Dynamics and steady-state properties of photorefractive poly(N-vinylcarbazole)-based composites sensitized with (2,4,7-trinitro-9-fluorenylidene)malononitrile in a 0-3 wt % range

This paper reports on the characterization of six low-T-g poly(N-vinylcarba zole)-based photorefractive (PR) composites sensitized with (2,4,7-trinitro -9-fluorenylidene)-malononitrile (TNFM) in different concentrations, rangin g from 0 to 3 wt %. At 780 nm, two-beam coupling gain coefficients, four-wa ve mixing diffraction efficiencies, and photoconductivities were measured v ersus electric field, writing beam intensity, and temperature. Dynamic meas urements pointed out that chromophore reorientation is not rate-limiting in any of the six samples. In samples with sensitizer concentrations up to 1. 24 wt %, increasing the sensitizer concentration leads to a faster grating buildup through a faster charge generation. The grating buildup in these sa mples is rate- limited by the photogeneration speed. We provide evidence th at the TNFM- anions, formed by photoreduction of TNFM, can act as a trap, s imilar to what has been observed in C-60-sensitized samples. As a result, a bove 1.49 wt % of TNFM, the larger amount of traps produced by photoreducti on of the sensitizer reduces the mobility of the charges. Then, the grating buildup speed becomes mobility limited, and smaller buildup rates are obse rved. Except for the sample with 3 wt % TNFM, increased writing beam intens ities or sensitizer concentrations give rise to a larger dynamic range. The different behavior of the sample with the largest sensitizer concentration is explained theoretically in terms of the trap density. The PR phase shif ts were found to decrease with increasing writing beam intensity or sensiti zer concentration. This provides evidence that the PR phase shifts are cont rolled by the charge mobility rather than by the photogeneration efficiency . PR measurements as a function of temperature and electric field evolve as predicted by theory. (C) 2001 American Institute of Physics.