Poling dynamics and relaxation behavior of the second harmonic generation signal from tricyanothiophene derivatives in PMMA

The effects of surface and trapped charge on the behavior of the relaxation and stability of a contact-poled nonlinear optical (NLO) polymer have been investigated. The second harmonic generation signal as well as the poling current in thin films of polymethylmethacrylate doped with a tricyanothiophene derivative chromophore was simultaneously measured. The relaxation behavior is correlated with the effect of trapped charge. The build-up of trapped charge is shown to be related to a memory effect in which the SHG relaxation time gradually increases as the polymer is subjected to multiple polings. It is also demonstrated that the charges trapped in the polymer matrix are released when the polymer is heated to temperatures approaching the glass transition temperature. The dynamics of the release of the trapped charge was found to depend upon the heating rate.