S. Yilmaz

Pulsed laser deposition of stoichiometric potassium‐tantalate‐niobate films from segmented evaporation targets

The preparation of epitaxial potassium‐tantalate‐niobate (KTa0.7Nb0.3O3, KTN) films on strontium‐titanate substrates by means of pulsed excimer laser evaporation in vacuum is reported. In the most successful deposition experiments, a segmented evaporation target consisting of a semicircular KTN single crystal and a semicircular potassium nitrate pellet was utilized−to our knowledge for the first time; it was thus possible to overcome the otherwise observed partial loss of the volatile potassium and to avoid potassium deficiency in the KTN films. Rutherford backscattering spectrometry and x ray diffraction results indicate that the samples have the desired stoichiometric composition as well as the orientation prescribed by the substrate crystal.

Comparison of quasi‐phase‐matching geometries for second‐harmonic generation in poled polymer channel waveguides at 1.5 μm

We have investigated three different quasi‐phase‐matching approaches to second‐harmonic generation (SHG) in DANS (4‐dimethylamino‐4′‐nitrostilbene) poled polymer channel waveguides at 1.5 μm. Periodic photobleaching and periodically poled electrodes deposited directly on the film produced unacceptably high propagation losses. However, periodic electrodes on the substrate gave low losses and useful SHG.

POLYMER WAVEGUIDES WITH OPTIMIZED OVERLAP INTEGRAL FOR MODAL DISPERSION PHASE-MATCHING

Modal dispersion phase-matched second harmonic generation is demonstrated in new poled polymer waveguide geometries with a nonlinear optical core consisting of two side-chain polymers with different glass transition temperatures. After poling above and between the respective glass transitions, the sign of the nonlinear optical coefficient is reversed in the two polymers, thereby improving the overlap integral. Conversion efficiencies up to 7%/W cm2 were achieved in the first experiments.

Dielectric, pyroelectric, and electro-optic monitoring of the cross-linking process and photoinduced poling of Red Acid Magly

Crosslinking of polymers with dipoles can be followed by means of dielectric, pyroelectric, and in situ electro-optic measurements. All three techniques provide similar insights as to the cross-linking process. In addition, electro-optic measurements allow for the determination of the resulting optical nonlinearity. After crosslinking is completed the polymer behaves like a side-chain polymer. In addition, photoinduced poling of an already cross-linked dipolar polymer network is demonstrated.

Phase-shift interference microscope for the investigation of dipole-orientation distributions.

A compact experimental setup for an electro-optical microscope is introduced. The microscope is based on phase-shift interferometry (a well-known tool for surface profilometry) that is modified for measuring electrooptic responses. Its feasibility is demonstrated with a two-dimensional map of the electro-optic activity of a periodically poled nonlinear-optical side-chain polymer.

Active-matrix-addressed spatial light modulators based on the longitudinal pockels effect in oblique-cut perovskites

Abstract Oblique-cut layers from the perovskite crystals potassium tantalate niobate and barium titanate and from the tungsten-bronze crystal strontium barium niobate are suggested as control layers for active-matrix-addressed electro-optic spatial light modulators. Since all these materials consist of oxygen octahedra as building blocks and since they all exhibit their highest electro-optic effect in the tetragonal (4mm) phase, a common theoretical treatment is used to determine the optimal oblique-cut angle. Preliminary experimental results on oblique-cut crystals are presented and a compact and efficient optical system for their use is proposed.

Electro-optic potassium-tantalate-niobate films prepared by pulsed laser deposition from segmented pellets

Thin films of potassium tantalate niobate (KTN) were prepared by means of pulsed excimer-laser deposition and investigated with a number of analytical techniques, including electrical and electro-optical measurements. For applications in longitudinal electro-optic modulators, a transparent electrode is required between substrate and electro-optic layers. Suitable electrode materials, which at the same time permit epitaxial growth of KTN, were identified and prepared. The resulting layered samples were not only of good epitaxial and optical quality, but also exhibited the expected maximum of the longitudinal electro-optic effect at temperatures between the phase transitions from cubic to tetragonal and from tetragonal to orthorhombic. However, the maximum achievable electro-optic phase shift was found to be limited to roughly τ/100 for KTN films in the thickness range around 1 μm. Therefore, much thicker films are probably necessary for most practical applications, which requires significant improvements in the long-term stability and homogeneity of the deposition process.

Spatial and thermal analysis of optical nonlinearity created by asymmetric charge injection

Abstract ‘In-plane’ poling of polymer/dye films, accompanied by charge injection can create strong optical nonlinearity perpendicular to the direction of the applied field. Pyroelectric analysis demonstrates that an anisotropic polarization distribution has been created in the polymer. Comparison with sandwich electrode poling experiments supports the hypothesis that the non-uniform polarization of in-plane poled films results from diffusion of injected charges into the polymer.

Piezoelectrical experiments on poled nonlinear optical polymers

Poled nonlinear optical polymers are molecular dipole electrets and must therefore exhibit pyro- and piezoelectricity. As a complement to pyroelectrical techniques, piezoelectrical investigations may offer additional possibilities for looking into the physical mechanisms of various effects found in poled polymers. First results of ongoing piezoelectrical experiments are described and discussed. In particular, the direct and the inverse piezoelectric responses of nonlinear optical guest-host and side-chain polymers were obtained from acousto-electric and interferometric measurements, respectively. This research may eventually shed new light on more fundamental aspects such as e.g. indirect contributions to the electro-optical response in poled polymers as well as on more practical features such as the application of piezo- and elasto-optical effects in polymer-waveguide devices.

Preparation of chi (2) -inverted waveguides with poled polymers for efficient second-harmonic generation

Modal dispersion phase-matched second harmonic generation is demonstrated in polymer-based waveguides with a nonlinear optical core consisting of two side-chain polymers with different glass-transition temperatures. For an optimized overlap integral, a step like nonlinearity profile (chi(2)-inverted structure) is required across the core thickness. The chi(2)-inverted structure is achieved by two consecutive thermally assisted poling steps above and between the respective glass-transition temperatures, with an opposite poling field in the second poling step. The achieved chi(2)-inverted structure is monitored by in- situ electro-optic measurements and proved by electro-optic and second harmonic generation thermal analysis. Conversion efficiencies up to 7%/Wcm2 were achieved in first waveguide second-harmonic generation experiments.