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 nonlinear, forms a grating whose diffraction efficiency is strongly dependent on the externally applied electric field. Two-beam coupling measurements show no energy transfer between the beams and grating translation measurements and reveal an index of refraction grating that is not 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 photorefractive in origin. This illustrates the importance of two-beam coupling measurements for characterization of potentially photorefractive polymers. A possible origin of the nonphotorefractive grating based on quadratic electroabsorption is discussed.