318 research outputs found
Acoustic black holes in a two-dimensional "photon-fluid"
Optical field fluctuations in self-defocusing media can be described in terms
of sound waves in a 2D photon-fluid. It is shown that, while the background
fluid couples with the usual flat metric, sound-like waves experience an
effective curved spacetime determined by the physical properties of the flow.
In an optical cavity configuration, the background spacetime can be suitably
controlled by the driving beam allowing the formation of acoustic ergoregions
and event horizons. An experiment simulating the main features of the rotating
black hole geometry is proposed.Comment: revised versio
Faraday spectroscopy of atoms confined in a dark optical trap
We demonstrate Faraday spectroscopy with high duty cycle and sampling rate
using atoms confined to a blue-detuned optical trap. Our trap consists of a
crossed pair of high-charge-number hollow laser beams, which forms a dark,
box-like potential. We have used this to measure transient magnetic fields in a
500-micron-diameter spot over a 400 ms time window with nearly unit duty cycle
at a 500 Hz sampling rate. We use these measurements to quantify and compensate
time-varying magnetic fields to ~10 nT per time sample.Comment: 6 pages, 8 figures Accepted in Phys. Rev.
Pinning and transport of cyclotron/Landau orbits by electromagnetic vortices
Electromagnetic waves with phase defects in the form of vortex lines combined
with a constant magnetic field are shown to pin down cyclotron orbits (Landau
orbits in the quantum mechanical setting) of charged particles at the location
of the vortex. This effect manifests itself in classical theory as a trapping
of trajectories and in quantum theory as a Gaussian shape of the localized wave
functions. Analytic solutions of the Lorentz equation in the classical case and
of the Schr\"odinger or Dirac equations in the quantum case are exhibited that
give precise criteria for the localization of the orbits. There is a range of
parameters where the localization is destroyed by the parametric resonance.
Pinning of orbits allows for their controlled positioning -- they can be
transported by the motion of the vortex lines.Comment: This version differs from the printed paper in having the full titles
of all referenced pape
Effects of spatial confinement on migratory properties of Dictyostelium discoideum cells.
Migratory environments of various eukaryotic cells, such as amoeba, leukocytes and cancer cells, typically involve spatial confinement. Numerous studies have recently emerged, aimed to develop experimental platforms that better recapitulate the characteristics of the cellular microenvironment. Using microfluidic technologies, we show that increasing confinement of Dictyostelium discoideum cells into narrower micro-channels resulted in a significant change in the mode of migration and associated arrangement of the actomyosin cytoskeleton. We observed that cells tended to migrate at constant speed, the magnitude of which was dependent on the size of the channels, as was the locomotory strategy adopted by each cell. Two different migration modes were observed, pseudopod-based and bleb-based migration, with bleb based migration being more frequent with increasing confinement and leading to slower migration. Beside the migration mode, we found that the major determinants of cell speed are its protrusion rate, the amount of F-actin at its leading edge and the number of actin foci. Our results highlighted the impact of the microenvironments on cell behavior. Furthermore, we developed a novel quantitative movement analysis platform for mono-dimensional cell migration that allows for standardization and simplification of the experimental conditions and aids investigation of the complex and dynamic processes occurring at the single-cell level
Dye lasing in optically manipulated liquid aerosols
We report lasing in airborne, rhodamine B-doped glycerol-water droplets with diameters ranging between 7.7 and 11.0 mu m, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of the intensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by changing the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium
Landau levels of cold atoms in non-Abelian gauge fields
The Landau levels of cold atomic gases in non-Abelian gauge fields are
analyzed. In particular we identify effects on the energy spectrum and density
distribution which are purely due to the non-Abelian character of the fields.
We investigate in detail non-Abelian generalizations of both the Landau and the
symmetric gauge. Finally, we discuss how these non-Abelian Landau and symmetric
gauges may be generated by means of realistically feasible lasers in a tripod
scheme.Comment: 13 pages, 9 figure
Slow light in degenerate Fermi gases
We investigate the effect of slow light propagating in a degenerate atomic
Fermi gas. In particular we use slow light with an orbital angular momentum. We
present a microscopic theory for the interplay between light and matter and
show how the slow light can provide an effective magnetic field acting on the
electrically neutral fermions, a direct analogy of the free electron gas in an
uniform magnetic field. As an example we illustrate how the corresponding de
Haas-van Alphen effect can be seen in a neutral gas of fermions.Comment: Slightly updated. Phys. Rev. Lett. 93, 033602 (2004
Thermocapillary manipulation of droplets using holographic beam shaping: Microfluidic pin ball
International audienceWe demonstrate that holographically generated optical patterns offer greater flexibility for the thermocapillary control of water droplets than Gaussian spots; droplets can be stopped in faster flows while using less optical intensity when the surface tension variations are created by line patterns instead of single spots. Further, experiments are performed making use of variable light patterns to achieve controlled droplet routing in a four-way cross microfluidic channel. Finally, multiple droplet storage is demonstrated as well as changing drop order. © 2008 American Institute of Physics
Soliton topology versus discrete symmetry in optical lattices
We address the existence of vortex solitons supported by azimuthally
modulated lattices and reveal how the global lattice discrete symmetry has
fundamental implications on the possible topological charges of solitons. We
set a general ``charge rule'' using group-theory techniques, which holds for
all lattices belonging to a given symmetry group. Focusing in the case of
Bessel lattices allows us to derive also a overall stability rule for the
allowed vortex solitons.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
Optical binding mechanisms: a conceptual model for Gaussian beam traps
Optical binding interactions between laser-trapped spherical microparticles
are familiar in a wide range of trapping configurations. Recently it has been
demonstrated that these experiments can be accurately modeled using Mie
scattering or coupled dipole models. This can help confirm the physical
phenomena underlying the inter-particle interactions, but does not necessarily
develop a conceptual understanding of the effects that can lead to future
predictions. Here we interpret results from a Mie scattering model to obtain a
physical description which predict the behavior and trends for chains of
trapped particles in Gaussian beam traps. In particular, it describes the
non-uniform particle spacing and how it changes with the number of particles.
We go further than simply \emph{demonstrating} agreement, by showing that the
mechanisms ``hidden'' within a mathematically and computationally demanding Mie
scattering description can be explained in easily-understood terms.Comment: Preprint of manuscript submitted to Optics Expres
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