Poling technique

Fig. S3. High quality, biaxially stretched PVDF ferroelectric film can be electrically poled to a remanent polarization of over 9 fxCcm with the Bauer cyclic poling technique. The polarization versus electric field of a typical sample is shown.

Kawai (1) and (2) (1969) found that polar polymer films such as PVDF, poly (vinyl fluoride), PVC, nylon 11, and polycarbonate exhibit a strong piezoelectricity when they are drawn and then polarized under a high cLc. field Ep at a high temperature Tp and cooled keeping the d.c. field. The piezoelectricity thus obtained depends on Ep, Tp, and poling period. An improved poling technique was reported by Edelman, Grisham, Roth, and Cohen (1970). 

Tg = 125-140°C, and was stable in a N2 atmosphere to 220°C. The 13C NMR spectrum revealed the presence of 7t-interaction between those chromophoric groups that are in isotactic relationship to each other ( 30-35% of total). The orientation of the chromophores in a PPNA sample (Tg = 125°C) was achieved in a thin film by the corona poling technique at temperatures above Tg. The subsequent freezing process resulted in a polymeric film that exhibited an initial high second-order nonlinear coefficient, d33=31 pm/V, as measured by Maker-fringe technique with 1.06 im fundamental. 

Since the first nonlinear susceptibility is a third-rank tensor, it is only non-zero in non-centrosymmetric media. To break the centrosymmetry of the macroscopic media, poling techniques using optical and electric fields have been developed, or use was made of the inherent polar ordering in Langmuir-Blodgett films and crystals with non-centrosymmetric point groups. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

For the DRl-MMA 35/65, N = 1.4 10 molecules/cm and P - 0.8 10 It implies a mean orientation ratio equal to 0.3, which is comparable to what can be achieved using the dc electric field poling technique such as corona poling. 

The chemical structure of the polyimide polymers (named PI-1 and PI-2) studied by Sekkat et al. is shown in Figure 12.12. They prepared the polymer samples by spin-casting onto glass substrates. PTl was cast from a cyclohexanone solution and PI-2 from 1,1,2,2- tetrachloroethane. The Tg values of PI-1 and PI-2 were determined to be 350°C and 252 C, respectively, by scanning calorimetry method. The thicknesses of the PI-1 and PI-2 films were, respectively, approximately 0.72 im and 0.14 im, and their respective optical densities were approximately 0.79 and 0.3 at 543.5 nm. Details of the preparation and characterization of the samples can be found in References 3 and 20. In their EFISH experiment, a typical corona poling technique was used to pole the samples, with a dc electric field about 2-3 MV/cm across a 1-2 lm thick polymer film. They used the SHG output from the EFISH experiment to in situ monitor the photochemical change in the third-order susceptibility of the PI-1 and PI-2 polymers. 

While the first technique takes advantage of the interaction between dipole moments and the applied static field, the two others take account of the cooperative effect of static and optical field (photoassisted poling) and purely optical field (all optical poling) and photoisomerization process. Between the DC poling techniques we note ... 

Fig. 23. Variation of the poling current I with the needle voltage 1 for different grid Vg tensions in corona poling technique [151]...

Photothermal poling [ 140,141 ] is a simple modification of the electrode poling technique. The only difference is the use of a laser beam, with wavelength lying in the material absorption band, to heat the thin film. The main advantage of this technique consists of a very localized poling. It has been used for the fabrication of bidirectionally poled polymer films [141]. 

A detailed study of the SHG from both spin-coated and evaporated films of trinitro-(l) [73,74], trhodo-(4) [74] and trioctylsulfonyl-(7) [74] subphthalocyanines has been carried out. In order to induce molecular ordering in hie spin coated film, a corona poling technique was used. Films were prepared by spin-coating from a solution of the corresponding SubPc in poly(methylmethacrylate) (PMMA). For comparison purposes, evaporated films were also prepared by vacuum sublimation onto amorphous silica plates. 

Materials with non-linear optical (NLO) properties are of particular interest to the telecommunications industry and specific to the technology of photonics [150]. The impregnation of polymeric materials with organic, azobenzene dyes is a possible route to the preparation of these materials. Poling techniques are required to avoid the polar molecules aligning antiparallel to each other, which avoids centro-symmetric arrangements [151]. A thorough review of the preparation and assessment of polymeric materials with azo-dyes for non-hnear optics has recently been prepared by Yesodha et al. [152]. 

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