1,538 research outputs found
Multidimensional optical fractionation with holographic verification
The trajectories of colloidal particles driven through a periodic potential
energy landscape can become kinetically locked in to directions dictated by the
landscape's symmetries. When the landscape is realized with forces exerted by a
structured light field, the path a given particle follows has been predicted to
depend exquisitely sensitively on such properties as the particle's size and
refractive index These predictions, however, have not been tested
experimentally. Here, we describe measurements of colloidal silica spheres'
transport through arrays of holographic optical traps that use holographic
video microscopy to track individual spheres' motions in three dimensions and
simultaneously to measure each sphere's radius and refractive index with
part-per-thousand resolution. These measurements confirm previously untested
predictions for the threshold of kinetically locked-in transport, and
demonstrate the ability of optical fractionation to sort colloidal spheres with
part-per-thousand resolution on multiple characteristics simultaneously.Comment: 4 pages, 2 figures. Accepted for publication in Physical Review
Letter
Tungsten nuclear rocket, phase II, part 1 Final report, Jan. 16 - Jun. 15, 1966
Critical experiments and nuclear analyses of tungsten water moderated nuclear rocket reacto
Colloidal Electrostatic Interactions Near a Conducting Surface
Charge-stabilized colloidal spheres dispersed in deionized water are supposed
to repel each other. Instead, artifact-corrected video microscopy measurements
reveal an anomalous long-ranged like-charge attraction in the interparticle
pair potential when the spheres are confined to a layer by even a single
charged glass surface. These attractions can be masked by electrostatic
repulsions at low ionic strengths. Coating the bounding surfaces with a
conducting gold layer suppresses the attraction. These observations suggest a
possible mechanism for confinement-induced attractions.Comment: 4 pages, 2 figure
Mie scattering by a charged dielectric particle
We study for a dielectric particle the effect of surplus electrons on the
anomalous scattering of light arising from the transverse optical phonon
resonance in the particle's dielectric constant. Excess electrons affect the
polarizability of the particle by their phonon-limited conductivity, either in
a surface layer (for negative electron affinity) or the conduction band (for
positive electron affinity). We demonstrate that surplus electrons shift an
extinction resonance in the infrared. This offers an optical way to measure the
charge of the particle and thus to use it in a plasma as a minimally invasive
electric probe.Comment: 5 pages, 5 figures, accepted manuscrip
Photoemission Electron Microscopy as a tool for the investigation of optical near fields
Photoemission electron microscopy was used to image the electrons
photoemitted from specially tailored Ag nanoparticles deposited on a Si
substrate (with its native oxide SiO). Photoemission was induced by
illumination with a Hg UV-lamp (photon energy cutoff eV,
wavelength nm) and with a Ti:Sapphire femtosecond laser
( eV, nm, pulse width below 200 fs),
respectively. While homogeneous photoelectron emission from the metal is
observed upon illumination at energies above the silver plasmon frequency, at
lower photon energies the emission is localized at tips of the structure. This
is interpreted as a signature of the local electrical field therefore providing
a tool to map the optical near field with the resolution of emission electron
microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter
Theory of Optical Tweezers
We derive a partial-wave (Mie) expansion of the axial force exerted on a
transparent sphere by a laser beam focused through a high numerical aperture
objective. The results hold throughout the range of interest for practical
applications. The ray optics limit is shown to follow from the Mie expansion by
size averaging. Numerical plots show large deviations from ray optics near the
focal region and oscillatory behavior (explained in terms of a simple
interferometer picture) of the force as a function of the size parameter.
Available experimental data favor the present model over previous ones.Comment: 4 pages, 3 figure
Interactions between sub-10 nm iron and cerium oxide nanoparticles and 3T3 fibroblasts : the role of the coating and aggregation state
Recent nanotoxicity studies revealed that the physico-chemical
characteristics of engineered nanomaterials play an important role in the
interactions with living cells. Here, we report on the toxicity and uptake of
the cerium and iron oxide sub-10 nm nanoparticles by NIH/3T3 mouse fibroblasts.
