Build-Up of the Polar Order

As already mentioned, the only techniques sensitive to the polar order are even order nonlinear optical techniques such as the already-described second harmonic generation and linear electro-optic effect (cf. Chapter 2). The hrst technique offers a high sensitivity to the fast electronic contributions to susceptibility and is widely used. As already mentioned, it also gives the opportunity to study the kinetics of the poling by in situ measurements [152]. 

The build-up of the polar order, imaged by the growth of the SFFG intensity, can be described by a triexponential function (Fig. 31)  

The sum of the poling saturation limit values for different processes (P = Pi + P2 + P3) represents the overall poling efficiency under the experimental conditions temperature, geometry, poling field, atmosphere. 

The superscriptsp and r in Eq. (28) refer to polymer thin film and reference harmonic intensities. The reference, with simultaneous measurements of SHG intensity from a poled, stabilized thin film, was used to correct for eventual laser intensity fluctuations. The fits of the measured temporal growth of SHG intensity with mono-, bi-, and triexponential functions are shown in Fig. 31. The best agreement is obtained when using a triexponential function (Eq. (28)). The time constants T2, and T3 depend on the polymer, temperature, and the chromophore itself. 

The different time constants describing the growth of the polar order during poling may be attributed to different rotational mechanisms linked to the chromophore and the polymer (Fig. 32), as was discussed by Noel and Kajzar [154]. The initial rapid growth of SHG signal is characterized by a time constant Tj of a few 

---