Nonlinear susceptibility, effect temperature

The fact that isomerization and relaxation processes are sensitive to the temperature of a specimen allows a combined optical-temperature control of azo-polymer third-order susceptibility. The temperature-tuned VPC is just one example of this. Combination of optical excitation and temperature variations may result in more effective governing linear and nonlinear optical properties of azo-dye polymer materials. 

In order to provide a means for the precise recalculation of nitrogen chemical shifts reported since 1972, it is necessary to have accurate values of the differences in the screening constants between neat CH3N02 and the large number of reference compounds which have so far been used. Table VII shows the results of precise, 4N measurements (61) which have been carried out in concentric spherical sample and reference containers in order to eliminate bulk susceptibility effects on the shifts. Since the technique adopted (61, 63) involves the accumulation of a large number of individually calibrated spectra with the subsequent use of a full-lineshape analysis by the differential saturation method, (63) the resulting random errors comprise those from minor temperature variations, phase drifts, frequency instability, sweep nonlinearity, etc. so that systematic errors should be insignificant as compared with random errors. 

In addition to photoconductivity, polysilanes have been found to exhibit marked nonlinear optical properties,95-97 suggesting that they may eventually be useful in laser and other optical technology. The third-order non-linear susceptibility, X3, is a measure of the strength of this effect. The non-linear properties of polysilanes, like the absorption spectra, seem to be dependent on chain conformation and are enhanced for polymers having an extended, near anti conformation (Table 5.5). The value of 11 x 10 12 esu observed for (n-Hex2Si) below its transition temperature is the largest ever observed for a polymer which is transparent in the visible region. 

The device performance of any application in the real world has to be guaranteed for a certain time without or only minor degradation. Some 10,000 hours of operation at temperatures up to 80 °C under illumination are required. Research about the lifetime of organic nonlinear optical material has only started in the past years as the chances for applications based on the second-order susceptibilities x have improved. Nevertheless, only little is known about the longtime effects in nonlinear materials under real world conditions. 

Fig. 5.14. The s — p and p — p components of the effective nonlinear optical susceptibility of 8CB evaporated onto a quartz substrate as a function of deposition time. The data are a superposition of measurements obtained for three different temperatures of the evaporation source. The scale is given relative with respect to the time necessary to reach the first nearly flat region of the predominant s — p SHG signal. Vertical lines denote borders between various growth stages.

Doping with active species (e.g., Au or Ag NPs, and CdSe or PbS QDs) can result in new NLO materials, such as those with significant third-order NLO susceptibility. The Z-scan method is used to characterize NLO effect in nanocomposite sol-gel films. Z-scan measures the nonlinear refiactive index and nonlinear absorption coefficient of a material [22]. The measurement setup is shown in Figure 22.3. A standard open-aperture Z-scan apparatus is used to measure the nonlinear extinction coefficients of materials, using a laser with a pulse duration of fs, ps, or at least ns as an excitation source, to avoid thermal effects. All the Z-scan measurements described in this chapter were carried out at room temperature. Sol-gel films doped with noble metal NPs (e.g., Au, Ag, and Cu) or QDs (e.g., CdS and PbS) have been investigated by Z-scan measurements [23,24]. 

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