Development of Chromophores Based on the Dicyanomethylene Acceptor Group
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INTRODUCTION A major challenge in the design of nonlinear optical chromophores for electro-optic (EO) devices is simultaneously achieving acceptable thermal stability, transparency, nonlinearity, and processibility in one compound. In 1991 we began the development of all-polyimide integrated EO process using 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4Hpyran (DCM) as guest dopant. In the process of developing this system, we determined that DCM exhibited a combination of important properties, such higher nonlinearity and thermal stability relative to Disperse Red 1 (DRI), low absorption at wavelengths near 850 nm, and photobleachability. The low loss of DCM is related to its use as a laser dye, 1 since laser dyes are designed to have sharp cut-offs on the longer wavelength side of their major absorption peak, so as to avoid reabsorbing emitted light. DCM-based Devices Our first DCM-based triple-stack Mach-Zehnder modulators were all-polyimide devices which in which Amoco Ultradel 4212 was incorporated in the core layer as a host for DCM and used in undoped form for the cladding layers. 2 These devices were functional from a proof-ofprinciple standpoint, but the r33 values obtained after poling were relatively low (3 to 4 pm/V). These low r33 values led to high device switching voltages which were impractical for use in Mat. Res. Soc. Symp. Proc. Vol. 392 © 1995 Materials Research Society
real systems. We believed these low r33 values were the result of higher electrical conductivity at poling temperature in the DCM/polyimide core layers relative to the undoped polyimide cladding layers. This conductivity mismatch leads to a decrease in the fraction of the poling field dropping across the core layer, rendering the poling process inefficient. Our most recent device demonstrations have substituted a polyacrylate for polyimide in the upper and lower cladding layers. Devices have been constructed on a silicon wafer using DCM/polyimide core and polyacrylate claddings with the layout and cross-section as shown in Figures 1 (a) and (b). 4-(Dlcyanomethylone)-2-methyl-6-{p-dimethylamlnostyryl)-4H-pyran
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Figure 1. Core Components (a) and Dimensions (b) of DCM-based Mach-Zehnder Modulator Using the DCM/polyimide core material system with polyacrylate cladding layers, the polymer layers were fabricated by spin coating, and the channel waveguide was defined by photobleaching. Microstrip electrodes were built 4 ptm thick by gold plating and end faces were prepared by sawing. The polyacrylate has been shown by Girton et al. 3 to have a significantly higher conductivity at poling temperature than did the polyimide. Poling has been enhanced in these systems resulting in higher r33 values and lower switching voltages as shown in Figure 2.
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