Second guest-host systems

In Fig. 3, the values for the electron-number-related static hyperpolarizability fiJN312 obtained for these ionic chromophores (open symbols) have been compared with the same values for the best dipolar, neutral chromophores reported so far (diamonds).31 32 These chromophores, with a reduced number of electrons N equal to 20, have dynamic first hyperpolarizabilities approaching 3000 x 10 30 esu at a fundamental wavelength of 1.064 pm, in combination with a charge transfer (CT) absorption band around 650 nm. It is clear that at this point, the neutral NLOphores surpass the available ionic stilbazolium chromophores for second-order NLO applications, however, only at the molecular level. The chromophore number density that can be achieved in ionic crystals is larger than the optimal chromophore density in guest-host systems. 

Extensive studies have been made on poled NLO guest-host systems . The limited solubility of the NLO species in a host matrix and the thermal relaxation of the induced nonlinear optical activity of the poled polymeric films are major disadvantages. Side chain polymers have attracted attention since a large number of NLO molecules may be covalently attached to the polymer chain and the problem of phase segregation of the NLO component is alleviated. In this article, we present the second and third order NLO properties of a new class of guest-host system which possess excellent doping features and photoreactive characteristics. 

Second Order NLO Active Polymers 2.4.1 Guest-Host Systems... 

One of the disadvantages of guest-host systems is that usually the LB multilayers deposit in the Y-type centrosymmetric mode, rendering the structure nlo-inactive. By using a second material, which may be either optically active or passive, to form ABAB-type multilayers, non-centro-symmetric films can be produced. Good Y-type deposition has been achieved with the polyethers (64) and (65), to produce four AB layers exhibiting the expected quadratic dependence of second-order nlo response with film thickness [85]. These materials use the hemicyanine-based unit as the pendant side-group, with opposite dipole orientation in the two structures to achieve an additive effect in Y-type LB multilayers. 

The decay of NLO response for a series of guest-host systems as a function of time at different temperatures was studied using the SHG technique [58,135]. These polymer samples were prepared based on a number of polymers and NLO chromophores. The relaxation of second-order optical nonlinearity can be reasonably well described by the Kohlrausch-Williams-Watts (KWW)... 

Equation (11) of Nordio et al [20] can be generalised to develop a theory with a non-traceless pseudotensor of second rank Wy, the chirality interaction tensor. Because the contribution of the interaction energy from the chiral/chiral potentials of the molecules is small in comparison to the chiral/achiral contribution, the helical twisting power is then given for a guest/host system as well as for a single component chiral nematic phase by ... 

Three major kinds of polymers have been studied for second-order NLO effects guest-host systems, side-chain polymers, and main-chain polymers. 

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