Microscopic nonlinearities of two-component organic crystals

We determine the molecular first-order hyperpolarizability β of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST, βzzz=(1540±250)×10-40 m4/V at λ = 1542 nm) and the cocrystal 4-{2-[1-(2-hydroxyethyl)-4-pyridylidene]-ethylidene}-cyclo-hexa-2,5-dien-1-one)–(methyl 2,4-dihydroxybenzoate) (M2-MDB, βzzz=(1510±250)×10-40 m4/V at λ = 1907 nm) in solution. These values are compared with the molecular first-order hyperpolarizabilities in the solid state, calculated with the measured nonlinear optical coefficient of the crystals. For both materials we show that the molecular nonlinearity in the solid state is considerably smaller than in solution. This lowering of the nonlinearity is attributed to the relatively strong Coulomb interactions in DAST and to the very strong hydrogen bonding in M2-MDB.