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.
Second harmonic generation in Langmuir-Blodgett multilayers of stilbazolium chloride polyethers
Abstract Multilayer Langmuir-Blodgett (LB) films of a new dye-substitutted polymer, dialkylaminostilbazolium poly(epichlorohydrin) are studied with second harmonic generation (SHG) and UV-visible absorption techniques. The synthesis resulted in a nearly quantitatively substituted stilbazolium chloride polyether (stilbazolium-PECH) with a “comb” structure. The dye can be compressed up to 47 mNm −1 to form a stable monolayer that is easily transferred to glass supports, and can be interleaved with behenic acid to form non-centrosymmetric structures. The second-order susceptibility of the stilbazolium-PECH is comparable in magnitude with that of a well-known hemicyanine dye and is thus potentially a useful material for fabricating films for non-linear optics. Multilayers of stilbazolium-PECH interleaved with behenic acid display enhancement of the SHG efficiency, although not quadratic. Unlike the hemicyanine dye, stilbazolium-PECH does not appear to form H aggregates in LB films. We have also investigated the effect of varying the number of behenic acid spacer layers between each pair of dye layers on SHG intensity and absorbance.
Determination of the nonlinear optical susceptibility χ(2) of surface layers by sum and difference frequency generation in reflection and transmission
The theoretical investigation of sum and difference frequency generation in thin surface layers with rotational symmetry leads to formulas which connect the generated light intensities to the surface second order nonlinear susceptibility tensor. A maximum of seven tensor components can be determined in the case of lowest symmetry. Measurements in transmission should be especially useful since they allow easy variation of both polarization and angle of incidence. On the other hand, large signal enhancements are expected for total internal reflection geometries. A consistent set of χ(2) tensor components for a thin layer of rhodamine-6G adsorbed on fused silica is found based on data from reflection and transmission measurements.