No abstract

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.

Isoxazolone (ISX) acceptor based chromophores terminated with hydroxyl functional groups were synthesized, spin cast into optical quality 1-2 µm thick films, poled in the presence of an electric field, and thermoset into polyurethane networks to lock in the poling-induced acentric chromophore order. Alkyl groups were introduced into the chromophore backbone (π-electron bridge) to increase chromophore solubility and to inhibit electrostatic interactions between these "high µβ" chromophores. Thermosetting polyurethanes containing alkylsubstituted chromophores exhibited much (1.7-1.9 times) higher electrooptic coefficients than polymers containing unsubstituted chromophores. Chromophores with hydroxyl groups on only the donor end of the molecule were processed into hardened "themoset" materials, exhibiting a dynamic thermal stability (temperature at which the macroscopic optical nonlinearity is first observed to decrease in a thermal ramping experiment) of poling-induced dipolar order of ∼80 °C, while chromophores with hydroxyl groups on both the donor and acceptor ends display a dynamic thermal stability of poling-induced acentric order of ∼110 °C. Optical loss in these materials originates from the scattering of incident light by chromophore aggregates, rather than from intrinsic (resonant) optical absorption of the chromophores. Alkyl groups in the modified chromophores serve as "bumpers" to sterically prevent aggregation of chromophores driven by strong, spatially anisotropic, intermolecular electrostatic interactions. Steric inhibition of aggregation serves not only to enhance poling efficiency (and thus, electrooptic coefficients) but also to minimize scattering losses.

We present a novel post-fabrication laser trimming technique to adjust the power splitting ratio of strip waveguide Y-branches made in thermally crosslinked electro-optic polymers. The trimming is based on the irreversible index change due to photobleaching. Our method uses simple equipment and the process takes only a few seconds. Waveguides made by both reactive ion etching and photobleaching are trimmable. An adjustable range of the splitting ratio as wide as is achieved with less than 0.2 dB of excess loss. This in situ trimming technique is effective for both the TE and TM modes of the waveguide and is very suitable for automated device processing.

No abstract