A Combined EIS-NVSS Survey Of Radio Sources (CENSORS) III: Spectroscopic observations

The Combined EIS-NVSS Survey Of Radio Sources (CENSORS) is a 1.4GHz radio survey selected from the NRAO VLA Sky Survey (NVSS) and complete to a flux-density of 7.2mJy. It targets the ESO Imaging Survey (EIS) Patch D, which is a 3 by 2 square degree field centred on 09 51 36.0, -21 00 00 (J2000). This paper presents the results of spectroscopic observations of 143 of the 150 CENSORS sources. The primary motivation for these observations is to achieve sufficient spectroscopic completeness so that the sample may be used to investigate the evolution of radio sources. The observations result in secure spectroscopic redshifts for 63% of the sample and likely redshifts (based on a single emission line, for example) for a further 8%. Following the identification of the quasars and star-forming galaxies in the CENSORS sample, estimated redshifts are calculated for the remainder of the sample via the K-z relation for radio galaxies. Comparison of the redshift distribution of the CENSORS radio sources to distributions predicted by the various radio luminosity function evolution models of Dunlop & Peacock 1990, results in no good match. This demonstrates that this sample can be used to expand upon previous work in that field.Comment: Accepted for publication in MNRAS. This version has some reduced resolution figures and 13 associated gif files. A version with all figures incorporated (at full resolution) is available at http://www.roe.ac.uk/~pnb/papers/censors_spectro.pd

Orientation of biological cells using plane-polarized Gaussian beam optical tweezers

Optical tweezers are widely used for the manipulation of cells and their internal structures. However, the degree of manipulation possible is limited by poor control over the orientation of trapped cells. We show that it is possible to controllably align or rotate disc shaped cells - chloroplasts of Spinacia oleracea - in a plane polarised Gaussian beam trap, using optical torques resulting predominantly from circular polarisation induced in the transmitted beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure

Nanotrapping and the thermodynamics of optical tweezers

Particles that can be trapped in optical tweezers range from tens of microns down to tens of nanometres in size. Interestingly, this size range includes large macromolecules. We show experimentally, in agreement with theoretical expectations, that optical tweezers can be used to manipulate single molecules of polyethylene oxide suspended in water. The trapped molecules accumulate without aggregating, so this provides optical control of the concentration of macromolecules in solution. Apart from possible applications such as the micromanipulation of nanoparticles, nanoassembly, microchemistry, and the study of biological macromolecules, our results also provide insight into the thermodynamics of optical tweezers.Comment: 5 pages, 3 figures, presented at 17th AIP Congress, Brisbane, 200

Forces from highly focused laser beams: modeling, measurement and application to refractive index measurements

The optical forces in optical tweezers can be robustly modeled over a broad range of parameters using generalsed Lorenz-Mie theory. We describe the procedure, and show how the combination of experimental measurement of properties of the trap coupled with computational modeling, can allow unknown parameters of the particle - in this case, the refractive index - to be determined.Comment: 5 pages, 4 figures, presented at 17th AIP Congress, Brisbane, 200

The final two redshifts for radio sources from the equatorial BRL sample

Best, Rottgering and Lehnert (1999, 2000a) defined a new sample of powerful radio sources from the Molonglo Reference Catalogue, for which redshifts were compiled or measured for 177 of the 178 objects. For the final object, MRC1059-010 (3C249), the host galaxy is here identified using near-infrared imaging, and the redshift is determined from VLT spectroscopy. For one other object in the sample, MRC0320+053 (4C05.14), the literature redshift has been questioned: new spectroscopic observations of this object are presented, deriving a corrected redshift. With these two results, the spectroscopic completeness of this sample is now 100%. New redshifts are also presented for PKS0742+10 from the Wall & Peacock 2.7 GHz catalogue, and PKS1336+003 from the Parkes Selected Regions. PKS0742+10 shows a strong neutral hydrogen absorption feature in its Lyman-alpha emission profile.Comment: 4 pages. LaTeX. Accepted for publication in MNRA

Towards efficient modelling of optical micromanipulation of complex structures

Computational methods for electromagnetic and light scattering can be used for the calculation of optical forces and torques. Since typical particles that are optically trapped or manipulated are on the order of the wavelength in size, approximate methods such as geometric optics or Rayleigh scattering are inapplicable, and solution or either the Maxwell equations or the vector Helmholtz equation must be resorted to. Traditionally, such solutions were only feasible for the simplest geometries; modern computational power enable the rapid solution of more general--but still simple--geometries such as axisymmetric, homogeneous, and isotropic scatterers. However, optically-driven micromachines necessarily require more complex geometries, and their computational modelling thus remains in the realm of challenging computational problems. We review our progress towards efficient computational modelling of optical tweezers and micromanipulation, including the trapping and manipulation of complex structures such as optical micromachines. In particular, we consider the exploitation of symmetry in the modelling of such devices.Comment: 5 pages, 4 figure

Inorganic Surface Passivation of PbS Nanocrystals resulting in Strong Photoluminescent Emission

Strong photoluminescent emission has been obtained from 3 nm PbS nanocrystals in aqueous colloidal solution, following treatment with CdS precursors. The observed emission can extend across the entire visible spectrum and usually includes a peak near 1.95 eV. We show that much of the visible emission results from absorption by higher-lying excited states above 3.0 eV with subsequent relaxation to and emission from states lying above the observed band-edge of the PbS nanocrystals. The fluorescent lifetimes for this emission are in the nanosecond regime, characteristic of exciton recombination.Comment: Preprint, 23 pages, 6 figure