Second-order nonlinear optical properties of langmuir-blodget mono - and multilayers of diazostilbene and polyenic molecules

Second harmonic generation (SHG) experiments were performed on Langmuir-Blodgett (L — B) mono- and multilayers of various charge-transfer molecules. From SHG measurements on monolayers, we evidenced large ß values for diazostilbene molecules (11 x 10-38 SI) and polyene derivatives (35 x 10-38 SI). The significant increase of ß observed when going from the diazostilbene compounds to the polyenic molecules can be accounted for by a much better polarizability of the polyenic chain as compared to the stilbene structures. Various configurations in multilayers were also investigated, in order to compare their respective performances. The experimental variation of the emitted second harmonic intensity l2ω with respect to the number of active layers N was studied for Z - and Y - type multilayers of a diazostilbene derivative, and for Y - type alternate multilayers of two diazostilbene compounds. In the latter configuration, the expected quadratic dependence l2ω = k N2 is ensured for large N values. The depart from this theoretical quadratic behaviour for small N's can be accounted for by ordering and structural considerations.