Highly Ordered Pseudo-Discotic Chromophore Systems for Electro-Optic Materials and Devices
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Highly Ordered Pseudo-Discotic Chromophore Systems for Electro-Optic Materials and Devices Nishant Bhatambrekar1, Scott Hammond1, Jessica Sinness1, Dr. Olivier Clot1, Harry Rommel1, Dr. Antao Chen2, Dr. Bruce Robinson1, Dr. Alex K-Y. Jen3, Dr. Larry Dalton1 1 Department of Chemistry, University of Washington, Seattle, WA-98195-1700 2 Applied Physics Laboratory, University of Washington, Seattle, WA 98105-5640 3 Department of Material Science and Engineering, University of Washington, Seattle, WA98195-2120 ABSTRACT In order to achieve the near-ferroelectric order desired in organic electro-optic (EO) chromophore systems, a pseudo-discotic chromophore is under investigation. Calculations suggest head-to-tail inter-chromophore dipole-dipole interactions should drive chromophores with an appropriate aspect ratio into ferroelectric columns similar to those seen in discotic liquid crystals (DLCs). Therefore, the liquid crystalline properties of these chromophores are being examined by differential scanning calorimetery (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Furthermore, the effect of this discotic behavior on the order and EO properties of the system are being examined both dynamically by second harmonic generation (SHG) and statically by attenuated total reflection (ATR). Additionally, these chromophores are being incorporated into waveguide-based photonic devices. INTRODUCTION In the past couple of decades, there has been a great attention towards improving materials for electro-optic devices. Organic based EO materials have been proven to be a much alternative to traditional inorganic crystals such as LiNbO3. One of the main advantages of the organic EO materials over inorganic crystals is the exceptionally high band widths.[1] Other advantages of organic materials include low dielectric constant, easy processability into electro-optic devices, and low operating voltages.[2] Despite of all the aforementioned advantages, their temporal instability is the main impediment to commercialization of organic EO materials and devices. The fundamental unit of the organic EO materials is a π-conjugated network in which an electron rich donor is connected to an electron withdrawing acceptor through a π-conjugated bridge. The electron density is transferred from the donor to the acceptor via the π-conjugated bridge in the presence of an external electric field. This changes the refractive index of the medium and hence the speed and the phase of light in the material. For example, in a typical Mach-Zender type waveguide device a phase shift can be introduced between the two arms by applying an external electric field to one of the arms. In devices made of high electro-optic coefficient (r33) organic materials, the operational voltages can be reduced to less than one volt.[2]
THEORY
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One challenging problem in improving electro-optic activity of these devices is to achieve unidirectional ordering of the EO materials such that all the dipoles point in the same direction. As in most
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