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F.J.ARANDA*, C.F. CHENG*, D.V.G.L.N. RAO*, J.A. AKKARA**, D.L. KAPLAN**, AND J.F. ROACH** *Physics Department, University of Massachusetts, Boston, MA 02125 **US Army Natick Research,Development and Engeneering Center, Natick, MA 01760 ABSTRACT Polymers of Benzidine were synthesized by hydrogen peroxide reaction catalyzed by horseradish peroxidase enzyme. The polymerization reaction was carried out at room temperature in a monophasic organic solvent with a small amount of water at pH 7.5. The technique of degenerate four-wave mixing (DFWM) with nanosecond and picosecond pulses was employed to measure the third-order nonlinear optical susceptibility X( 3 ). The samples were studied in solution in Dimethyl sulfoxide:Methanol in volume ratio 4:1. The observed values are of order 10-9 to 10-8esu. Measurements on a thin film agree approximately with the extrapolated values fron solution measurements. Picosecond time resolved measurements indicate a pulse-width limited response followed by a small slow component. Investigation of the total energy transmission as a function of the incident intensity and fluence at 532 nm for pico- and nanosecond pulses indicates reverse saturable absorption. As we observe the nanosecond and picosecond curves to be superimposed for the intensity plot but not for fluence, we conclude that the nonlinearity is predominantly due to two-photon absorption. Numerical analysis of the data yields a value of 12.25 cm/GW for the two-photon absorption coefficient 13.The imaginary component of X(3) obtained is 5 x 10-9 esu. The material appears to be a good candidate for applications in optical power limiting and switching. INTRODUCTION Organic compounds with 7r electron delocalization are receiving much attention 1 in recent years in an effortto identify materials with large third-order nonlinearity and fastresponse forvarious applications in switching, optical information processing, etc. Polymers have well known fabrication advantages including molding and fiber forming by injection extrusion, and thin film fabrication by spin coating or plasma deposition.The origin of the enormous variety of these 7r-electron systems comes from the characteristic valence four of the carbon atom. The conjugation arises from the electrons in p-orbitals which do not participate in sp-hybridization to form the skeleton of the saturated bonds. It is therefore indicated by purely steric and topological consideration that carbon conjugated polymeric structures can be either one- or two-dimensional. That means that conjugation can notbe present if the four electrons are involved in four different bonding directions as a diamond structure. It is known that conjugated 7r-electron backbone structure in general enhances the

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Mat. Res. Soc. Symp. Proc. Vol. 374 ©1995 Materials Research Society

nonlinearity oforganic materials. Sometheoretical issues like the dependence of X(3) upon the extent of electron delocalization are discussed in the literature. In this paper we report measurements of third-order nonlinear properti