In this paper we report on the synthesis of a number of photorefractive materials with high glass transition temperatures. We have developed a series of fully-functionalized polymethacrylates with both photoconducting and NLO-active moieties. Secondly, low molecular weight glasses based on triphenylamine and carbazole units together with different NLO-chromophores have been designed which show no tendency towards crystallization. We now report on the synthetic approach to these novel materials and their properties with regard to photorefractive applications.

Photobleaching experiments on thin films of azo chromophore functionalized methacrylates were carried out to investigate bleaching kinetics and its underlying mechanisms. The polymer film's spatial index variation was measured and compared with a theoretical model of bleaching dynamics. From IR-spectroscopy and GPC measurements it was found, that bleaching causes a destruction of the chromophore and a broadening of the polymer molecular weight distribution. The generated low molecular weight fractions from both polymer and chromophore photodecay evaporate during annealing above T-g, resulting in a reduction of the film's thickness. Surface roughness increases during photobleaching, causing an increase in waveguide loss. Film surface can be smoothed completely by annealing, thus lowe,ring the optical loss induced by bleaching.

We investigated both time and temperature dependence of chromophore movements by means of in situ second-harmonic generation (SHG) measurements. Using real-time detection of the SHG intensities during and after poling of various nonlinear-optical polymer films we observed differences between main-chain and side-chain polymers regarding the nonlinear coefficients d33 and the glass transition dynamics. We present experimental data on new main-chain polymers with polar stilbene chromophores attached in the most common transverse position to the backbones (MC-T) or incorporated in a linear fashion where they are a part of the main-chain (MC-L). The side-chain polymer (SC) presented here has been developed for photorefractive applications and is a copolymer containing Disperse Red 1 and a photoconducting carbazole unit in the side chain. The influence of the macromolecular structure on the chromophore dynamics is striking. This is verified by means of in situ corona poling experiments and thermal experiments based on temperature ramps on poled polymers. MC-T and especially MC-L polymers show a better thermal stability of the chromophore orientation relative to the glass transition temperature than SC polymers; however, chromophores of main-chain polymers are, as can be expected, less mobile during the poling process. We obtained resonance enhanced nonlinear optical coefficients d33 of 100 pm/V in a SC polymer, 90 pm/V in a MC-T polymer, and 30 pm/V in a MC-L polymer.

Chromophore relaxation is still one of the major problems with NLO polymers. One strategy to overcome this problem is the synthesis of polymers with high glass transition temperatures. We have prepared a number of polymers by copolymerization of NLO-active methacrylates with the bulky adamantyl methacrylate. This leads to polymers with glass transition temperatures up to 190 degrees C. The synthesis and properties of these polymers are reported in detail. The polymers show a good long term stability of the electrooptic coefficient (r(33)) at elevated temperatures. So polymer 3c with a bisazo chromophore shows only a 25% EO-coefficient decay after one month at 120 degrees C.

Chromophore relaxation is still one of the major problems of NLO polymers. Two strategies have been developed to overcome this problem. One of them is crosslinking after poling. We synthesized new polymers with methylfuryl acryloyl side groups. Upon irradiation with UV- light, they undergo a (2 + 2) cycloaddition which leads to a highly crosslinked material. Compared to cinnamoyl gorups frequently used in such polymers, the absorption of the novel crosslinker is shifted to longer wavelengths. This allows to use a cutoff filter during irradiation and therewith minimizes chromophore decomposition. The second way to suppress chromophore relaxation is the use of polymers with high glass transition temperatuers. We have prepared a number of polymers by copolymerization of NLO-active methacrylates with the bulky adamantyl methacrylate. This leads to polymers with glass transition temperatures up to 190 degrees C. The synthesis and properties of these polymers are reported in detail. The polymers shown a good long term stability of the electro-optic coefficient (r33) with its relaxation approximately following a KWW-function.

