindsofdefectmodes
correspondingtoleft-andright-handedcircu-
larlypolarizedlightcouldexistoutsidethePBG
oftheCLC.Ontheotherhand,oneadditional
peakwasobservedat678.6nm,whichcorre-
spondstotheband-edgewavelengthof the
CLC.Bydetailedconsiderationofthepolariza-
tionstatesoftransmittedlight,theadditional
peakwasclearlydistinguishedfromtheother
defectmodepeaks. Suchapeakwasnot
observedina1-DPCwithauniformdefect,
such as an isotropic medium or nematic
LCs.100,106)Namely,thispeakisadefectmode
peculiartothehelixdefectinthe1-DPC,and
isassociatedwithphoton localizationorigi-
natingfromtheband-edgeeffectoftheCLC
helix.Notethatthisdefectmodepeakisvery
sharpandtheFWHMofthispeakis0.05nm,
which ismore than four timessmaller than
thatofotherdefect-modepeaks (0.23nm).
From thepeakwidth, theQ-factorof the
additionalmodeatthebandedgeoftheCLC
wasestimatedtobe14000,whichwasmuch
higherthanthoseoftheotherdefectmodes.
Inordertoclarify theappearanceofthe
high-Qdefectmode in thedouble-periodic
structure,wehaveperformeda theoretical
estimationof theelectric fielddistribution
in thethreetypesof1-Dperiodicstructures
describedabove,bya finitedifference time
domain(FDTD)method.Figure29showsthe
calculatedelectricfielddistributionsandrefrac-
tiveindicesintwotypesofperiodicstructures.
Thewavelengthof the incident light to the
periodic structures corresponds
tothehigh-Qdefectmodewave-
length. It shouldbenoted that
light is strongly localized in the
double-periodicstructureandthe
maximumelectric field intensity
ismorethan15timesasmuchas
thatofasimpleCLC.Lightislocal-
izedatthecenteroftheCLClayer
in thedoubleperiodic structure
shown inFig.29(a)and its field
pattern is similar to that in the
CLCshown inFig.29(b),which
indicatesthat light inthedouble-
periodic structure is confinedby
the band-edge effect of CLC.
Additionally, lightconfinement is
effectivelyenhancedbytheouter
periodic structure because the
wavelengthof light iswithin the
PBGof theouterperiodic struc-
ture. Namely, because of the
contributionsofboththeband-edgeeffectof
CLCandthedefect-modeeffectoftheouter
periodicstructure, lightisstronglylocalizedin
thedouble-periodicstructure.
Wehave investigated the laser action
ina1-DHPCwithaCLCdefect.Atahigh
pumpingenergyof18nJ /pulse,asshownin
Fig.30(a),onlyonesharplasingpeakappears
at 643.5 nm. The calculated transmission
spectrumofthissystemisalsoshowninFig.
30(b).UponcomparingFigs.30(a)and30(b),
the lasingpeak is seentocoincidewith the
wavelengthofthepeculiardefectmode.Note
�0JSAPInternationalNo.17(January2008)
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thatthelaseractionwasasinglemodebased
on one additional mode, although many
modesexistbecauseofthehighQ-factor.The
thresholdoflaseractioninthe1-DHPCwitha
CLCdefectwaslowerthanthatinsimpleCLC
withouta1-DPC.103)Thisresult isattributed
tothestrongopticalconfinementduetothe
highQ-factoroftheadditionalmode.Similar
resultshavebeenconfirmed in the1-DPC
containingFLCasadefect.107)
�. Tunable Two-Dimensional Photonic Crystal
Various typesof two-dimensional (2-D)
PCshavebeenstudied.108-111) In2-DPCs,the
PBGanddefect statesalsoplay important
roles.Weare interested inthecharacteristics
ofphotonsinwaveguidesin2-DPCs.
WithaY-shapedwaveguide inLC-infil-
trated2-DPCs,switchingofthelightpropaga-
tiondirectionanditstemporalbehaviorhave
been theoreticallyclarified.Detailsarenow
understudy.
�. SummaryIn thisarticle, thehistoricalbackground
andrecentprogressinorganicelectronicsand
photoniccrystalshavebeendiscussed.Novel
characteristicsoforganicelectronicdevices
mainlyutilizingconductingpolymersandrelated
withopticaleffectsandtunablephotoniccrys-
talscomposedofaperiodicstructureofoptical-
wavelengthordercombinedwith functional
organicmaterialshavebeendemonstrated.
Thesenewdevicesexhibit tremendous
novel characteristics, with which various
newapplicationswillbedeveloped.Weare
convincedthattheseorganicelectronicdevices
andphotoniccrystalswillbecomethekeytech-
nologyinsustainingthe21stcentury.
Itshouldalsobenotedthatthesedevices
arealso importantfromenvironmentalview-
points.
