ELECTRIC FIELD-DEPENDENT NONPHOTOREFRACTIVE GRATINGS IN A NONLINEAR PHOTOCONDUCTING POLYMER

The molecularly doped polymer system polyvinylcarbazole:Lophine 1:2,4, 7-trinitro-9-fluorenone, which is both photoconductive and optically n onlinear, forms a grating whose diffraction efficiency is strongly dep endent on the externally applied electric field. Two-beam coupling mea surements show no energy transfer between the beams and grating transl ation measurements and reveal an index of refraction grating that is n ot phase shifted relative to the light intensity pattern at any value of the externally applied field. These results lead to the conclusion that, even though diffraction is field dependent, the grating is not p hotorefractive in origin. This illustrates the importance of two-beam coupling measurements for characterization of potentially photorefract ive polymers. A possible origin of the nonphotorefractive grating base d on quadratic electroabsorption is discussed.