Improved second-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 group1,2 the extent of molecular aggregation3,4and the type of packing5–7 Mixed films, in which the dye molecules are interspersed with an inert phase, have been used to reduce aggregation and consequently enhance SHG3,8, 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.
Absorption and second-harmonic generation of monomer and aggregate hemicyanine dye in Langmuir-Blodgett films.
Absorption spectra and second-harmonic generation of Langmuir–Blodgett monolayers containing a hemicyanine dye with a large second-order hyperpolarizability are reported. The dye is found to be in the form of H aggregates in a pure-dye film and mostly monomeric in a 1:4 dye–arachidic acid film. This observation explains a report in the literature that mixed dye–arachidic acid films display greater second-harmonic generation efficiency than a pure-dye film. It may also be a factor in the reported failure of second-harmonic generation to increase quadratically with the number of layers in noncentrosymmetric multilayer films.
Hemicyanine monolayer orientation studied by second-harmonic generation.
Using a novel scheme of polarization-dependent excitation and detection, the molecular orientation of hemicyanine monolayers has been studied. Wavelength-dependent measurements of the average tilt angle of molecules within the monolayer reveal details of aggregate formation and mobilization of these aggregates by addition of diluting agents such as arachidic acid.