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
LiquidCrystals
Liquid crystals
termotropic liotropic
Crystal LiquidCrystal Liquid
temperature
crystalLiquid crystal
liquid
tem
pera
ture
Concentration of solvent
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)
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.
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