Invited Review Paper
JSAPInternationalNo.17(January2008)��
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Invited Review Paper
��JSAPInternationalNo.17(January2008)
Katsumi YoshinowasborninShimaneJapanin1941.Hegraduated in1964 fromDe-partmentofElectricalEngineer-ing, Faculty of Engineering,OsakaUniversity,wherehere-ceivedPh.D.degree. In1969hebecamearesearchassociateat theDepartmentofElectri-
calEngineering,OsakaUniversity,andin1972,hewaspromotedtoanAssistantProfessorin1972,toanAs-sociateProfessorin1978andtoFullProfessorin1988.From1974to1975hewasavisitingscientistat theHahn-MeitnerInstitutfürKernforschunginBerlin.From2005,hehasbeenaProfessorofEmeritusofOsakaUniversity.HehasbeenalsoaprofessorofShimaneUni-versityandNagasakiInstituteofAppliedScienceandanAdvisorofShimaneInstituteforIndustrialTechnology.From2007hehasbeenanDirectorGeneralofShimaneInstituteforIndustrialTechnology. HewasthevicepresidentoftheInstituteofElectri-calEngineersofJapanandthepresidentoftheJapa-neseLiquidCrystalSociety.HereceivedtheAppliedPhysicsAward(1984),OsakaScientificAward(1990),BookofYearAward(1997)andOutstandingAchieve-mentAwardfromTheInstituteofElectricalEngineersofJapan(1998),FellowfromTheInstituteofElectronics,InformationandCommunicationEngineers(2001),TheBestPaperAwardfromtheSocietyofElectricalMate-rialEngineering(2001),theBestPaperAwardfromTheJapaneseLiquidCrystalSociety(2002),JapaneseLiquidCrystalSocietyAward(2003),FellowfromIEEE(2004),TheBestPaperAwardfromJapaneseInstituteofElec-tricalEngineer(2004)andTheContributionAwardforPolymerSciencefromtheSocietyofPolymerScience,Japan.
Yutaka Ohmori graduatedin1972 fromDepartmentofElectricalEngineering,FacultyofEngineering,OsakaUniver-sity,Osaka, Japan,wherehereceived theDoctorofEngi-neeringdegree. In1977,hejoinedNipponTelegraphTele-phonePublicCorporation(now
NTTCorporation),whereheworkedmainlyresearchonopticalsemiconductordevices.In1989,hebecameanassociateprofessorinElectricalEngineering,FacultyofEngineering,OsakaUniversity. In2000,hebecameaprofessor inCollaborativeResearchCenterforAd-vancedScienceandTechnology,ElectronicMaterialsandSystemsEngineering,OsakaUniversity,andnowheisaprofessorofCenterforAdvancedScienceandInnova-tion(CASI),whereheworkedonopticalandelectricaldevicesutilizingorganicmaterialsincludingconductingpolymers.ProfessorOhmoriisafellowoftheInstituteofElectricalandElectronicsEngineers,Inc.(IEEE),andamemberoftheJapanSocietyofAppliedPhysics(JSAP),theInstituteofElectronics,InformationandCommuni-cationEngineers(IEICE),AmericanPhysicalSociety(APS),TheInternationalSocietyforOpticalEngineering(SPIE),andtheMaterialsResearchSociety(MRS).
Masanori Ozaki receivedhisB.E.,M.E., andD.E.degreesfromOsakaUniversityin1983,1995,and1988,respectively.He joinedDepartmentofElec-tronicEngineering inOsakaUniversityasaresearchassoci-atein1988,andwaspromotedtoanAssociateProfessor in
1994andtoaProfessorin2005.Hehasbeenengagedintheresearchonphysicalpropertiesandapplicationsoforganicfunctionalmaterials,particularlyliquidcrystalsandconjugatedpolymers.HestayedinPhysicsDepart-mentatUniversityofUtahfrom1994to1995asavis-itingscientistandstudiedspectroscopyofconjugatedpolymers.HeobtainedBestPaperAwardofJapanLiq-uidCrystalSociety.Hiscurrentresearchinterestsarena-no-structuredorganicmaterialsandtheirapplicationtoelectronicandphotonicdevices.
Akihiko FujiiwasborninOsa-ka,Japanin1969.HereceivedhisB.E.,M.E.,andD.E.degreesfromOsakaUniversityin1993,1995,and1997,respectively.After one-year JSPS fellow-ship,he joinedDepartmentofElectronicEngineeringinOsakaUniversityasaresearchassoci-
atein1998,andwaspromotedtoanAssociateProfes-sorinDivisionofElectrical,ElectronicandInformationEngineering,GraduateSchoolofEngineering,OsakaUniversityin2006. Hehasbeenengaged inresearchonopticalandelectricaldevicesutilizingorganicmaterialsincludingdyemoleculesandconductingpolymers. He isamemberof the JapanSocietyofAppliedPhysics,theinstituteofElectricalEngineersofJapan,theInstituteofElectronics,InformationandCommunicationEngineers,andtheSocietyofPolymerScience,Japan.
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