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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.