Martin Sprave

High electric field conduction mechanisms in electrode poling of electro‐optic polymers

We investigated electrical conduction phenomena, which occurred during high electric field electrode poling of side chain χ(2) polymers. Electric current and second harmonic intensity were measured simultaneously in poling experiments performed on samples with and without an additional inorganic poly–methyl–siloxane layer. Current densities appeared to be limited by a Schottky‐type potential barrier at the electrode/insulator interface. The field dependence of the current density was found to be Schottky charge emission for medium field strengths (EPOL≤100 V/μm) whereas it was dominated by Fowler–Nordheim tunneling at higher poling fields. In the presence of the siloxane layer, a significant suppression of tunneling current was observed. This leads to a reduced probability of singular dielectric breakdown events and shifts the limit of avalanche breakdown to higher internal effective poling field strengths.

Nonlinear optical active polymethacrylates

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.

Nonlinear Optical Active Polymethacrylates with High Glass Transition Temperatures

Abstract 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 °C. The synthesis and properties of these polymers are reported in detail. The polymers show a good long term stability of the electrooptic coefficient (r33) at elevated temperatures. So polymer 3c with a bisazo chromophore shows only a 25 % EO-coefficient decay after one month at 120 °C.

High Tg NLO-polymers by functionalization of reactive precursors

A new copolymer system with high, but still variable glass-transition temperatures was synthesized. The fixation of various NLO-chromophores is performed after the radical is performed after the radical copolymerization by a polymer analogous reaction. This was accomplished by copolymerizing various N-substituted maleimides with methylvinyl- or vinylisocyanate, so that a polymer with a 1:1 composition of the two monomers and reactive isocynate side-groups is obtained. With different substituents attached to the imide-ring, Tg can be adjusted between 100 and 200 degrees C. Two effects of the substituent at the imide-nitrogen are observed. Bulky gorups reduce the mobility of the chain and lead to high Tg values. A similar tendency is found for smaller less flexible substituents. In this case the intermolecular forces by hydrogen-bonding (urethanes) contibute ot the effects for high Tgs. A variety of different NLO-chromopores can be fixed on the polymer backbone by reaction of a hydroxyalkyl-spacer with the isocyanates, so that a urethane linkage between the chromophore and the polymer is formed. Based on the experience obtained with this first class of NLO-materials, new functionalized polymers were prepared. In order to further increase the Tg-values, while keeping a good processability (solubility), chemical modifications of the spin-coated polymer were prepared. In order to further increase the Tg-values, while keeping a good processablility (solubility), chemical modifications of the spin-coated polymer were performed. Monomers functionalized with tertiary butyl groups (esters or carbonates) allow it, to split off isobutene from the final (and processed) polymer, thereby reducing its solubility and increasing its Tg-value.

Electrical conduction processes and high-electric-field poling in nonlinear optical polymers

We report on electrical conduction phenomena occurring during high electric field electrode poling of side chain (chi) (2) polymers. Electric current and second harmonic intensity were measured simultaneously in poling experiments performed on samples with and without an additional inorganic poly-methyl-siloxane layer. Current densities appeared to be limited by a Bardeen-type potential barrier at the electrode/insulator interface. The field dependence of current density was found to be Schottky charge injection for medium field strengths (EPOL <EQ 100 V/micrometers ) whereas it was dominated by Fowler-Nordheim tunneling at higher poling fields. In the presence of the buffer layer, a significant suppression of tunneling was observed which leads to a reduced probability of singular breakdown events and shifted the limit of avalanche dielectric breakdown to higher internal effective poling fields.

χ(2)-materials: high-field poling

This paper gives a brief introduction to second order nonlinear optics and polymeric χ(2)-Materials and then focuses on high field poling and electrical conduction.

Conductive low-index buffer layer for electro-optic polymer waveguides

An inorganic methyl-siloxane based buffer material has been investigated that is suitable for nonlinear optical waveguides used in integrated electro-optic devices. Slab waveguide structures were fabricated to study the influence of buffer layers on poling mechanisms. Poling currents and second harmonic intensities were measured simultaneously under various poling conditions. Maximum poling efficiency for sandwich structures was found to be close to Tg being identical to samples without buffer. Poling currents first decreased with time following a t-4 dependence and finally reached a steady-state value. Resistivities were determined from saturation currents for different temperatures and electric field strengths. An Arrhenius-like temperature dependence of the resisitivity was found for both buffer materials and NLO polymer in its glassy state. For both materials a Schottky-type dependence of the resisitivity on the elctric field strength was observed at poling tempertures. Charge injection currents into buffer were found to be one order of magnitude smaller than into the NLO polymer. Electro-optic coefficients of poled samples with and without buffer layer were measured providing that even higher degrees of chromophore orientational order can be achieved in samples with buffer due to an increased electrical breakdown limit. A low index of refraction and a high transmittance were determined for the whole visible to near infrared spectral region. Channel waveguides wer realized by photobleaching.