DESIGN OF OPTIMIZED PHOTOREFRACTIVE POLYMERS - A NOVEL CLASS OF CHROMOPHORES

It is demonstrated that the microscopic mechanism of the photorefracti ve (PR) effect in organic composites with; low glass transition temper atures involves the formation of refractive index gratings through a,s pace-charge held-modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpr etation is presented. The second-order dipole orientation term contain ing the anisotropy of the first-order optical polarizability alpha(-om ega;omega) is shown to yield the dominant contribution to the Kerr sus ceptibility chi((3))(-omega;omega,0,0). a class of special chromophore s having negligible second-order polarizabilities: beta(-omega;omega,0 ) and large dipole moments mu has been identified in order to optimize this term. These chromophores are not subject to the efficiency-trans parency tradeoff typically encountered with second-order nonlinear opt ical (NLO) chromophores, providing highly transparent materials with l arge PR Kerr response. Contrary to previous approaches in this field, the best-performing PR polymers are then expected to employ chromophor es that would be-useless for second-order applications (negligible bet a). We report PR of the material 30% 2,6-di-n-propyl-4H-pyran-4-yliden emalononitrile (DPDCP): 15% N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl)b enzidine (TPD):55% poly(methyl methacrylate) (PMMA):0.3% C-60 as an il lustration of this principle. A 100 mu m thick film of this material e xhibits a steady-state diffraction efficiency of eta=25% and net two-b eam coupling of Gamma=50 cm(-1) at a bias field of 100 V/mu m and a wa velength of 676 nm. The macroscopic Kerr susceptibility of the materia l is related to molecular electronic properties of the chromophore DPD CP which were independently determined by experiments in solution and by quantum chemical calculations. (C) 1996 American Institute of Physi cs.