EFISH

In the first part of this paper several novel approaches to the alignment problem of second-order nonlinearity will be presented. The molecular information which guided and motivated these efforts, and others described in this symposium, derives from third-order nonlinear optical experimentation (EFISH and others). In the second... 

Two straightforward third-order nonlinear optical phenomena which are used to characterize ft and y are EFISH and THG (2)... 

Similarly one must be cautious in other third-order experiments. While EFISH of condensed phase materials looks like simple SHG if the sample is totally confined to the region of constant field between the electrodes, there is an important difference between this case and the more frequently performed and described study of frequency doubling crystals (31). In the latter case nonvanishing E2w occurs only in the crystal where x 0. However, in EFISH P2" also occurs in air due to its nonvanishing x Contributions to E2w upstream and downstream from the sample must... 

Other less definite yet important effects such as profile changes due to nonlinear refractive index alteration in spatially nonuniform high power beams must be carefully considered. As example, the use of nonidentical liquids and optical paths prior to and in, say, EFISH cells and the usual quartz calibration cells could cause potentially inaccurate x determinations. Obviously these types of considerations are important when precise experimentation to test fine models of molecular behavior are intended, but have not stood as obstacle to uncovering the important general trends in molecular nonlinearity enhancement. 

Correlations between molecular orientations, that is, between terms with different X, are disregarded (except in EFISH where the dipole correlation factor g is employed in the ji v term (40) - a practice which assumes the tensorial behavior of the vector components of to show the same intermolecular correlations as the permanent dipole) single terms for each molecular type are adopted. 

Electric discharge ozone generator, 77 798 Electric-field-induced second harmonic generation (EFISH), 20 515 Electric field intensity, exponents of dimensions, 8 585t Electric field perturbations, 74 616 Electric fields... 

Using these and other design principles, very large increases in the valnes of chro-mophores have been obtained since the original work on the simple red monoazo dyes of the disperse dye class. These new NLO chromophores are known as high pP chromophores , a selection of which are shown in Figure 5.31, the pP values quoted being measnred by electric field induced second harmonic at (EFISH) 1.9 It... 

EFISH electric field-induced second harmonic generation... 

Electric Field Induced Second-Harmonic Generation. An essential aspect of the development of materials for second-order nonlinear optics is the determination of the p tensor components. The technique that has been developed to accomplish this is called electric field induced second harmonic generation (EFISH) (13,14). 

In the EFISH method, the molecule of interest is dissolved in an appropriate solvent and put into a cell of the type shown in Figure 9. Electrodes above and below the cell provide the means for a D.C. electric field, which orients the solute (and solvent) molecules through its interaction with the molecular dipoles. Similar to the poled polymer approach, the average molecular orientation is increased along the field direction and an oriented gas model used to extract p. 

The EFISH experiment is formally a process since it involves the... 

If the unknown crystal can be shaped into a wedge similar to the EFISH cell geometry, a similar sinusoidally varying signal is observed from which tensor elements can be extracted provided a reference sample of known properties is available. Quartz crystals have been characterized extensively (17) and can be obtained in the form of a wedge. They are often used as a reference material for both EFISH measurements and other crystals. 

Figure 9. Diagram of the essential features of a cell used for EFISH experiments. (Reprinted with permission from Williams, D. J. Angew. Chem. Int. Ed Engl 1984, 23, 690. Copyright VCH Publishers.)...

Third-Order NLO Techniques. There is a wider range of third-order techniques commonly used to characterize materials, including electric field induced second harmonic generation (EFISH) (15, 16), third harmonic generation (THG) (17) and degenerate four wave mixing (DFWM) (18). EFISH and DFWM will be discussed briefly then... 

Many other organic and some organometallic materials have been examined by the powder SHG method and/or tested by EFISH techniques. They will not be discussed in detail here but are listed by structural classes for completeness. Several compendia of materials responses have been published. (38,54,55) Clearly, the largest single class of second order materials consists of donor-acceptor substituted aromatics. The class has been extended to stilbenes and diarylacetylenes and... 

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