Static Field Poling

One of the important tasks in getting efficient materials based on functionalized polymers for second-order NLO applications is the creation of macroscopic non-centrosymmetry. This is done by orienting dipole moments in a privileged direction, which is defined by an external electric field. Several techniques have been developed, which use the interaction strength of dipole moment with an external electric field to orient them. These are  

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  

In the contact poling a large poling field is created through electrodes with the polymer thin film placed in between (Fig. 21). The film is heated up to the glass 

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]. 

In all cases the use of an electrode is required, with all possible negative aspects such as charge injection and light absorption. As a consequence, it implies the necessity of using buffer layers, in such applications such as frequency conversion in periodically poled systems [149], in which otherwise it is unnecessary. Moreover, the poling fields are limited due to the microcircuits connected with the point effect. This leads also to unwanted and prohibitory increase of the optical propagation losses. 

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Fig. 24. Static field poling mechanism of dipolar molecules. Before applying electric field the dipole moments are randomly distributed. At higher temperatures the dipole moments of NLO chromophores are mobile and orient in the direction of the applied external field. The orientation is frozen by cooling to low temperature and/or photo- or thermal crosslinking...

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

The poles con espond to excitation energies, and the residues (numerator at the poles) to transition moments between the reference and excited states. In the limit where cj —> 0 (i.e. where the perturbation is time independent), the propagator is identical to the second-order perturbation formula for a constant electric field (eq. (10.57)), i.e. the ((r r))Q propagator determines the static polarizability. 

Fig. 27 indicates the apparent piezoelectric constant e of roll-drawn PVDF as a function of static bias field E0 (Oshiki and Fukada, 1972). The value of e at E0=0 represents the true piezoelectric constant e. The curve exhibits a hysteresis and the polarity of e changes according to the poling history. If the piezoelectricity in /)-form PVDF originates from the polarization charge due to spontaneous polarization, inversion of polarity of e would mean the inversion of the polarization by the external field and hence /S-form PVDF may be a ferroelectric material, as was first suggested by Nakamura and Wada (1971). 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

Pole — (i) the locus of the charge in a static electric field, or (ii) the electrically isolated terminus of a conducting path. (This latter usage, though regarded as old-fashioned, still survives in technical electrochemistry, and in circuit theory.) The inadequacy of the word in connection with electrolysis was noted by - Faraday, and, after conferring with -> Whewell, he adopted the word -> electrode [i]. 

The calculation of the molar polarizabilities, often involves statistical mechanical averaging over orientational distributions of the molecules. An important example is the distribution function w caused by dipole orientation in an externally applied static electric field E° because it describes the process of electric poling of NLO-phores. To second order in the field, the dipolar contributions to this (normalized) function are given by (100),... 

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