Co-crystal containing a merocyanine dye, perfectly aligned for electro-optics

For high speed photonic applications there is a great interest for materials with large electrooptic coefficients. Due to the low dielectric constant and the mainly electronic contribution to the electro-optic effect, organic materials are especially suited. In crystals the highest possible density and best alignment of the chromophores can be achieved. The technique of co-crystallizing a nonlinear chromophore with a guest molecule allowed us to force crystallization in a perfect arrangement for electro-optics by the use of short hydrogen bonds. An example of such a crystal is the highly nonlinear merocyanine chromophore (4-{2-[1-(2-hydroxyethyl)-4-pyridylidene]- ethylidene}-cyclo-hexa-2,5-dien-1-one) co-crystallized with MDB (methyl 2,4-dihydroxybenzoate) (Fig. 1). Considering the microscopic nonlinearity of the chromophores and their alignment, one can expect an electro-optic coefficient of several hundred pm/V. We determined the electro-optic coefficient as high as (41 ± 4) pm/V at a wavelength of 1064 nm. The origin of this nonlinearity can be interpreted by a combination of the molecular nonlinearity and a distortion of the electron distribution caused by the strong hydrogen bonding between the molecules. To underlie this we will present experimental results the electro-optic and electro-absorptive wavelength dispersion as well as infrared spectroscopy.