Coating strategies include low-molecular weight ligands (citric acid) and
polymers (poly(acrylic acid), MW = 2000 g mol-1). Electrostatically adsorbed on
the surfaces, the organic moieties provide a negatively charged coating in
physiological conditions. We find that most particles were biocompatible, as
exposed cells remained 100% viable relative to controls. Only the bare and the
citrate-coated nanoceria exhibit a slight decrease of the mitochondrial
activity for cerium concentrations above 5 mM (equivalent to 0.8 g L-1). We
also observe that the citrate-coated particles are internalized by the cells in
large amounts, typically 250 pg per cell after a 24 h incubation for iron
oxide. In contrast, the polymer-coated particles are taken up at much lower
rates (< 30 pg per cell). The strong uptake shown by the citrate-coated
particles is related to the destabilization of the dispersions in the cell
culture medium and their sedimentation down to the cell membranes. In
conclusion, we show that the uptake of nanomaterials by living cells depends on
the coating of the particles and on its ability to preserve the colloidal
nature of the dispersions.Comment: 9 figures, 2 table
Light scattering by an elongated particle: spheroid versus infinite cylinder
Using the method of separation of variables and a new approach to
calculations of the prolate spheroidal wave functions, we study the optical
properties of very elongated (cigar-like) spheroidal particles. A comparison of
extinction efficiency factors of prolate spheroids and infinitely long circular
cylinders is made. For the normal and oblique incidence of radiation, the
efficiency factors for spheroids converge to some limiting values with an
increasing aspect ratio a/b provided particles of the same thickness are
considered.
These values are close to, but do not coincide with the factors for infinite
cylinders. The relative difference between factors for infinite cylinders and
elongated spheroids (a/b \ga 5) usually does not exceed 20 % if the following
approximate relation between the angle of incidence and
the particle refractive index m=n+ki takes the place: \alpha \ga 50 |m-1| + 5
where 1.2 \la n \la 2.0 and k \la 0.1. We show that the quasistatic
approximation can be well used for very elongated optically soft spheroids of
large sizes.Comment: 12 pages, 7 figures, Accepted by Measurement Science and Technology
(special OPC issue
Depolarization volume and correlation length in the homogenization of anisotropic dielectric composites
In conventional approaches to the homogenization of random particulate
composites, both the distribution and size of the component phase particles are
often inadequately taken into account. Commonly, the spatial distributions are
characterized by volume fraction alone, while the electromagnetic response of
each component particle is represented as a vanishingly small depolarization
volume. The strong-permittivity-fluctuation theory (SPFT) provides an
alternative approach to homogenization wherein a comprehensive description of
distributional statistics of the component phases is accommodated. The
bilocally-approximated SPFT is presented here for the anisotropic homogenized
composite which arises from component phases comprising ellipsoidal particles.
The distribution of the component phases is characterized by a two-point
correlation function and its associated correlation length. Each component
phase particle is represented as an ellipsoidal depolarization region of
nonzero volume. The effects of depolarization volume and correlation length are
investigated through considering representative numerical examples. It is
demonstrated that both the spatial extent of the component phase particles and
their spatial distributions are important factors in estimating coherent
scattering losses of the macroscopic field.Comment: Typographical error in eqn. 16 in WRM version is corrected in arxiv
versio
An analytical model for the detection of levitated nanoparticles in optomechanics
Interferometric position detection of levitated particles is crucial for the
centre-of-mass (CM) motion cooling and manipulation of levitated particles. In
combination with balanced detection and feedback cooling, this system has
provided picometer scale position sensitivity, zeptonewton force detection, and
sub-millikelvin CM temperatures. In this article, we develop an analytical
model of this detection system and compare its performance with experimental
results allowing us to explain the presence of spurious frequencies in the
spectra
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