Electrooptic Coefficients in Electric Field Poled Polymer Waveguides
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ELECTROOPTIC COEFFICIENTS .IN ELECTRIC FIELD POLED POLYMER WAVEGUIDES BARTON A. SMITll, S. IIIIRMINGIIAUS**, and .. I). SWAI.FN *iBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120. "Universitit Konstanz, (Jniversititsstral~e 10, Fakultit [hr Physik, 7750 KONSTANZ, Germany
Abstract Planar wavcguides were made from a linear epoxy copolymer of bisphcnol-A diglycidyl ether and amino-nitro-tolanc. The refractive indices, ni-M and n-E, and the clectrooptic coeflicients, r 33 and r, 3, of the poled nonlinear optical polymcr films were determined by measurements of the wavcguide modes of these films at a wavelength of 632.8nm, and the results arc reported for both electrode and corona poling. The ratio of the second order susceptibilities was found to exceed significantly the theoretically expected value of 3. Restricted motion of the nonlinear chromophorcs during poling provides a possible explanation of this discrepancy.
Introduction Polymers containing nonlinear optical (NI.O) moieties arc considered promising for the development electro-optic devices [I- 3 ]. Thcir advantage, as compared to NIO crystals, is that optical waveguides adjacent to a solid substrate can be prepared with high optical quality by spin casting. I-However, since these films lack crystalline alignment, the NI.O moieties are randomly oriented, i.e., their arrangement is ccntrosymmctric. For first order elcctrooptic effects or second harmonic generation (SI IG), the centrosyminctry must be removed. This is convcniently done by heating the film to a temperature close to its glass transition and applying an electric field. The NI.O moieties are then oriented (poled) due to their strong electric dipole moment. The usual description of the poling process assumes that the dipoles (i.e., the N1.O moieties) arc surrounded by a homogeneous, hydrodynamically isotropic medium and that the dipoles are in thermal equilibrium with the applied electric field. A most important result of' this approach is that in the limit of small alignment, the ratio of the electrooptic coefficients, ra/r, as well as the ratio of the SI IK; coefficients,-d,;d.3 , should be equal to 3.
Sample Preparation Polymer wavcguidcs were made of poly(bisphenol-A-amino-nitro-tolanc) (Bis-A-ANT), a linear epoxy copolymer of bisphcnol-A-diglycidyl-cther and amino-nitro-tolane, by spin casting from cyclohexanone solution onto the substrates. These consisted of glass slidcs coated with a ý--50nmn thick gold film necessary for the optical experiments, as discussed below. The samples were then baked in a nitrogen atmosphere at 150'C for at least 24 hours to remove the solvent. Final thicknesses of the wavcguides ranged fr'om 0.5 to 0.8 tnm. Poling was performed at temperatures around 115'C, which is somewhat bclow the glass transition temperature of 125'C [4], where the sample current increases duc to ionic conductivity. We wanted to avoid high currents (>I/gA/cmn2) that cause the optical quality of the filns to deteriorate. Corona poling was performed with a cor
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