Preparation and Properties of the Novel Nanocrystal and Polymer Composite Films for Electro-Optic Applications
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Preparation and Properties of the Novel Nanocrystal and Polymer Composite Films for Electro-Optic Applications
Quan Ren1,3, Dong Xu2, Yuktak Chow3, Hauping Chan3 and Paklim Chu3 1 Department of Optics, Shandong University, Jinan 250100, P. R. China 2 State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China 3 Optoelectronics Research Centre, City University of Hong Kong, Hong Kong, P. R. China ABSTRACT Novel nanocrystals and polymer composite thin films for the applications of electro-optical devices were prepared. We used PbTiO3 nanocrystals as a nonlinear chromophore (guest) in the NLO polymer system. The transparent polymer polyetherketone (PEK-c) was chosen as the polymer host due to its high glass transition temperature. The PbTiO3 nanocrystals were synthesized successfully by the chemical solution decomposition method. The size of the PbTiO3 nanocrystals was estimated to be 30-40 nm. The weight ratio of PbTiO3 nanocrystals to polymer polyetherketone was 15 %. The composite thin films of PbTiO3 / PEK-c were prepared by spin-coating technique. The films were then electrically poled to induce asymmetry in the material by heating the films to a temperature close to their glass transition and applying an electric field. The refractive indices of the un-poled composite PbTiO3 / PEK-c film were found to be 1.55165 for 633 nm and 1.65352 for 414 nm by using the Metricon prism coupling system. In the poled composite PbTiO3 / PEK-c film sample, the TE- and TM-indices differences were found to be 0.02945 for 633 nm and 0.03915 for 414 nm, showing very good optical anisotropy properties. Dielectric constant ε of the film was determined to be 7.32 at room temperature at 100 KHz using an Impedance Analyzer. The electro-optic coefficient, γ33, of poled PbTiO3 / PEK-c composite film was measured to be 18.34 pm/V at 633 nm under room temperature by the simple transmission technique. The factor F2 = n7 γ2 /ε, figures of merit, was estimated to be 1546. The relaxation time constant was calculated to be 5836 min. INTRODUCTION There is a great interest in nonlinear optical (NLO) composite polymer materials suited to different types of photoelectronics devices for applications to optical communication technologies. The compounding of active NLO units in a polymer allows a high NLO effect to be added to traditional polymers while maintaining a low dielectric constant after poling in an electric field. In addition, the resulting composite materials can easily be processed into thin films for waveguide applications. They are potentially competitive materials for integrated optical devices such as electro-optic modulators and switches [1-2]. Lead titanate (PbTiO3) is a perovskite-type ferroeletric material. Its single domain crystal has a high spontaneous polarization Ps (~81 µC cm-2), a dielectric constant ε(~107) and a pyroelectric coefficient p (6×10-8 C cm-2 K-1). It also has a high electro-optic figure of merit F2 (~574) [3]. However, the synthesis of large, perfect, and pure PbTiO3 single
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