Synthesis and Characterization of New Photorefractive Polymers with High Glass Transition Temperatures

Polyamides based on 2‘,5‘-diamino-4-(dimethylamino)-4‘-nitro-stilbene (DDANS) and aliphatic diacids represent a well-established class of polymers for nonlinear optical (NLO) applications, in which the NLO units are fixed in the polymer backbone with their dipole moments oriented transversly to the polymer main chain. The introduction of a hole transport (HT) agent, [4-((diphenylhydrazono)methyl)phenyl]diethylamine (DEH) or 4-(bis(2-hydroxyethyl)amino)benzaldehyde 1,1-diphenylhydrazone (BBDH), was found to generate or enhance photorefractivity in these materials. Using appropriate chemical pathways, the NLO and HT moieties were connected in various ways, and polymer systems with dramatically different properties were obtained. Thus, (i) polyamides based on DDANS and aliphatic diacids, (ii) polyamide esters based on DDANS, BBDH, and adipic acid, (iii) polymer blends of the DDANS polyamides and a polyester based on BBDH and adipic acid, and (iv) DDANS polyamides doped with DEH have been prepared. All types of polymer systems were processed into transparent thin films by spin coating and could be oriented at elevated temperatures, applying a corona discharge poling technique. Second-order NLO coefficients (d 33) of up to 45 pm/V at a fundamental wavelength of 1542 nm have been measured. The poling efficiency was found to depend strongly on the content of the hydrazone moiety and the polymer system, decreasing with increasing content of hydrazone. Preliminary temperature-dependent two-beam coupling experiments demonstrated photorefractivity for DDANS-based systems, even at temperatures below the glass transition.