Second-order susceptibility magnitude

To fit the experimental results, it is necessary to fix the overall phase. This can be done, for example, by defining h as a real quantity (/ / = 0). The values found for the coefficients /, g, and h can then subsequently be used to calculate the values of the components of the second-order susceptibility, X(2). This is done in detail for a Langmuir-Blodgett film of a poly(isocyanide) in the following section. Note that both phase and magnitude of all tensor components are relative values. The absolute phase cannot be determined... 

Materials that have a nonzero second-order susceptibility will produce light at twice the incident frequency. The magnitude of this effect is small, and has been a practical consideration only since the advent of lasers. If the symmetry of a crystal or other medium is such that it has a center of inversion, no SHG effect will be observed. However, surfaces by their very nature break this inversion symmetry. Hence, an SHG signal may arise at the electrode-solution interface even though both bulk phases may be considered centrosymmetric [66], The magnitude of the SHG signal is sensitive to surface conditions (e.g., electrode potential, ionic or molecular adsorption, etc.). Surface spectroscopy is also feasible since the SHG signal will be enhanced if either the incident frequency (to) or SHG (2co) corresponds to an electronic absorption of a surface species [66]. 

We multiply by the bond density, iV/2, to get the polarization density, which corresponds to a second-order susceptibility of magnitude... 

In the case of bulk materials or films the second-order susceptibility values, can be obtained by means of the investigation of the SHG [1-3]. The Kurtz-Perry technique [33] is often used to compare the intensity of the SHG of a powder sample with that of a reference sample of known such as quartz or urea. Although this technique is limited (the magnitude of the response is also dependent on particle size), it is a simple and rapid method for screening a large number of powder materials. 

The zero-field oe-o type SHG, in our opinion, may be accounted for by a multipolar (the quadrupolar) mechanism known for the solid crystals. The effective second-order susceptibilities calculated from the experimental data on the oe-o synchronism are shown in the Table. Their order of magnitude is typical of the quadrupolar SHG mechanism. So, the phenomenon of the zero-field second harmonic generation in nematic and... 

Figure Bl.5.3 Magnitude of the second-order nonlinear susceptibility x versus frequency co, obtained from the anliannonic oscillator model, in the vicinity of the single- and two-photon resonances at frequencies cOq and coq 2> respectively.

The viability of the various soil consortia was dramatically affected by multiple freeze-thaw cycles, with CFU/ ml typically decreasing by five orders of magnitude after 48 cycles (Figure 2). This was then repeated for a second 48 cycles after reculturing the survivors (not shown). The overall decrease in viability did not simply result from a reduction in numbers since the appearance of the colonies indicated that there was a shift in the complexity of the populations. Evidence for this differential susceptibility to freeze-thaw treatments is shown by the lack of E. coli colonies after 24 cycles and the absence of P. chloroaphis after 48 cycles (Figure 2). After isolating DNA from 16 colonies that survived this rigorous freeze-thaw selection in one series of experiments, five distinct sequences were identified. The bacteria from which these five sequences were isolated were then used... 

Second, the g-value contribution [Eq. (II.la)] and the susceptibility contribution [Eq. (II.lb)] usually have the same order of magnitude but opposite sign. Thus, part of the high accuracy with which the g- and -values may be... 

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