Nonlinear Optics in Novel Polymer Systems.

Polymeric nonlinear optical (NLO) materials have recently attracted considerable attention and been the subject of intensive investigations. Polymeric NLO materials possessing large second and third order NLO properties, ultrafast response times, high optical damage threshold, transparency over a broad wavelength range, and capability to be easily processed into good optical quality thin films, offer significant advantages over the traditional inorganic materials for applications in fabricating integrated optical devices, such as waveguide electro-optic (EO) modulators and optical frequency doublers, and optical signal processing devices. This dissertation presents the experimental investigations on novel NLO polymers synthesized in the Laboratory of Electronic and Photonic Materials at University of Massachusetts Lowell. Progress made for the past few years on polymeric NLO materials is reviewed, especially with regard to the second order NLO properties of the polymeric materials. Two novel stable second order NLO polymer systems, an interpenetrating polymer network (IPN) formed via thermal crosslinking and a sol-gel process, and a photocrosslinkable conducting polymer, upon poling and crosslinking, exhibited large and stable second order NLO properties measured for these polymers by using the second harmonic generation (SHG) technique. For the IPN system, the SHG measurements as a function of time at several elevated temperatures indicate the superb stability of the second order NLO properties. For the conducting NLO polymer, the NLO property of the poled and photocrosslinked polymer film is stable at room temperature. The wavelength shifting of a Q-switched Nd:YAG laser by stimulated Raman scattering is also described. Measurements were made on the third order NLO properties of a dye doped photocrosslinkable guest-host polymer system at different dye concentrations with a modified Michelson interferometer. By functionalizing the dye to make it more compatible to the polymer host, no evidence of phase segregation was observed for dye concentrations up to 67 wt %. Photoconductivity in three photocrosslinkable second order NLO polymers was studied. Optical excitation of the NLO chromophores is the origin of photocarrier generation and the donor groups in the NLO chromophores are conjectured to participate in the charge transport process as well. These photoconducting second order NLO polymers promise to exhibit the photorefractive effect. Photovoltage generation in a poled and photocrosslinked guest-host polymer system was also studied. The physical origin of this photovoltaic effect is a change in the polarization of the medium due to photoexcitation. A novel approach to achieve practical photovoltaic devices is presented. Photoinduced orientation and birefringence were observed, and thermally erasable holographic gratings with long term stability were demonstrated in a photoresponsive second order NLO polymer for reversible optical data storage.