LiquidCrystalsandtheirapplications

inphotonics

Discovery:• F. Reinitzer (1888), discovered an

anomal two-step melting property of synthesized cholesteryl benzoate. At first changed into a milky substrate (at 145°C) and later into opaque (179°C).

• The „liquid crystal” term was given by Otto Lehman

LiquidCrystals

• Statebetweensolidcrystalandisotropicliquid,

• Flowabilitywithfar-rangeorderingofthestructure,

• Liquidcrystalphase– mezophase,• Generationofliquidcrystalphases

– Thermotropic– Liotropic

PAA

LiquidCrystals

Liquid crystals

termotropic liotropic

Crystal LiquidCrystal Liquid

temperature

crystalLiquid crystal

liquid

tem

pera

ture

Concentration of solvent

Thermotropic liquid crystals

DiscoticRod-like

ThermotropicphasesN- nematic N:allmoleculesareorientedparalleltoeachother.Nematics isopticallyuniaxial

- nematic N*(cholesteric):chiral moleculesareorientedinlayers,ineachlayerthemoleculesareorientedinonedirection,theversor ineachlayerischangingperiodically.Cholesterics N*isuniaxial andopticallyactive.

ThermotropicphasesS- SmecticphasesS:moleculesareorientedinlayers,andthemolecule’saxesareorientedunder90degreesinrespecttothenormal,• smecticsSA iS*A:moleculesareorientedinlayers,paralleltoeachotherandtothenormaltothelayersurface.SmecticsSA isuniaxial,andSmecticsS*A isuniaxialandopticallyactive.

ThermotropicphasesS

• smecticsSB iS*B :thesame,butthemoleculesformsahexagonalstructure.

• smecticsSC :likeinSA,buttheanglewithnormalisdifferentfromzero.

• ChiralsmecticsC*:alignedinlayers,withdifferenttiltofthedirector,

Polymorphism

• BetweensolidphaseandisomorphicmayexistoneLCphase(monomorphism),two(dymorphism),upto6phases(hexamorphism)

• ExampleSG-SF-SB-SC-SA-N

OpticalpropertiesVeryhighbirefringence(naturalandinduced)

-60 -40 -20 0 201,44

1,46

1,48

1,50

1,52

1550

1110

Reduced temperature T-TN-I

ne

nino

Refra

ctive

indi

ces

C5H11 O

O

OCH3C

Opticalproperties

• Opticalactivity• Linearandcirculardichroism,• Strongselectivelightreflectionfromstructuresandopticallyactivetextures.

• ElectroopticKerreffect,100 timesstrongerthaninnitrobenzene– Helicalpitchchange(à reflectedlightcolor)

Texturesunderpolariscope

AligningLCmolecules

• Rubbingmethod,• Termo-alignment,• Photo-alignment,

Textures• Homeotropictexture:opticalaxisisparalleltothedirectionofobservation,so

theviewisdark(underthepolarizationmicroscope)• Planartexture:opticalaxislaysintheplaneofliquidsurface.• Marbletexture:isotropic,separateddomains,ineachdomaintheversormay

havedifferentdirection.• Filamentoustexture:insidethethicksamplesathinlineswithnonregularshape

areobserved.• Nodaltexture:inthinsamplesanorderdeffectsareobserved.• Confocaltexture,• Polygonaltexture,• Fingerprinttexture.

FromtexturestoLCcells• Twothinglassplates,withelectreodes,andliquidcrystallayerbetweenthem.

• Anchoringphenomenon:theLCmoleculesareanchoredontheglassplates.

LCdisplays

Off On

Liquid crystals in photonic crystalfibers

Thehighest leveloftunabilityofpropagation andpolarizationproperties byexternalfields

Thermal,externalac&dcfields,opticalfield sensitivity

Variety ofLC materials andLCstructures;influenceofmolecularordering

AdvantagesofbothmTIR andPBG phenomena

Varietyof PCFstructures(birefringence, SM,nonlinearity,etc.)

LiquidCrystalPhotonic CrystalFiber LiquidCrystalsPhotonic Fibers

3 main types of Photonic Crystal Fibers

Isotropic with solid core Hollow-core Birefringent with solid core

• 2-dimmensional photonic crystals with deffect along the fiber length insidethe core region,

• made of one type of glass material with periodic matrix of air micro-holesforming a structure of photonic crystal,

Photonic Crystal Fibers

D

d

Λ

•Pitch, Λ• Large hole diameter, D• Small hole diameter, d• Diameter of holey region• Filling factor, d/Λ

→ 4.4 µm→ 4.5 µm→ 2.2 µm→ 40 µm→ 0.5

Commercially available birefringent PCF from NKTPhotonics

Parameters

Photonic Crystal Fibers

m-TIR (modified Total Internal Reflection) – characteristic for PCFs with solid core,

PBG (Photonic Band Gap) – characteristic for „hollow-core” PCFs,

Propagation mechanisms in PCF

OrientationofLCmoleculesinPCF

planar axial radial

Isotropic molecules orientation in PLCFs:

transverseTransverse molecules

orientation in PLCFs observed under influence of external

electric field

Light propagation in PLCF

Peltier module

Halogen lightsource PCF with LC

Spectrometer

Polarizing microscope

Temperature control

PCF with 3 rings of holes filled with PW500 LC (2.5 cm)

Transmision spectra under influence of temperature

PBG TIR

Photonic Band Gap Index guiding

Tcr T

1023 PCF + 1550 LC, temperature-induced switching

Wolinski T.R., et al, Measurement Science and Technology 17, 985–991 (2006)

DP2000PCF+6CHBT;PVCilayerirradiatedwithUVlightpolarizedparallelly tothefiberaxis– resultssimilartoPCFwithoutphotoaligninglayer– planarmoleculesorientation

E-tuninginphotoalignedPLCFs

Otherapplications

• thermography– medicine– heating

• LCblinds• Electronicpaper(e-paper)

Termogrphy

Termography