Effect of plasticizer and temperature on the photorefractive phase shift in fully functionalized polymethacrylates

We have analyzed the evolution of the photorefractive phase shift and the p hotoconductivity in three fully functionalized photorefractive polymethacry lates with increasing temperature and plasticizer concentration. Apart from a strong increase in refractive index modulation amplitude, the polymers s how a larger phase shift and photoconductivity as the measurement temperatu re is raised. When the measurement temperature is kept at 21 degreesC, the photorefractive performance can be improved by reducing the glass transitio n 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 plastici zer (IP), diffraction efficiencies and gain coefficients are improved subst antially. 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-ethylcarba zole, the concentration of the charge transport molecules is increased. Tog ether with the lowering of T-g, this produces a larger phase shift. In all experiments, a good correlation between photoconductivity and photorefracti ve phase shift was observed.