Poled second-order susceptibilities

The susceptibility tensors measure the macroscopic compliance of the electrons. Since the second order polarization is a second rank tensor, SHG is zero in a centrosymmetric or randomly oriented system. To make the material capable of SHG, the NLO dopants must be oriented noncentrosymmetricaly in the polymer matrix (2-3). When modeling the poled, doped films using a free gas approximation, the poled second order susceptibilities are given by (2.19)... 

In the limit of the oriented gas model with a one-dimensional dipolar molecule and a two state model for the polarizability (30). the second order susceptibility X33(2) of a polymer film poled with field E is given by Equation 4 where N/V is the number density of dye molecules, the fs are the appropriate local field factors, i is the dipole moment, p is the molecular second order hyperpolarizability, and L3 is the third-order Langevin function describing the electric field induced polar order at poling temperature Tp - Tg. 

The electro-optic coefficient in a poled polymer film can be related to the second-order susceptibility as shown in Equation 5. 

Detailed information about the components of the second-order susceptibility y2)(-2w to, w) can be obtained from second harmonic measurements on well-defined samples such as single crystals or oriented thin films, the latter obtained by procedures such as the asymmetric Langmuir-Blodgett deposition technique or electric-field poling of NLO chromophore-doped polymers.31 In the case of single-crystal samples, the second harmonic is... 

The all optical poling experiment consists of two periods the writing period (so-called seeding) and the readout period of the induced second-order susceptibility x - During the seeding period, the mutually coherent fundamental (O-beam and second harmonic 2o)-beam simultaneously irradiate the sample, and due to similar one- and two photon absorptions, the x grating is printed... 

The macroscopic second-order susceptibility x of these materials is determined by measuring the SHG signal intensity. By optimizing the poling process. 

To understand and optimize the electro-optic properties of polymers by the use of molecular engineering, it is of primary importance to be able to relate their macroscopic properties to the individual molecular properties. Such a task is the subject of intensive research. However, simple descriptions based on the oriented gas model exist [ 20,21 ] and have proven to be in many cases a good approximation for the description of poled electro-optic polymers [22]. The oriented gas model provides a simple way to relate the macroscopic nonlinear optical properties such as the second-order susceptibility tensor elements expressed in the orthogonal laboratory frame X,Y,Z, and the microscopic hyperpolarizability tensor elements that are given in the orthogonal molecular frame x,y,z (see Fig. 9). 

The requirement of non-centrosymmetry is not restricted to the molecular level, but also applies to the macroscopic nonlinear susceptibility, which means that the NLO molecules have to be organized in a non-centrosymmetric alignment. The first measurements of the macroscopic second-order susceptibility, have been performed on crystals without centrosymmetry [5]. However, many organic molecules crystallize in a centrosymmetric way. Other condensed oriented phases such as Langmuir-Blodgett (LB) films and poled polymers therefore seem to be the most promising bulk systems for NLO applications. 

We have successfully demonstrated the concept of simultaneous polymerization and poling of photocrosslinkable systems to produce high quality films with large second-order nonlinear optical susceptibilities and low scattering losses. The ability to process these materials under mild conditions may help avoid potential problems encountered in... 

The results are to some extent inconclusive and suggest that a two-photon state may have to be included. Also reported here are some further major improvements in molecular second order nonlinearities of particular importance to poled-polymer electrooptic applications (EO). Thus, it is found that appropriate replacement of benzene moieties with that of thiazole in certain azo dyes results in a factor of three increase in i-p, the molecular dipole ( io) projected molecular second order nonlinear optical susceptibility, p. 

For dipolar chromophores that are the subject of this chapter, only one component of the molecular hyperpolarizability tensor, Pzzz, is important. Thus, the summation in Eq. (8) disappears. Electric field poling induces Cv cylindrical polar symmetry. Assuming Kleinman [12] symmetry, only two independent components of the macroscopic second-order nonlinear optical susceptibility tensor... 

Second order, polyazomethine possesses both donor and acceptor groups at the molecular level, which when aligned gives a poled polymer. Tatsuura et al. fabricated an as-deposited poled polymer film by applying anin-situ electric field during CVD and reported on the second order nonlinear optical properties of the obtained film. The polymer film refractive index was reported to be about 1.68 at 632.8 nm. Second order NLO susceptability, p, was found to be... 

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