Abstract

We have analyzed the evolution of the photorefractive phase shift and the photoconductivity in three fully functionalized photorefractive polymethacrylates with increasing temperature and plasticizer concentration. Apart from a strong increase in refractive index modulation amplitude, the polymers show a larger phase shift and photoconductivity as the measurement temperature is raised. When the measurement temperature is kept at 21 °C, the photorefractive performance can be improved by reducing the glass transition temperature of the polymers by adding an external plasticizer. Two types of plasticizer were used. When doped with 10 or 20 wt% of an inert plasticizer (IP), diffraction efficiencies and gain coefficients are improved substantially. Due to a dilution of the charge transporting groups, however, the photoconductivities and the phase shifts start to decrease when more than 10 wt% of inert plasticizer is added. A detailed analysis of this behaviour is presented. When the carbazole-functionalized copolymers with carbazole as the only charge transporting group are doped with 20 wt% of N-ethylcarbazole, the concentration of the charge transport molecules is increased. Together with the lowering of T_g, this produces a larger phase shift. In all experiments, a good correlation between photoconductivity and photorefractive phase shift was observed.