234 research outputs found
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
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
Two controversies in classical electromagnetism
This paper examines two controversies arising within classical electromagnetism which are relevant to the optical trapping and micromanipulation community. First is the Abraham-Minkowski controversy, a debate relating to the form of the electromagnetic energy momentum tensor in dielectric materials, with implications for the momentum of a photon in dielectric media. A wide range of alternatives exist, and experiments are frequently proposed to attempt to discriminate between them. We explain the resolution of this controversy and show that regardless of the electromagnetic energy momentum tensor chosen, when material disturbances are also taken into account the predicted behaviour will always be the same. The second controversy, known as the plane wave angular momentum paradox, relates to the distribution of angular momentum within an electromagnetic wave. The two competing formulations are reviewed, and an experiment is discussed which is capable of distinguishing between the two
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