Switchable Bragg Gratings

107 Pages·2005·7.78 MB·English

Universit(cid:224)degliStudidiNapoliFedericoII
-
DottoratodiRicercainFisicaFondamentaleedApplicata
18 ciclo
(cid:14)
Dott. Francesco Vita
Switchable Bragg Gratings
Based on Soft Materials
IlCoordinatore
Prof. A.Tagliacozzo
________________________
Novembre2005

iii
Table of contents
Introduction ...................................................................... 3
1 Liquid crystals................................................................. 7
1.1 Betweenliquidsandcrystals ...................................................7
1.2 Physicsofnematics ...........................................................7
1.2.1 Theorderparameter ....................................................7
1.2.2 Continuumtheory ......................................................8
1.2.3 External(cid:2)eldeffect................................................... 10
1.2.4 FreØderickszeffect ................................................... 11
1.2.5 Reorientationdynamics............................................... 13
1.3 Opticsofliquidcrystals...................................................... 15
1.4 Polymerdispersedliquidcrystals............................................. 16
1.4.1 Introduction.......................................................... 16
1.4.2 Preparationandmorphology .......................................... 16
1.4.3 Opticalproperties .................................................... 17
1.4.4 Switchingproperties.................................................. 18
2 Holographic gratings ........................................................ 19
2.1 Introductionandclassi(cid:2)cation ............................................... 19
2.2 Rigorousformulationofthediffractionproblem .............................. 20
2.2.1 TheFloquetheoremandtheBraggcondition........................... 20
2.2.2 Thesystemofcoupleddifferentialequations ........................... 23
2.3 Diffractionfromthingratings ................................................ 24
iv
2.4 Diffractionfromthickgratings ............................................... 25
2.4.1 Kogelniktheory ...................................................... 25
2.4.2 Diffractionef(cid:2)ciencyfortransmissiongratings......................... 26
2.4.3 Diffractionef(cid:2)ciencyforre(cid:3)ectiongratings............................ 28
2.5 Diffractionfromthickanisotropicgratings.................................... 28
2.5.1 MontemezzaniandZgonik’stheory.................................... 28
2.5.2 Transmissiongratings................................................. 30
2.5.3 Re(cid:3)ectiongratings.................................................... 32
2.6 Criteriaforthin-thickgratingclassi(cid:2)cation ................................... 32
3 Holographic gratings in LC-polymer composites ........................ 33
3.1 Introduction................................................................. 33
3.2 Productionprocedure........................................................ 35
3.2.1 Set-up ............................................................... 35
3.2.2 Samplepreparation ................................................... 36
3.2.3 Exposure............................................................. 37
3.3 Materials ................................................................... 38
3.3.1 Monomers ........................................................... 38
3.3.2 Liquidcrystals ....................................................... 41
3.3.3 Photoinitiators........................................................ 42
3.3.4 Reactivediluent ...................................................... 43
3.3.5 Surfactants ........................................................... 44
3.4 Formationdynamics......................................................... 44
3.5 Non-dropletgratings: PolicrypsandPoliphem ................................ 47
3.5.1 AnewclassofLC-polymergratings................................... 47
v
3.5.2 Policryps............................................................. 48
3.5.3 Poliphem............................................................. 49
4 H-PDLC gratings: characterization and modeling ...................... 53
4.1 Introduction................................................................. 53
4.2 Angularmeasurements ...................................................... 54
4.2.1 Experimental......................................................... 54
4.2.2 Data(cid:2)tting........................................................... 56
4.3 Morphologicalmodel........................................................ 58
4.4 Commentsontheeffectsoftheopticaldispersion ............................. 62
4.5 Measurementsunderelectric(cid:2)eld ............................................ 64
5 Policryps and Poliphem characterization ................................. 67
5.1 Introduction................................................................. 67
5.2 Thermalproperties .......................................................... 68
5.3 Diffractionef(cid:2)ciencyandswitchingproperties................................ 69
5.4 Bandwidthandlosses........................................................ 71
5.5 Switchingtimes ............................................................. 73
5.5.1 Policrypsdynamicmeasurements ..................................... 73
5.5.2 AmodelforPolicrypsdynamics: switchingoff ........................ 76
5.5.3 AmodelforPolicrypsdynamics: switchingon......................... 78
6 Optical characterization of soft materials................................. 83
6.1 Introduction................................................................. 83
6.2 Experimentaltechniques..................................................... 84
6.2.1 Spectroscopicellipsometry............................................ 84
6.2.2 Half-leakyguidedmodetechnique .................................... 86
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6.3 Opticaldispersioninliquidcrystal ........................................... 87
6.3.1 HLGMmeasurementsonliquidcrystals ............................... 87
6.3.2 Ellipsometricmeasurementsonliquidcrystals ......................... 89
6.3.3 E7and5CBdispersionspectra ........................................ 91
Bibliography..................................................................... 95
vii
Acknowledgments
The present work has been carried out at the Integrated Optics Laboratory of the Uni-
versity of Naples, under the precious guide of Prof. Abbate. From him I received advice,
knowledge,stimulation,andopportunities.
I am also very grateful to Dr. Marino and Dr. Tkachenko: we worked together every
dayandtheyde(cid:2)nitelyhadapartinallmyresearches.
The present work would have been impossible without a fruitful collaboration with
manyscienti(cid:2)cgroups,whichprovidedmewithmaterials,ideasandexperience. Mysincere
thanksgoesto:
Prof. SimoniandhisgroupatthePolythecnicUniversityofMarche(Ancona,Italy);
(cid:15)
Prof. UmetonandhisgroupsattheUniversityofCalabria(Cosenza,Italy);
(cid:15)
Prof. Stumpe and Dr. Slussarenko at the Fraunhofer Institute for Applied Polymer
(cid:15)
Research(Potsdam,Germany);
Prof. CrawfordandhisgroupattheBrownUniversity(ProvidenceRI,USA).
(cid:15)
Andaftertheprofessionalacknowledgments,thepersonalones. Againaspecialthanks
to Giancarlo, Antigone, and Vladimir: more than colleagues, they were close friends; it was
a pleasure to share with them most days of the last three years. I hope to keep having coffee
breakstogether!
I must thanks two special persons, Ferdinando and Giovanni: they do not even know
whatliquidcrystalsare(althoughIhavetriedtoexplainitsomanytimes),buttheirfriendship
wasreallyimportantformeintheseyears;sothereisabitoftheminthiswork.
Finally and above all, I will be eternally grateful to my parents and my brother Luca:
things change with time, but their presence is still fundamental in my life. Without them, I
wouldneverbearrivedatthismoment.

Tomyfamily
Therealvoyageofdiscovery
consistsnotinseekingnewlandscapes
butinhavingneweyes
MarcelProust