## 2 Electro-Optic Properties of Polymers
### Inducing of the Macroscopic Electro-Optic Properties
In the side-chain systems, the chromophores can rotate around the attaching point and it is easier to orient them in the poling procedure. Nevertheless, the orientation is maintained longer than in the guest-host systems. For all these reasons, side-chain copolymers are widely used in optical application but also guest-host systems are used because of the relatively easy fabrication process. In order to induce non-centrosymmetry after the films deposition, several poling techniques can be used. The most common ones are:  
_Contact Poling_ or _Electrode Poling_. Chromophores such as DR1 show a large static dipole moment \(\mu_{0}\), due to their rod shape with donor and acceptor at the opposite sides. As a result, it is possible to use a large static electric field to orient these polar molecules along the field and thus induce an asymmetric configuration. For contact poling, the sample must be prepared by spinning the polymeric solution on a substrate that was previously covered by an electrode. Then, after spinning and drying, the polymeric film must be topped with another electrode, via sputtering deposition or other techniques, in order to obtain a sandwich structure with the copolymer sandwiched between the two electrodes. Then, the sample is ready for the poling procedure; the temperature of the sample is increased to the glass transition one (\(T_{g}\)), where the polymer matrix reaches a large mobility. A large static electric field is then applied between the two electrodes for a sufficient time, in order to align the nonlinear molecules; then with the field still on, the temperature is slowly decreased to Room Temperature and the field is subsequently removed. At Room Temperature the orientational mobility of the molecules is sufficiently low to maintain the chromophores alignment for a long time and consequently giving rise to a long lasting electro-optic coefficient. With such a method it is not possible to apply a very large field, because unavoidable small imperfections in the polymer film can lead to a decrease of the dielectric rigidity of the material and, as a consequence to electrical discharge with complete damage of the sample. _Corona Poling_. In such a technique [23] it is not necessary to apply electrodes on the polymeric material, as a consequence, it is possible for example to deposit, by spin coating, a polymeric film directly on a bare substrate. In the corona poling procedure the sample is put on a metal plate that plays simultaneously the role of heater and bottom electrode. The top electrode is given by a layer of ions which are deposited on the film surface. In order to produce the ions, a tungsten needle is placed near the surface of the polymer film at a potential of about 1-10 kV with respect to the substrate. The needle generates an intense electric field that ionizes the surrounding atmosphere (usually an inert gas, nitrogen, for example) and the generated ions are pushed on the film surface where they accumulate. As a consequence, the charges produce a quite uniform high intense field that orients the nonlinear dipoles. As in the case of electrode poling, the temperature is increased close to \(T_{g}\) for a suitable time interval during the poling procedure, then it is lowered to Room Temperature, still maintaining the voltage drop between needle and substrate. Corona poling is more efficient than electrode poling because it can generate very intense fields. Also during corona poling procedure, there is the possibility of having electrical discharge due to the high field, but in this case, since the ions are not mobile on the surface of the material, the discharges are localized and the sample does not damage the whole surface. The method is, however, the subject of criticism for material characterization purposes: in fact, the real effective poling field remains unknown and it is not well reproducible. _Photoassisted Poling._ In the previously described techniques it is necessary to pole the sample at a high temperature in order to have sufficient chromophore mobility.