THERMOSETTING POLYURETHANES WITH STABLE AND LARGE 2ND-ORDER OPTICAL NONLINEARITY

A two-component, thermally-curable second-order nonlinear optical material, based on polyurethane, is developed. Tough, chemically-inert films were prepared from this material. Resonance-enhanced second-harmonic generation coefficients (d33) of approximately 120 pm/V at a 1.064-mu-m fundamental wavelength were obtained. Using a Mach-Zehnder interferometer, an electrooptic coefficient of r13 = 13 pm/V was measured at a 633-nm wavelength with a 3-kHz modulation frequency, which is in good agreement with that derived from the SHG d31 (d31 = d33/3) coefficient employing a two-level model Second-harmonic generation (SHG) revealed a long-term stability of dipole orientation in an electrically-poled and thermally-cured film. The dipole alignment of the poled and thermosetted film showed no detectable relaxation at room temperature for over 3000 h, as monitored by SHG measurement. A high poling stability was also observed when the film was kept in an oven at 90-degrees-C for 3000 h, and 70% of the signal still remained. The versatility of the reaction scheme and the ease of processing makes this material appropriate for further development for device application.