Electro-optic composite polymers

Composition of electro-optic polymers

The photorefractive effect, as a light induced refractive index change, is based on two main steps. The first one consist of a photoinduced charge generation. These charges can migrate due to various transport mechanisms and can be retrapped in energy levels of dopants. Due to this redistribution of charges an internal space charge field is generated. The second step is a translation of the internal space charge field into a refractive index change by an electro optic effect. Therefore a material which shall show photorefractivity has to fulfill following requirements:
  1. Light induced generation of free charges
  2. Electro optic response resulting in a refractive index change
  3. Traps for redistributed charges
  4. Increased mobility of one charge carrier (holes or electrons)
In polymers these qualities can realized by two different approaches. The first strategy is to synthesize a fully functionalized material in which all requirements for a photorefractive material are given by one substance. This approach can be reached by linking functional groups to a polymer itself or by introducing small glas-building molecules into a host polymer. The second strategy is based on guest host systems in which all requirements on the material are given by different components. These composites have the advantage of a great flexibility in the chemical synthesis and can therefor easily changed in there physical and optical properties. Because of the easiness of fabrication of photorefractive devices using composite polymers and there high optical nonlinearities we have focused our research on this potentiality. Therefore we will discuss this approach more detailed in the following.
Hoppingmodell in Polymeren
Charge transport model in polymers: Hopping of charge carriers in the HOMO of the charge transfer agent.
The first step for photorefractivity, the light induced generation of free charge carriers is realized by sensitizers, which have a brought absorption band in the visible region of the spectrum. The selection of the sensitizers is determined by the working wavelength of the final photorefractive device. The underlying physical process consist of absorption of a photon and generation of electron-hole pairs which can dissociate in an external electric field. The charge transporting function is provided by a network of charge transport agents (CTA) which are close enough together in space to provide for hopping motion. This hopping process is often supported by a high external electric field which can "`pull"' charge carriers from molecule to molecule. In many cases the polymer itself acts as the CTA for positive charge carriers (holes).
For the development of an internal space charge field it is important that charges are not only transported but getting really restored. Therefore so called trapping sites are needed, which are energetic deep levels preventing the charge carriers to participate in transport and recombination with complementary charges. Up to now these traps are microscopically unknown in polymers, but they can be provided by local inhomogeneities of the polymer matrix or impurities. Also sensitizers can act as traps, but this point has to be object of future investigations.
After separation of charges, resulting in a space charge field, a field induced refractive index change is needed for photorefractivity. This quality is in polymers given by chromophores which are introduced into the polymer matrix. These molecules can show a linear electro-optic effect (Pockels effect) like inorganic photorefractive crystals. For this the rod-like dipole molecules have to be oriented in an electric field in order to break the optical isotropy. In most polymeric photorefractive composite a second effect is more important for the refractive index change. This is a quadratic electro-optic or Kerr effect, which yields a change of birefringence by orientation of the chromophores in the internal space charge field (fig. xxx). This often called orientational enhancement effect needs the possibility of easy rotational orientation of chromophores in the polymer matrix under the influence of an electric field. This can be realized by setting the glass temperature of the composition near to the working temperature of the device by introducing plasticizers. A second strategy for increasing the birefringence changes is to increase the static dipole moment and the anisotropy of polarizabilities of the molecules. Often this approach is realized by rod-like push-pull systems with double bondings as bridge elements. Because of the key role of chromophores in the fabrication process of photorefractive devices this compound is, in contrast to the others, synthesized in our group.
Kerreffekt in Polymeren
Change of birefringence by orientation of chromophores with high anisotropic polarizabilities. Top: Isotropic distribution of chromophores in the polymer matrix. Middle: Orientation of chromophores in an external electric field (needed for linear electro optic effect). Bottom: Orientation in an electric field consisting of the sum of internal space charge field and external applied field.
The final properties of the composite are not only influenced by the different compounds but also the interaction of the different molecules have to be taken into account. The sensitizers are not only responsible for the working wavelenght but also for the quantum efficiency and the conductivity of the device. The chromophores can lead to an unwanted absorption or can contribute to the charge transport. The optimal material would show high refractive index changes with short response times. For this, high concentrations of chromophores ans sensitizers in a weak polymer matrix are needed. This objective stands in contrast to the solubility of chromophores and sensitizers which show crystallization even ad moderate concentrations. Therefore the selection as well as the composition of all compounds have to be chosen carefully in order to achieve the wanted properties. The optimization of the photorefractive polymers for a special application is always a labor-intensive experimental procedure.

Fabrication of polymeric photorefractive devices

Structure of a phtorefractive device
As described in the previous section, our photorefractive polymers are based on a composition of several functional molecules together with a polymer matrix. Experimental all compounds are given in a solvent in a defined molar ratio in order to get a homogeneous blend. This composite is than either directly or after evaporation of the solvent pressed between to ITO coated glass slides. The optical transparent ITO-electrodes are important for the application of an external electric field to the polymer. This step has to be executed very carefully because of the very high voltages of 150 V/µm applied to the polymer, which is often only 100 µm thick.

Characterization of the polymer properties

Apart from mechanical properties of the composites, as the glass temperature, which can be measured by DSC-methods, tho optical properties are the main research object of our work. As fundamental properties the absorption and the optical quality of the photorefractive devices are very important, but of special interest is the nonlinear and photorefractive behavior of the materials. Therefore the orientation of chromophores, the light induced refractive index changes and energy transfers by wave coupling are under investigation. The dependences of refractive index change and phase shift of theses index changes on the external electric field, the light intensities and the composition of the material are the basis for the optimization of photorefractive devices for applications in optical information processing.
Polymer in Halter
Photograph of a photorefractive polymer sample in a mount for optical investigations.


The main goal of our research in this field is the optimization of photorefractive polymers for optical information processing and especially nonlinear motion detection microscopy. In the past, most work wore done on polymers working at wavelength in the red. In order to take advantage of the full resolution in a microscope system we are working on the realization of photorefractive polymers with high energy coupling in the blue wavelength region. Additionally these polymers have to be thermal stable and of excellent optical quality.