Non-centrosymmetry and second-harmonic generation in Z-type Langmuir–Blodgett films

Abstract

THE principal requirement for materials that exhibit second-harmonic generation—frequency-doubling of light—is that they have a non-centrosymmetric structure. For optically active organic materials, this can be achieved by the use of the Langmuir–Blodgett (LB) technique^1,2: amphiphilic molecules comprising a hydrophilic head (the chromophore) and a hydrophobic tail are aligned at an air–water interface, and the resulting ordered monolayer is transferred to an appropriate solid substrate. For practical applications, the non-centrosymmetric structure of the individual mono-layers needs to be maintained across many such layers, but this has been achieved previously for relatively few optically active molecules^3–5. Most amphiphiles pack centrosymmetrically, with the layers adopting a head-to-head and tail-to-tail 'Y-type' structure⁶. To enforce a non-centrosymmetric structure, it becomes necessary to intersperse the optically active layers with another material^7–12. 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 containing more than 100 layers.