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IntroductiontoNonlinearOptical(NLO)PolymericalMaterialsWuChengxunCollegeofMaterialSci.&Tech.DongHuaUniv.21世纪被称为高度信息时代。电子信息传输方式固有的缺陷，包括速度、容量、空间相容性、信息检测精度等方面，都制约了电子信息系统在未来的信息时代中的广泛应用。与电子传递信息相比，光子传递信息具有许多优点，如良好的并行性（即光信号可以交叉传送）、高频率（信息载量大）、宽带宽、高速度、高密度（光束可聚焦）以及抗电磁波干扰等。非线性光学材料正是未来迈向信息时代的关键性材料。一概念与理论•1.1.1非线性光学效应：非线性光学是研究在强光作用下物质的响应与场强之间的非线性关系的科学。这种与场强相关的光学效应称为非线性光学效应。源于分子和材料在光场中的非线性极化。•1.2.2非线性产生的物理机制：电子云的贡献—畸变分子的振动和转动及晶格的振动分子的重新取向和分布电致伸缩温度效应1.1非线性光学的一些基本概念•1960Laser•CriterianecessaryforNLOmaterials:•Largenonlinearsusceptibilities(χ(2)orχ(3));•Easeofprocessing;•Lowabsorptionlosses;•Highopticaldamagethreshold,etc.非线性光学（nonlinearoptics,NLO）：是研究在强光作用下物质响应与场强之间的非线性关系的科学。在强激光的作用下，材料的电极化强度P不再与入射光的场强E成简单的线性关系，而成幂级数关系：P=χ(1)E+χ(2)E·E+χ(3)E·E·E+…其中χ(2)为二阶非线性极化率，对应着二阶非线性光学效应。NLO聚合物材料NLO聚合物材料具有明显高于无机晶体的非线性系数、皮秒至飞秒的响应速度、很高的光学损伤阈值，尤其是它的分子可修饰性、高分子可加工性以及所制成器件的可集成性等一系列其它材料无法比拟的优点，更在于其所展现出来的在高速集成光学器件方面的巨大潜力。NLO聚合物材料通常是通过将具有大非线性光学效应的NLO生色团引入到聚合物中而得到的。•Nonlinearoptics:•P=P(0)+χ(1)E+1/2*χ(2)EE+1/6*χ(3)EEE+...•P---theinducedpolarization•P(0)---thepermanentpolarization•E---electricfieldstrength•χ(1)---thelinearsusceptibility•χ(2)andχ(3)---thesecond-andthird-ordersusceptibility(orthefirstandsecondordernonlinearsusceptibility)•Molecularpolarizabilities:•μi=μg,i+αijEi+βijkEjEk+γijklEjEkEl+...•μi---induceddipole•μg,i___thegroundstatedipolemoment•αij---thelinearpolarizabilitytensor•βijk---thesecond-orderpolarizabilityorfirst-orderhyper-polarizability•γijkl---thethird-orderpolarizabilityorsecondhyper-polarizability.式中：α、β和γ分别表示分子的线性、二阶极化率和三阶极化率，β和γ也常称作分子一阶和二阶超极化率；χ（1）、χ（2）和χ（2）分别为介质宏观的线性、二阶和三阶非线性极化率。电光聚合物之二阶非线性光学特性就源于上述泰勒展开式中第二项的贡献。由于Β和χ（2）均为三阶张量，由张量定义可知，材料要显示宏观二阶光学非线性，其分子和其聚合物都应是非中心对称的。三阶光学非线性则不需要非中心对称。分子诱导电极化：材料宏观电极化：EEEEEEpEEEEEEPP)3()2()1(0•FirstorderNLOeffects:•SecondHarmonic•OpticalparametricOscillation•Linearelectric-opticeffect(n=n0+n2*I)•Sumanddifferencefreq.Generation•Opticalrectification.•SecondorderNLOeffects:•Frequencytripling•Two-photonabsorption•Coherentspectroscopy•Quadraticelectro-optic•Fourwavemixture•OpticalKerreffect(n2=3χ(3)/4cn02)•Self-focusing•Self-phasemodulation•Thephoto-refractiveeffect•CascadingoffirstorderNLOeffectsforapplicationsinsecondorderNLO.1.2各种非线性光学效应及其应用效应次数效应应用一次χ（1）折射率光纤，光波导二次χ（2二次谐波发生（ω+ω2ω）倍频器光整流（ω–ω0）光混频（ω1+ω2ω3）紫外激光器参量放大（ωω1+ω2）红外激光器Pockets效应（ω+0ω）电光调制器三次χ（2）三次谐波产生（ω+ω+ω3ω）三倍倍频器直流二次谐波产生（ω+ω+02ω）分子非线性电极化率测定Kerr效应（ω+0+0ω）超高速光开关光学双稳态（ω+ω–ωω）光学存储器、光学运算元件光混频（ω1+ω2+ω3ω4）拉曼分光1.3非线性光学材料的分类•以LiNbO3和KDP为代表的具有良好透明性和硬度的氧化物和铁电单晶类；•外延生长的约束结构形式Ⅲ-Ⅳ族半导体材料；•有机和聚合物材料。