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.
Efficient phase-matched second-harmonic generation of blue light in an organic waveguide
THE development of materials for the efficient frequency-doubling of low-power semiconductor lasers is a technologically important goal for which several important challenges remain. The ease of processing and high intrinsic optical nonlinearity of organic materials1,2 makes them attractive for such applications, and it has been demonstrated that molecular-assembly techniques can achieve the macroscopic non-centrosymmetric order required for bulk nonlinear optical activity3,4. But to achieve efficient power conversion, a promising material must also exhibit low attenuation of light at high power densities, and match phase velocities over relatively long distances for light at the fundamental and second-harmonic frequencies5. Here we report the fabrication by Langmuir– Blodgett deposition of a low-loss optical waveguide in which precise control of the film thickness, together with inversion of the nonlinear susceptibility across the film, are used to simultaneously achieve phase matching and improve the optical–field overlap between the propagating (fundamental and second-harmonic) waveguide modes. The resulting structure converts low-power near-infrared laser light efficiently to blue light.
Local public health agency performance and community health status.
This study describes the results of an analysis of the relationship between local public health agency performance and health outcomes. Findings support the model that local public health agency performance affects community health status, and depending on the outcome, performance contributions vary. New hypotheses are offered that could help determine how system performance affects health outcomes.