Organic Salts with Large Second-Order Optical Nonlinearities

This paper presents a review of recent work on the development of organic salts for second-order nonlinear optical applications. In particular, salts in which the cation has been designed to have a large molecular hyperpolarizability and in which variation of the counterion facilitates preparation of crystals with the required noncentrosymmetric packing are discussed. In many cases, this approach has led to materials with large powder second harmonic generation (SHG) efficiencies. One salt, N,N-dimethylamino-N'-methylstilbazolium p-toluenesulfonate, DAST, exhibited an SHG efficiency >1000 times that of a urea powder reference. A common layered-polar-sheet crystal packing motif was observed for DAST and several salt crystals that have been examined crystallographically. A possible explanation for the high incidence of noncentrosymmetric packing in these structures is discussed. Finally, the growth and properties of DAST single crystals are reviewed.