SHG and orientation phenomena in chromophore DR1-containing polymer films

Corona poling of two different methacrylate-type co-polymers with the chromophore Disperse Red 1 methacrylate (MDR1) results in the orientation of molecular dipoles and gives a nonlinear optical response in the form of second harmonic generation (SHG). The intensity of the SHG signal was used to measure the effectiveness of poling-induced alignments, which depend also on fundamental order phenomena in the co-polymers. The two matrix polymers varied in length and polarity of their side chains, which affect strongly side chain tacticity and chromophore aggregation. In situ SHG measurements during and after poling showed that the relaxation rate in methacrylic acid (MAA) co-polymers could be lowered by heat-induced cross-linking of the polymer chains. Order parameters derived from SHG measurements were found to be greater than 4, even when no changes were recorded in the optical spectra after poling. When changes were observed in the optical spectra, it was found that these were due to a decrease in the molar extinction coefficient for hydroxy-propyl-methacrylate (HPMA) based co-polymers, and solely due to the poling-induced decrease in film thickness in methacrylic acid based co-polymers (MAA).

Research highlights
► DR1-containing methacrylate-type (MA) co-polymers were thermally poled.
► The resulting SHG signal gives order parameters greater than 4.
► Cross-linking reduces SHG-relaxation in methacrylic acid (MAA) co-polymers.
► Absorption changes in MAA-co-polymers reflect poling-induced film thickness changes.
► Poling reduces the molar extinction coefficient of hydroxy-propyl-MA co-polymers.