Optical nonlinearity orientational

The first issue of the construction of noncentrosymmetric LB films with highly efficient optical nonlinearity is how one overcome the difficulty in realizing a high-degree orientational order of polar molecules, which possess high... 

For obtaining the information on fabrication of noncentrosymmetric LB films with highly efficient second-order optical nonlinearity, six azobenzene-linked amphiphiles were synthesized as a model compound, and their molecular hyperpolarizabilities (3, monolayer-formation at the air-water interface, and molecular orientation and second-order susceptibilities of the azobenzene-linked amphiphiles LB films were evaluated. The molecular structures of the azobenzene-linked amphiphiles are shown in Fig.2. 

Using the alternating deposition of the amphiphiles with a carboxyl substituent and arachidic add, noncentrosymmetric LB films (hetero Y-type) were prepared, and molecular orientation and second-order optical nonlinearity in the LB films were evaluated with the linear dichroism [4] and the second-harmonic generation (SHG) measurements, respectively. The SHG measurement procedure is mentioned in the section 1.3. 

Here, we demonstrate that oriented PAV films with well-developed -conjugated system can be fabricated through the regulation of orientation of precursor polymer chains by use of the Langmuir-Blodgett technique, and that large and anisotropic third-order optical nonlinearity was observed in the oriented PAV films. 

Third-order optical nonlinearity of the oriented MOPPV LB film was evaluated by the THG measurement [30]. The detailed procedure is described in the section 2.1. 

Second-order optical nonlinearities result from introduction of a cubic term in the potential function for the electron, and third-order optical nonlinearities result from introduction of a quartic term (Figure 18). Two important points relate to the symmetry of this perturbation. First, while negative and positive p both give rise to the same potential and therefore the same physical effects (the only difference being the orientation of the coordinate system), a negative y will lead to a different electron potential than will a positive y. Second, the quartic perturbation has mirror symmetry with respect to a distortion coordinate as a result, both centrosymmetric and noncentrosymmetric materials will exhibit third-order optical nonlinearities. If we reconsider equation 23 for the expansion of polarization of a molecule as a function of electric field and assume that the even-order terms are zero (i.e., that the molecule is centrosymmetric), we see that polarization at a given point in space is ... 

The synthesis of materials for device applications has very different requirements. Here, the most important questions are What does the device do and what factors will affect its performance The magnitude of the desired optical nonlinearity will always be one of many criteria that will ultimately dictate the material of choice. In many instances the magnitude of the nonlinearity will not be the most important parameter. Depending on the device applications, other considerations such as optical transparency, processability, one- and two-photon optical stability, thermal stability, orientational stability, and speed of nonlinear response will all be important. Our current understanding of NLO materials suggests that these variables are frequently interrelated and that there is often no ideal NLO material. The material of preference for a given application will typically be one that is the best compromise for a variety of variables. Tutorials by G. Stegeman and R. Zanoni, and by R. Lytel outline some of the NLO device applications and the related materials issues. 

In Equation 6, n (a>.) is the intensity independent refractive index at frequency u).,.0 Tlie sum in Equation 5 is over all the sites (n) the bracket, < >, represents an orientational averaging over angles 0 and . Unlike for the second-order effect, this orientational average for the third-order coefficient is nonzero even for an isotropic medium because it is a fourth rank tensor. Therefore, the first step to enhance third order optical nonlinearities in organic bulk systems is to use molecular structures with large Y. For this reason, a sound theoretical understanding of microscopic nonlinearities is of paramount importance. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

Acceptor species concentrations, equations, 400-401 Acentric materials biomimetic design, 454-455 synthesis approaches, 446 Ar-(2-Acetamido-4-nitrophenyl)pyrrolidene control of crystal polymorphism with assistance of auxiliary, 480-482 packing arrangements, 480,481-482/ Acetylenes, second- and third-order optical nonlinearities, 605-606 N-Acetyltyrosine, phase-matching loci for doubling, 355,356/, t Acid dimers, orientations, 454 Active polymer waveguides, applications, 111... 

Since the dipoles of chromophore molecules are randomly distributed in an inert organic matrix in amorphous PR materials, the material is centrosymmet-ric and no second-order optical nonlinearity can be observed. However, in the presence of a dc external field, the dipole molecules tend to be aligned along the direction of the field and the bulk properties become asymmetric. Under the assumption that the interaction between the molecular dipoles is negligible compared to the interaction between the dipoles and the external poling field (oriented gas model), the linear anisotropy induced by the external field along Z axis at weak poling field limit (pE/ksT <[Pg.276]

Because chromophores orientation is important for creating anisotropy and optical nonlinearities, intensive studies have been performed to understand induced molecular orientation and relaxation processes in polymers. To gain further insight into the physics of thin polymer films and the effects of molecular orientation in solid polymers, studies at high pressure could be beneficial. Pressure as a thermodynamic parameter is widely used to study... 

Second harmonic is a convenient in situ probe of the dynamics of photoisomers in a polymer environment if the photoisomers have a second-order optical nonlinearity. A corona discharge is used to apply a large electrostatic field to align the molecules. When a change in the orientation of the photoisomers is induced through photoisomerization, second harmonic generation is used to follow the changes in the state and orientation of the photoisomers. 

Polymers show interesting properties and can be used in optics and optoelectronics3.4.5. Specially oriented polymers can exhibit optical nonlinear properties and can be used for optical processing purposes. For quadratic NLO polymers the potential applications are mainly fi uency doubling leading to a blue source for optical recording and an electrooptical modulator for optical communications. 

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