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.
Enhancement and electric charge-assisted tuning of nonlinear light generation in bipolar plasmonics.
We propose and experimentally demonstrate a new plasmonic nonlinear light generation (NLG) structure, termed plasmonic-enhanced, charge-assisted second-harmonic generator (p-CASH), that not only achieves high second-harmonic generation (SHG) enhancement (76-fold), large SHG tunability by bias (8%/V), wide tuning range (280%), 7.8 × 10(-9) conversion efficiency, and high stability but also exhibits a SHG tuning, that is bipolar rather than unipolar, not due to the third-order nonlinear polarization term, hence fundamentally different from the classic electric field induced SHG-tuning (EFISH). We propose a new SHG tuning mechanism: the second-order nonlinear polarization term enhanced by plasmonic effects, changed by charge injection and negative oxygen vacancies movement, and is nearly 3 orders of magnitude larger than EFISH. p-CASH is a bipolar parallel-plate capacitor with thin layers of plasmonic nanostructures, a TiOx (semiconductor and nonlinear) and a SiO2 (insulator) sandwiched between two electrodes. Fabrication of p-CASH used nanoimprint on 4″ wafer and is scalable to wallpaper-sized areas. The new structure, new properties, and new understanding should open up various new designs and applications of NLG in various fields.
Inequality trends in health and future health risk among English children and young people, 1999–2009
Objective To investigate trends in health inequality among children and young people between 1999 and 2009, using outcomes consistent with the current NHS reforms. Design/data Secondary analysis of participants aged 0–24 in the Health Surveys for England (HSE) undertaken in 1999, 2004, 2006 and 2009. Main outcome measures Changes in the absolute and relative risks of four health outcomes by deprivation tertiles, based on occupation of the head of household ▸ self/parent-reported general health, ▸ presence of a long-standing illness (LSI), ▸ obesity ▸ smoking. Results No indicator showed a reduction in relative or absolute inequality between 1999 and 2009. For children (0–12 years), the relative risk comparing the most and least deprived tertiles increased significantly for poor general health (1999:1.6 (95% CI 1.2 to 2.2); 2009:3.9 (2.4 to 6.2), while the absolute difference in LSI prevalence(%) increased from 1.3 (−2.9 to 5.5) to 7.4 (3.6 to 11.4). Among young people (13–24 years), the absolute difference in LSI prevalence increased from −5.9 (−10.9 to −1.1) to 3.1 (−4.1 to 10.7). Absolute inequality in having tried smoking among children aged 8–15(%) increased significantly in the first half of the decade before decreasing in the second half (1999:3.3 (−1.1 to 7.7); 2004:14.1 (9.6 to 18.8); 2009:4.1 (0.1 to 8.8)). However, the increase in absolute inequality for smoking prevalence among young adults (16–24 years) was maintained throughout the decade (1999:−7.0 (−15.6 to 1.3); 2004:11.6 (3.7 to 20.0); 2009:8.2 (−0.3 to 16.9)). Conclusions The national programme between 1999 and 2009 was not successful in reducing inequality in four key indicators of health status and future health risk among children and young people. Some inequality measures for general health, LSI prevalence and smoking increased over this time.