NONCENTROSYMMETRY AND 2ND-HARMONIC GENERATION IN Z-TYPE LANGMUIR-BLODGETT-FILMS

THE principal requirement for materials that exhibit second-harmonic g eneration-frequency-doubling of light-is that they have a non-centrosy mmetric structure. For optically active organic materials, this can be achieved by the use of the Langmuir-Blodgett (LB) technique1,2: amphi philic molecules comprising a hydrophilic head (the chromophore) and a hydrophobic tail are aligned at an air-water interface, and the resul ting ordered monolayer is transferred to an appropriate solid substrat e. For practical applications, the non-centrosymmetric structure of th e individual monolayers needs to be maintained across many such layers , but this has been achieved previously for relatively few optically a ctive molecules3-5. Most amphiphiles pack centrosymmetrically, with th e layers adopting a head-to-head and tail-to-tail 'Y-type' structure6. To enforce a non-centrosymmetric structure, it becomes necessary to i ntersperse the optically active layers with another material7-12. Here we demonstrate an alternative strategy for achieving macroscopic non- centrosymmetric order in LB multilayers. We find that the addition of a second hydrophobic end-group to the hydrophilic chromophore reduces the tendency of the molecules to invert during deposition, and a head- to-tail 'Z-type' structure can be readily obtained for films containin g more than 100 layers.