IMPROVED 2ND-HARMONIC GENERATION FROM LANGMUIR-BLODGETT-FILMS OF HEMICYANINE DYES

THE phenomenon of second-harmonic generation (SHG), whereby a material under illumination generates light at twice the incident frequency, is finding increasing use in optical signal processing. The principal requirement for SHG is a non-centrosymmetric structure; in organic materials, this can be achieved through the use of Langmuir-Blodgett films, which offer control of structure at the molecular level. SHG has been demonstrated in films composed of hemicyanine dyes of the general formula D-C6H4-CH = CH-C5H4N+-RX-, where D is an electron donor, R is usually a hydrophobic alkyl chain and X- is a counterion such as Br- or I-. The frequency-doubling properties of these films are sensitive to the choice of donor group 1,2, the extent of molecular aggregation 3,4 and the type of packing 5-7. Mixed films, in which the dye molecules are interspersed with an inert phase, have been used to reduce aggregation and consequently enhance SHG 3,8, but these films have the potential disadvantage of phase separation. Here we show that the use of an amphiphilic anion as both counterion and spacer molecule gives rise to an ordered segregation of the hemicyanine chromophores, and greatly enhanced SHG.