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In previous work, we have successfully developed techniques to fabricate low-loss rib waveguides in a crystalline organic compound, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA). Here, we demonstrate apparently the first optical directional coupler using an index-matching layer in similar crystalline organic semiconductor waveguides. We also observed switching of the guided light between the waveguides in the PTCDA couplers by pumping the surface of one waveguide in the coupling region with 0.514 micrometer wavelength light. A directional coupler typically consists of a pair of parallel channel waveguides with a gap between them. The gap width and the refractive index discontinuity between the guides and the gap are small enough to allow the guided light to couple from one waveguide to the other by the overlapping evanescent tails of the guided modes. However, there is a large refractive index discontinuity of 1.017 for TE modes at the interface between a PTCDA waveguide and an air gap at lambda = 1.06 micrometer. The large index discontinuity causes a small coupling coefficient and a long coupling length. The best way to increase the coupling coefficient between two rib waveguides is to fill the gap with a layer whose index is only slightly lower than the adjacent guides. In our experiments, we employed AZ 1400 photoresist as the index-matching layer material for PTCDA couplers. AZ 1400 photoresist has a relatively high index of 1.61 and low absorption at lambda = 1.06 micrometers. In this case, for a coupler having 0.5 micrometer thick, 2 micrometer wide waveguides and 1 micrometer gap, Kappa approx. - 0.50 mm-1, which is about 40 times greater than that for the coupler without an index-matching layer. Correspondingly, the coupling length is 3.14 mm., This Article is from 'Integrated Photonics Research'. Volume 10, AD-A255 423, p112-113.