Relaxation of Second-Harmonic Generation in Guest/Host Polymers Poled by Indium-Tin Oxide Sandwich Electrodes

Second-harmonic generation (SHG) is used to monitor reorientation dynamics of 4-(dimethylamino)- 4′-nitrostilbene doped at 1 wt % in poly(ethyl methacrylate) within a temperature range of T_G ± 30 K. Special emphasis is put on a systematic analysis of experimental conditions and their severe impact on SHG data for samples which are poled in an indium-tin oxide (ITO)-sandwich-electrode configuration. These effects include thermal history, local heating effects due to laser focus, importance of dopant concentration, physical aging, details of the poling procedure, and charge-injection effects. The fast initial response of the SHG signal upon field switching is attributed to space charge effects. We show how the problem of SHG reproducibility can be overcome with appropriate experimental conditions. Analysis of the reliable data is based on a Gaussian distribution of energy barriers for the dopant relaxation which yields superior fits compared to a stretched exponential function. The Arrhenius-like behavior of the mean SHG decay rate points toward a decoupling of the dopant motion from the bulk α-process of the polymer host. Additionally, a novel repoling technique and its implications are discussed.