3,404 research outputs found
Towards absolute calibration of optical tweezers
Aiming at absolute force calibration of optical tweezers, following a
critical review of proposed theoretical models, we present and test the results
of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous
(MD) model, taking account of spherical aberration at the glass/water
interface. This first-principles theory is formulated entirely in terms of
experimentally accessible parameters (none adjustable). Careful experimental
tests of the MDSA theory, undertaken at two laboratories, with very different
setups, are described. A detailed description is given of the procedures
employed to measure laser beam waist, local beam power at the transparent
microspheres trapped by the tweezers, microsphere radius and the trap
transverse stiffness, as a function of radius and height in the (inverted
microscope) sample chamber. We find generally very good agreement with MDSA
theory predictions, for a wide size range, from the Rayleigh domain to large
radii, including the values most often employed in practice, and at different
chamber heights, both with objective overfilling and underfilling. The results
asymptotically approach geometrical optics in the mean over size intervals, as
they should, and this already happens for size parameters not much larger than
unity. MDSA predictions for the trapping threshold, position of stiffness peak,
stiffness variation with height, multiple equilibrium points and `hopping'
effects among them are verified. Remaining discrepancies are ascribed to focus
degradation, possibly arising from objective aberrations in the infrared, not
yet included in MDSA theory.Comment: 15 pages, 20 figure
Ultracold atomic Fermi-Bose mixtures in bichromatic optical dipole traps: a novel route to study fermion superfluidity
The study of low density, ultracold atomic Fermi gases is a promising avenue
to understand fermion superfluidity from first principles. One technique
currently used to bring Fermi gases in the degenerate regime is sympathetic
cooling through a reservoir made of an ultracold Bose gas. We discuss a
proposal for trapping and cooling of two-species Fermi-Bose mixtures into
optical dipole traps made from combinations of laser beams having two different
wavelengths. In these bichromatic traps it is possible, by a proper choice of
the relative laser powers, to selectively trap the two species in such a way
that fermions experience a stronger confinement than bosons. As a consequence,
a deep Fermi degeneracy can be reached having at the same time a softer
degenerate regime for the Bose gas. This leads to an increase in the
sympathetic cooling efficiency and allows for higher precision thermometry of
the Fermi-Bose mixture
Light self-trapping in a large cloud of cold atoms
We show that, for a near-resonant propagating beam, a large cloud of cold
87Rb atoms acts as a saturable Kerr medium and produces self-trapping of light.
By side fluorescence imaging we monitor the transverse size of the beam and,
depending on the sign of the laser detuning with respect to the atomic
transition, we observe self-focusing or -defocusing, with the waist remaining
stationary for an appropriate choice of parameters. We analyze our observations
by using numerical simulations based on a simple 2-level atom model.Comment: 3 pages, 4 figures, submitted to Optics Letter
Understanding Optical Trapping Phenomena: a Simulation for Undergraduates
Optical trapping is an attractive and multidisciplinary topic that has become the center of attention to a large number of researchers. Moreover, it is a suitable subject for advanced students that requires a knowledge of a wide range of topics. As a result, it has been incorporated into some syllabuses of both undergraduate and graduate programs. In this paper, basic concepts in laser trapping theory are reviewed. To provide a better understanding of the underlying concepts for students, a Java application for simulating the behavior of a dielectric particle trapped in a highly focused beam has been developed. The program illustrates a wide range of theoretical results and features, such as the calculation of the force exerted by a beam in the Mie and Rayleigh regimes or the calibration of the trap stiffness. Some examples that are ready to be used in the classroom or in the computer lab are also supplied
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