Optical-Field-Induced Orientational Order

Three synthetic approaches to donor-acceptor-substituted conjugated molecules with enhanced orientability in electric fields, potentially applicable to the preparation of electro-optic polymers via electric field poling, are summarized. The three approaches are parallel attachment of chromophores to a common framework, embedding the chromophore in a zwitterion, and head-to-tail oligomerization of chromophores. The oligomerization method as well as the use of dyes as curing agents are briefly discussed in relation to the stability of electric field-induced polar order in polymer matrices. 

There are mote dye molecules oriented along the optical field inducing order... 

The contribution of photo-induced nonpolar orientation through even order parameters (see Equation 8A.9 in the appendix, page 286) to the effects observed in the EFISH decrease is negligible. The same decrease is observed for a TE- or TM-polarized probe regardless of the pump polarization (not shown). The photochemically induced molecular shape change of the NLO dye blows out the strong optical field driven anharmonic movement of the... 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2 > = > - - third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect co = co + 0), or the quadratic electrooptic (Kerr) effect ( > = > - - 0 -I- 0). These electrooptic effects are discussed extensively in the article Electrooptical Applications (qv). EFISH is... 

To summarize this section, we note that the orientational order in nematic elastomers induces a chain anisotropy, which in turn determines the macroscopic shape of the sample. The manipulation of these shape spheroids by temperature and by electrical, mechanical, and optical fields is at the origin of many of the effects... 

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