We carried out photobleaching experiments on thin films of azo chromophore functionalized side chain polymers at different wavelengths in the UV and visible sprectral regions. Photobleaching alters the polymer layer due to two different mechanisms: In the first place, bleaching results in lowering of the index of refraction by destruction of the highly polari­zable NLO chromophore electronic π system and thus shifting the absorption band. Secondly, as a result of outgassing of molecular fragments and ablation, the polymer layer thickness is significantly reduced. Both effects strongly depend on the wavelength spectrum of the bleaching source and in addition exhibit remarkably different time responses. Limiting the wavelength spectrum the proper way we were able to either concentrate on chromophor modification alone or on both chromophor modification and thickness modification. This allows us to tailor the resulting geometrical profile of channel waveguides as well as the index difference to the surrounding cladding. Figure 1 (left) shows an AFM-micrograph of the geometrical dimensions of the top surface of a channel waveguide photobleached in a NLO-sidechain polymer as described. A very smooth profile (Figure 1 right side) is realized with a rms-value of the surface roughness of only 0.3 nm. In addition to the modification of the surface topography, the refractive index in the bleached region of the polymer is shifted by up to 10-2.

Photorefractive polymers have gret potential in applications such as all‐optical switching and optical communication. Here, new charge‐transporting polymers are presented based on a polysiloxane backbone with pendant carbazole groups, which provide both transport and room‐temperature poling properties without the need for additional plasticizing components. A quantitative evaluation of the phase behavior of holographic gratings based on the new materials is also presented.

A number of novel nonlinear optically (NLO) active polymethacrylates were prepared from the NLO active methacrylates 2a-d with azobenzene side groups and the bulky comonomer 1-adamantyl methacrylate. The polymers exhibit unusually high glass transition temperatures between 160 degrees C and 190 degrees C. The copolymerization parameters of the monomer pair 1-adamantyl methacrylate (1)/Disperse red methacrylate 2b (r(1) = 1,1 +/- 0,2, r(2) = 0,8 +/- 0,2) show that the two monomers are incorporated almost statistically into the polymer chain. Polymers 3a-d are soluble in common organic solvents and excellent films can be obtained by spin coating. After poling in an electric field of 120 V/mu m polymer 3b shows a large electrooptic (EO) coefficient (r(33)) of 25 pm/V at 633 nm. Within two weeks, only a negligible decay of 7% of the EO coefficient was observed at room temperature. On-line monitoring of the second harmonic generation (SHG) at 100 degrees C showed a fast initial drop (10%) of the SHG signal and subsequently a slow decay of 20% within 10 h. Afterwards, the signal remained almost constant for further 5 h at 100 degrees C. The novel polymers can thus be considered as easy processible NLO materials with a high thermal stability of the chromophore orientation obtained by poling.

Four high Tg side chain polymers have been investigated based on polyimide and acrylate backbones functionalized with DR1 and heterocyclic thiophene chromophores. UV-VIS studies revealed chromophore stability up to 210 degree(s)C. Maximum poling efficiency was found approximately 15 K above Tg. From dielectric relaxation studies and from poling dynamics it is obvious that chromophore reorientation follows an Arrhenius law at temperatures well above Tg leading into a WLF-behavior in the vicinity of Tg. Relaxation of the EO-coefficient could be interpreted in terms of a KWW function. The average relaxation times strongly deviate from the WLF-function at temperatures well below Tg and can be described with an Arrhenius law. Activation energies here are significantly smaller than those in the high temperature limit above Tg, indicating that the chromophore dipoles are incompletely coupled to the polymer (alpha) process. Very good stability was observed for the polyimide P3 with average relaxation times of 104 years at 50 degree(s)C and 4 months at 120 degree(s)C. EO coefficients of up to 12 pm/V at 1541 nm were realized using a poling field strength of 1 MV/cm. EO-coefficient at constant poling field was found to be approximately linearly dependent on chromophore content. Channeled waveguides were fabricated by selective reactive ion etching with small losses of 1 dB/cm at 1318 nm. However, loss varies strongly between 1 dB/cm and 3 dB/cm depending on film quality.