Jan Vydra

Photodecay mechanisms in side chain nonlinear optical polymethacrylates

The chemical and physical properties of photobleached nonlinear optical (NLO)‐polymethacrylates were studied in order to investigate photobleaching mechanisms. We have shown by three key measurement techniques (infrared‐spectroscopy, mass spectrometry, and gel permeation chromatography) that the predominant bleaching process is an irreversible decomposition of the NLO moieties accompanied by a broadening of the polymer molecular weight distribution. The quantum yield of the chromophore degradation determined by the evaluation of the bleaching kinetics fit is in agreement with typical data from comparable chromophores. It was found that bleaching causes an increase in surface roughness of NLO‐polymer films, causing an increase in waveguide loss. Smoothing of the film surface by an annealing step leads to a lowering of bleaching induced waveguide loss.

Mapping of the lateral polar orientational distribution in second-order nonlinear thin films by scanning second-harmonic microscopy

We present a nondestructive experimental technique for the determination of the lateral distribution of the polar order in second-order nonlinear optical thin films. The sample, which consists of a poled polymer film is scanned through the focus of an infrared laser beam in a second-harmonic generation setup and the second-harmonic intensity is monitored stepwise. In combination with a conventional electro-optic (EO) characterization it is possible to create an EO-coefficient map of the sample. The resolution of this mapping technique can be significantly increased by using high numerical aperture microscope optics for the illumination of the poled polymer. This method, for instance, allows the evaluation of poling inhomogeneities due to high-field poling and field distortions at the edges of poling electrodes.

DETERMINATION OF THE GLASS TRANSITION TEMPERATURE OF NONLINEAR OPTICAL PLANAR POLYMER WAVEGUIDES BY ATTENUATED TOTAL REFLECTION SPECTROSCOPY

We present an experimental technique for nondestructive measurement of the glass transition temperature (Tg) of thin nonlinear optical (NLO) polymer films by attenuated total reflection spectroscopy. At Tg the thermal expansion coefficient and temperature dependence of the refractive index ∂n/∂T change discontinuously. This change can be detected with high sensitivity by recording thickness and refractive index of a polymer waveguide coated on a glass/gold substrate as a function of temperature. Measurements were made on various high Tg NLO polymers (Tg up to 170u2009°C). The resulting Tg values are compared with those determined by differential scanning calorimetry of the bulk material.

Photobleaching in side chain NLO-polymers

Photobleaching experiments on thin films of azo chromophore functionalized methacrylates were carried out to investigate bleaching kinetics and its underlying mechanisms. The polymer films spatial index variation was measured and compared with a theoretical model of bleaching dynamics. From IR-spectroscopy and from 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 Tg, resulting in a reduction of the films thickness. Surface roughness increases during photobleaching, causing an increase in waveguide loss. Film surface can be smoothed completely by annealing, thus lowering the optical loss induced by bleaching.

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.

Electro-optical properties and poling stability of high glass transition polymers

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.

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.

Mapping of the lateral polar orientational distribution in second order nonlinear thin films by scanning second-harmonic microscopy (SSHM)

We present a novel nondestructive experimental technique for the determination of the lateral distribution of the polar order in second order nonlinear optical (NLO) thin films. The sample, which consists of a poled polymer film, is scanned through the focus of an infrared laser beam in a second harmonic generation (SHG) setup and the second harmonic intensity is monitored stepwise. In combination with a conventional electrooptic (EO) characterization it is possible to create an EO-coefficient map of the sample. The resolution of this mapping technique can be significantly increased by using high numerical aperture (NA) microscope optics for the illumination of the poled polymer. This method, for instance, allows the evaluation ofpoling inhomogeneities due to high field poling and field distortions at the edges ofpoling electrodes.