Introduction On Organic Nonlinear Optical MaterialsYong SongDepartment of Physics, University of Cincinnati,Cincinnati,OH,45220AbstractOrganic materials are important materials for the fast processing of in-formation and for optical storage applications. This paper tries to have aquick look at their properties and some effects.1. IntroductionRecent developments in the field of nonlinear optics(NLO) has pushingorganic second-order nonlinear optical materials into practical applications.Nonlinear optical materials are the materials in which light waves can interactwith each other[1]. This paper presents an brief introduction on the organnonlinear optical materials.2.Molecular PolarizabilitiesThe following equation describe the normal nonlinear optical effects:Pi=P0,i+ ε0(χ(1)ijEj+ χ(2)ijkEjEk+ χ(3)ijklEjEkEl+P0is the spontaneous polarization and χ(n)is the nth order susceptibilitytensor. ε0Is the vacuum permittivity. E is the applied electric field.In organic materials the optical properties are determined by the molec-ular polarizabilities. The following equation expressing the molecular dipolemoment p as:pi= µg,i+ ε0(αijEj+ βijkEjEk+ γijklEjEkElwhere µg,iis the ground state dipole moment, αijis the linear polarizabil-ity, βijkis the second-order polarizability or first-order hyperpolarizability,γijklis the third-order polarizability or second-order hyperpolarizability. Theelectric field is the one at the location of the molecule.1Fig1 shows the different of optical effects between first and second ordernonlinear optical materials[1].Fig1 First and Second Order nonlinear optical effectsPure first order nonlinear optical effects are in the left and the puresecond order effects are in the right hand side. The middle column shows theeffects related to both types of nonlinearity:the photorefractive effect and thecascaded optical Kerr effect.The tensors χ(2)ijkand βijkdescribe the second-order nonlinear optical andelectro-optic effects. They are third rank tensors, in the electric dipoleapproximation, contain nonvanishing elements only for noncentrosymmet-ric molecular and crystalline structures, respectively. Third order nonlinearoptical effects are described by the tensors χ(3)ijkland γijkl. No symmetryrequirements are imposted on these effects to occur[2].3 Organic Nonlinear Optical Effects3.1 Bonding Properties of Atoms In The MoleculeWe assume that the molecules function independently of each other andonly their net orientations within the crystal lattice are important to thecontribution of model.The bonding with the carbon atom and other elements has two types:σ bond and π bond. σ bond is confined along the inter-nuclear axis of thecarbon -carbon bond. And π bonds are regions of delocalized electroniccharge distribution above and below the inter-atomic axis(Fig2) [3]Fig2 Electronic bonding in (a) H2(σ bond) and (b) C2(π bond)3.2 Why Organic Can Has NLO PropertiesThe polarizability of organic materials is generally the contribution fromthe lattice components(atoms,molecules) because of the weak intermolecular2bonding(Van der Waals, dipole-dipole interactions, hydrogen bonds). Non-linear optical effects of molecular crystals depend on the polarizability of theelectrons in the π bonding orbitals.The optical nonlinearity can be increased if you add conjugated bonds(increasingL–The length of the conjugated system) or substitute donors(can donate elec-trons into the π electron system) and acceptors(can accept electrons into theπ electron system)(Fig3). The addition of the functionality at the ends ofthe π system can enhance the asymmetric electronic distribution. [4]Fig 3 organic materials for nonlinear optical effects of second-order. D-Donor A-Acceptor . substituted molecules with π electron ring systems. (i)one ring systems (ii) two ring systemsStudies show that conjugated organic molecules with large delocalized πelectron systems have very large nonlinear optical effects. The attachment offunctional groups with electron-accepting and donating character at oppositeends of the conjugation bridge leads to an essentially one-dimensional chargetransfer, enhancing especially the second order non-linearity[1].A further simplified model (the Equivalent Internal Field, EIF model) of afree electron gas corresponding to the delocalized π electron density of a con-jugated approximation β ∼ L3,which shows the strong nonlinear dependenceof the hyperpolarizability on the lenth of the conjugated π system.3.3 Organic Nonlinear Optical and Electro-Optic EffectsThe Fig 4 is the schematic representation of important nonlinear opticaland electro-optic effects[1]. Let us discuss the first five effects briefly.Fig4 Important nonlinear optical and electro-optic effects3.3.1 Sum Frequency GenerationSum frequency generation is the mixing of two incident light waves offrequencies ω1and ω2creating a wave of ω3=ω1+ω2.3.3.2 Second-Hrmonic Generation(SHG)Second-harmonic generation(SHG) or optical frequency doubling is justa special case of sum frequency generation. Only one light wave of frequencyω is incident, generating a wave of twice the frequency.3Sum frequency and second-harmonic generation are standard techniquesto create a new coherent output from existing laser systems and especiallyto access the short wavelength range towards the ultraviolet region.3.3.3 Difference Frequency GenerationDifference frequency generation(DFG) is treated as the interaction of twoinput beams of frequencies ω3and ω1resulting in an optical field with thefrequency ω2=ω3-ω1.Because of the Conservation of Energy and Momentum, all the nonlinearprocesses discussed have in common that they conserve energy, which wasalready implicitly assumed: ω3=ω1+ω23.3.4 Optic Parametric Oscillation/GenerationOptic parametric generation is a special case of difference frequency gen-eration, where only the pump beam is incident on the nonlinear materialgenerating two beams at the frequencies ω1and ω2.In order to enhance the efficiency of either process, the nonlinear mediumcan be placed inside a cavity with highly reflecting mirrors for the frequenciesω1and/or ω2.4. Conclusion:The organic materials exhibit extremely large nonlinear optical and electro-optic effects. The electronic nonlinearities are essentially based on the molec-ular units. Due to the important advantages of the organic materials, theywill be widely used in the field of organic chemistry, materials science, physicsand