Nonlinear Optical Polymers For Electrooptical Devices
Although organic crystals may be used to experimentally verify the large nonlinearities and short response times of organics, such crystals are not acceptable for device applications due to significant fabrication difficulties. Further, the bulk material nonlinearity is a function of molecular orientation and symmetry which may not be controlled during the crystallization process.
Nonlinear optical polymers have been synthesized at Hoechst Celanese for which the active NLO unit is attached to the polymer backbone as a pendant side chain. Control of orientation and symmetry of the unit is achieved by poling in an external electric field at elevated temperatures resulting in second order susceptibilities larger than inorganic crystals. The polymers have attractive secondary properties (i.e., optical transparency, high glass transition temperatures which are controlled by adjusting the side chain length and nature of the polymer backbone, low dielectric constants, and flat frequency respose). Further, single mode waveguides may be fabricated by spin coating. Deposition of electrodes on the waveguide permits application of an external field which changes the material's index of refraction due to the linear electrooptical effect. Thus, a host of electrooptical waveguide devices may be constructed which operate at low voltages and very high frequencies.