Progress toward device-quality second-order nonlinear optical materials. 1. Influence of composition and processing conditions on nonlinearity, temporal stability, and optical loss

We report the incorporation of a new trifunctionalized disperse red-type chromophore into a polyurethane thermosetting system. Six polymers with different chromophore loading and NCO/OH ratios were synthesized. This system exhibited very good poling efficiencies, and the optimum chromophore loading density ranged from 30 to 40%. Decreasing chromophore loading caused an attenuation of macroscopic nonlinearity, due to the lowering of chromophore concentration in the polymer; while increasing chromophore loading above 40% also attenuated the nonlinearity, presumably because of chromophore-chromophore electrostatic interactions starting to compete with the electric field-chromophore dipole interactions at high chromophore concentration. Materials with a NCO/OH ratio equal to or greater than unity have much higher temporal stability of dipole alignment than materials with a NCO/OH ratio less than 1. By anchoring the chromophores at both the donor and acceptor ends, the temporal stability of dipole alignment was enhanced significantly. For such materials, the optical loss of unpoled materials with nonstoichiometric NCO/OH ratios was observed to be high; however, this loss could be reduced to less than 1 dB/cm by adjusting the NCO/OH ratio to unity. It is generally believed that poling induced optical loss is a serious problem associated with second-order NLO materials prepared by electric field poling. In our study, we found this type of optical loss could be reduced to only 0.2 dB/cm by careful control of processing conditions.