1.3.1按材料性质分：1.3.2按材料用途分：极化聚合物光折变聚合物有机聚合物光限幅材料。•Materialsforχ(2):•Noncentro-symmetryisrequired,otherwiseχ(2)=0.•IthasproveddifficulttofindmaterialswithlargeNLOcombinedwithlow-enoughlinearandnonlinearlosses.Systematicstudieshaveshownthatconjugatedorganicmoleculeswithlargedelocalizedelectronsystemshowverylargenonlinearopticaleffects.Theattachmentoffunctionalgroupswithelectro-acceptinganddonatingcharacteratoppositeendsoftheconjugationbridgeleadstoanessentiallyone-dimensionalchargetransfer,whichwillgiveaverydominatingvalueβzzz(z-directionreferstothechargetransferdirection),andallothertensorelementsinβcanbeneglected.•BasicmolecularunitsandstructuresessentialforNLOorganicmaterials:•Theelectrondensityofπbondsismuchmoremobilethanthatoftheσbonds.Theelectrondistributioncanalsobeskewedbysubstituents,resultinginthehyper-polarizabilityinresponsetoanexternallyappliedelectricfield.•Two-levelapprox.Model:•Highestoccupiedmolecularorbit(HOMO)andlowestoccupiedmolecularorbit(LOMO).•TheresultingβCT(CT---Chargetransfer)isafunctionoftheenergygapbetweenthetwostates,theoscilatorstrengthoftheCTtransition,thedipolemomentassociatedwiththattransitionandthefundamentallaserphotoenergy.•TheequivalentinternalfieldmodelofafreeelectrongascorrespondingtothedelocalizedelectrondensityofaconjugatedsystemoflengthL:β~L3•AnoptimizedNLOchromophorecanthenbeexpectedtohaveanextendedconjugatedsystem(largeL),alow-energytransition(longwavelengthabsorption)withahighextinctioncoefficient,andalargedipolemomentbetweentheexcitedandgroundstateelectronicconfigurations(chargeasymmetry)introducedbydonorandacceptorgroups.•Thechargetransfermoleculeshaveaveryasymmetricresponsetoanappliedelectronicfield.However,thepresenceofconjugationanddonoranacceptorgroupsgenerallyintroducesaundesirableeffect,theso-calledtransparencyefficiencytrade-off,alsoleadstoashiftoftheabsorptionedgetowardslongerwavelengths(red).•Therearefewrationalstrategiesforoptimizingγijkl.Well-definedstructure-propertiesrelationsforchromophoresarestillabsent,althoughbondlengthalterationhasrecentlybeenidentifiedasonestructuralparameterforγijkl.•Largeχ(2)requirenoncentrosymmetricpackingoftheorganicmolecules.•Technologicalapproaches:•PolararrangementofthemoleculesinsinglecrystalsorLangmuir-Blodgettfilms.•Chromophoresasguestsandpolymersashostsorsol-gels•Polingprocess•Filmsformedbymolecularbeamepitaxy(MBE)•Bridge:benzeneanalogs•Donor:•Amino,dialkylamine,diphenylamino,methoxy,ketonedithioacetal,julolidinyl,etc.•Acceptor:•Nitro,