Second-order optical nonlinearity structure

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. 

Second order optical nonlinearity can be induced in polymeric systems containing dipolar (donor-acceptor) chromophores. The chromophore can be a molecular species attached to the host chain or it can be incorporated in the polymeric structure itself. In general, a good chromophore has an electron donating group connected to an electron... 

Multifunctional monomers have been synthesized whose polymers are exhibiting both photoconductivity and nonhnear optical (NLO) properties. Second-order optical nonlinearity requires the total system does not possess a center of symmetry. Therefore, the monomers have a rather complicated structure. The functionalities can be introduced by standard reactions in organic chemistry. 

For the production of second harmonic generation, which is the most important function of second-order optical nonlinearity, it is theoretically necessary to have a non-centro symmetric structure in the range of the incident radiation. Table 1 shows the magnitude of second-order optical nonlinearity in some single crystals. 

Ruiz, B., Z. Yang, V. Gramlich, M. Jazbinsek, and P. Gunter. 2006. Synthesis and crystal structure of a new stilbazolium salt with large second-order optical nonlinearity. J. Mater. Chem. 16 2839-2842. 

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

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. 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

These structure-function relationships provide extremely useful guidance for the future rational design of molecules and polymers with even higher optical nonlinearities. For non-centrosymmetric molecules such as 95, very high first hyperpolarizabilities /3 that determine the second-order nonfinear optical properties were also measured [140]. 

A specific set of experiments which must be mentioned, being directly associated with the main topic of this paper, is the work of Bergman, et. al. (22) dealing with the second-order nonlinear optical properties of polyvinylidene fluoride (PVF2). Nonvanishing the second-order nonlinear electric dipole susceptibility, is expected in PVF2 since it exhibits other properties requiring noncentrosymmetric microscopic structure. These properties appear... 

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