Microscopic nonlinearities of two-component organic crystals

We determine the molecular first-order hyperpolarizability beta of the high ly nonlinear organic salt 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tos ylate (DAST, beta (zzz) = (1540 +/- 250) x 10(-40) m(4)/V at lambda = 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, beta (zz z) = (1510 +/- 250) x 10(-40) m(4)/V at lambda = 1907 nm) in solution. Thes e values are compared with the molecular first-order hyperpolarizabilities in the solid state, calculated with the measured nonlinear optical coeffici ent of the crystals. For both materials we show that the molecular nonlinea rity in the solid state is considerably smaller than in solution. This lowe ring of the nonlinearity is attributed to the relatively strong Coulomb int eractions in DAST and to the very strong hydrogen bonding in M2-MDB. (C) 20 01 Optical Society of America.