Large Cubic Nonlinear Optical Properties of Organic Semiconductor Superlattices
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LARGE CUBIC NONLINEAR OPTICAL PROPERTIES OF ORGANIC SEMICONDUCTOR SUPERLATTICES Samson A. Jenekhe*, Wen-Chang Chen*, Saukwan Lo** and Steven R. Flom*** *Department of Chemical Engineering, University of Rochester Rochester, New York 14627 "**Honeywell Systems and Research Center Minneapolis, Minnesota 55418 ***Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Abstract We have measured extremely large second hyperpolarizabilities(-yxxxx) in solutions of two recently prepared organic polymer semiconductor superlattices. These block copolymers are of alternating aromatic and quinoidal moieties and structurally differ by a side group substituent. The values observed are 1.6 x 10-29 esu for the parent copolymer (PBTBQ) and 3.7 x 10-30 esu for its acetoxy derivative (PBTABQ). The corresponding values of X(3) are estimated to be 2.7 x 10-7 esu and 4.5 x 10- 8 esu. The measurements, made by picosecond degenerate four wave mixing at 532 nm, showed that the dynamics of the larger X(3) valued copolymer were faster than the 30 ps resolution of the instrument while the derivative exhibited a slower response. The large magnitude and rapid response of the cubic optical nonlinearities in these novel materials suggest their potential for further development and photonic device applications. Introduction The origins of large nonlinear optical (NLO) responses and the development of materials for application in photonic devices is the focus of much of the current interest in the third order NLO properties of conjugated polymers" 9 . These NLO properties are macroscopically characterized through measurements of the third order electric susceptibility tensor, X(l). Magnitudes typically observed in most of the currently known conjugated polymers range from about 10-12 to 10-9 esu when probed using nonresonant conditions. This range is insufficient for most proposed, practical device applications.'° Several strategies can be followed to increase the third order NLO response. Near resonant or resonant enhancement increases the magnitude of X(3). For example, the three photon enhanced X(') of polyacetylene was recently 9 measured to be about 10-1 esu at 2.1 yrm (,0.6 eV). Other approaches being pursued to further enhance the magnitude of X(3) include derivatization of existing NLO polymers8 and preparation of new conjugated polymer structures that might enhance X(3) by optimization of the currently known critical molecular parameters or offer new mechanisms of optical nonlinearity. Here the focus will be the NLO properties of a new type of conjugated polymer structure. Much recent research has shown that conjugated polymers can be effectively modeled as quasi-one-dimensional semiconductors'"4. Recently1"-17, periodic multiblock conjugated copolymers were proposed as quasi-one--dimensional organic s emi condu cto r superlattices that may exhibit novel electronic, linear optical, electro-optical, and nonlinear optical properties. These structures have chemical formulas of (AXBy), where A and B are the repeating un
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