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.
Large second-order optical polarizabilities in mixed-valency metal complexes
THE potential development of optoelectronic devices based on the nonlinear polarization of molecular materials has aroused much recent interest1,2. The search for large second-order electric susceptibilities (that is, proportional to the square of an applied electric field) has concentrated on acentric organic or organometallic chromophores with an organic π-electron system coupling electron donor and acceptor groups3–6. It is conceivable that mixed-valence compounds characterized by an intervalence charge-transfer (IVCT) transition7, in which the donor and acceptor centres are both metal atoms, might also have the potential to provide a large second-order response8, but this possibility has not been widely explored. Here we report the first hyperpolarizability, β, of a bimetallic complex ion, [(CN)5Ru-μCN-Ru(NH3)5]- (I in Fig. 1), and a novel organometallic analogue, [(η5-C5H5)Ru(PPh3)2-μ-CN-Ru(NH3)5]3+ (II). Measurements of β (which is related to the bulk second-order response) in solution at a wavelength of 1,064 nm using the newly developed hyper-Rayleigh scattering technique9,10 give values greater than 10–27 e.s.u., which are among the largest reported for solution species. The ease with which the energy of the IVCT transition can be modified suggests that there may be considerable potential for this class of chromophore in nonlinear optical devices.
The MR-TOF-MS isobar separator for the TITAN facility at TRIUMF
At TRIUMF’s Ion Trap for Atomic and Nuclear Science (TITAN) a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) will extend TITAN’s capabilities and facilitate mass measurements and in-trap decay spectroscopy of exotic nuclei that so far have not been possible due to strong isobaric contamination. This MR-TOF-MS will also enable mass measurements of very short-lived nuclides (half-life > 1 ms) that are produced in very low quantities (a few detected ions overall). In order to allow the installation of an MR-TOF-MS in the restricted space on the platform, on which the TITAN facility is located, novel mass spectrometric methods have been developed. Transport, cooling and distribution of the ions inside the device is done using a buffer gas-filled RFQ-based ion beam switchyard. Mass selection is achieved using a dynamic retrapping technique after time-of-flight analysis in an electrostatic isochronous reflector system. Only due to the combination of these novel methods the realization of an MR-TOF-MS based isobar separator at TITAN has become possible. The device has been built, commissioned off-line and is currently under installation at TITAN.