345 research outputs found
Theoretical study of a cold atom beam splitter
A theoretical model is presented for the study of the dynamics of a cold
atomic cloud falling in the gravity field in the presence of two crossing
dipole guides. The cloud is split between the two branches of this laser guide,
and we compare experimental measurements of the splitting efficiency with
semiclassical simulations. We then explore the possibilities of optimization of
this beam splitter. Our numerical study also gives access to detailed
information, such as the atom temperature after the splitting
Controlled deflection of cold atomic clouds and of Bose-Einstein condensates
We present a detailed, realistic proposal and analysis of the implementation
of a cold atom deflector using time-dependent far off-resonance optical guides.
An analytical model and numerical simulations are used to illustrate its
characteristics when applied to both non-degenerate atomic ensembles and to
Bose-Einstein condensates. Using for all relevant parameters values that are
achieved with present technology, we show that it is possible to deflect almost
entirely an ensemble of Rb atoms falling in the gravity field. We
discuss the limits of this proposal, and illustrate its robustness against
non-adiabatic transitions
Narrow band amplification of light carrying orbital angular momentum
We report on the amplification of an optical vortex beam carrying orbital
angular momentum via induced narrow Raman gain in an ensemble of cold cesium
atoms. A 20\% single-pass Raman gain of a weak vortex signal field is observed
with a spectral width of order of 1 MHz, much smaller than the natural width,
demonstrating that the amplification process preserves the phase structure of
the vortex beam. The gain is observed in the degenerated two-level system
associated with the hyperfine transition of cesium. Our experimental observations are explained
with a simple theoretical model based on a three-level system
interacting coherently with the weak Laguerre-Gauss field and a strong coupling
field, including an incoherent pumping rate between the two degenerate
ground-states.Comment: 9 pages, 4 figure
Off-axis retrieval of orbital angular momentum of light stored in cold atoms
We report on the storage of orbital angu- lar momentum (OAM) of light of a
Laguerre-Gaussian mode in an ensemble of cold cesium atoms and its re- trieval
along an axis different from the incident light beam. We employed a
time-delayed four-wave mixing configuration to demonstrate that at small angle
(2o), after storage, the retrieved beam carries the same OAM as the one encoded
in the input beam. A calculation based on mode decomposition of the retrieved
beam over the Laguerre-Gaussian basis is in agreement with the experimental
observations done at small angle values. However, the calculation shows that
the OAM retrieving would get lost at larger angles, reducing the fidelity of
such storing-retrieving process. In addition, we have also observed that by
applying an external magnetic field to the atomic ensemble the retrieved OAM
presents Larmor oscillations, demonstrating the possibility of its manipulation
and off-axis retrieval.Comment: 9 pages, 4 figure
Bose-Einstein condensation in dark power-law laser traps
We investigate theoretically an original route to achieve Bose-Einstein
condensation using dark power-law laser traps. We propose to create such traps
with two crossing blue-detuned Laguerre-Gaussian optical beams. Controlling
their azimuthal order allows for the exploration of a multitude of
power-law trapping situations in one, two and three dimensions, ranging from
the usual harmonic trap to an almost square-well potential, in which a
quasi-homogeneous Bose gas can be formed. The usual cigar-shaped and
disk-shaped Bose-Einstein condensates obtained in a 1D or 2D harmonic trap take
the generic form of a "finger" or of a "hockey puck" in such Laguerre-Gaussian
traps. In addition, for a fixed atom number, higher transition temperatures are
obtained in such configurations when compared with a harmonic trap of same
volume. This effect, which results in a substantial acceleration of the
condensation dynamics, requires a better but still reasonable focusing of the
Laguerre-Gaussian beams
Mesure de la charge topologique d'un faisceau twisté à l'aide d'une roue ajourée
National audienceNous présentons une nouvelle méthode pour mesurer la charge topologique d'un faisceau twisté. Nous utilisons une roue percée de trous sur sa périphérie et réguliÚrement espacés. La forme de la figure de diffraction, liée au nombre de trous, permet d'identifier l'ordre du faisceau. Cette technique est bien adaptée pour mesurer des charges topologiques élevées
Negative experimental evidence for magneto-orbital dichroism - supplemental information
A light beam can carry both spin angular momentum (SAM) and orbital angular
momentum (OAM). SAM is commonly evidenced by circular dichroism (CD)
experiments {\em i. e.} differential absorption of left and right-handed
circularly polarized light. Recent experiments, supported by theoretical work,
indicate that the corresponding effect with OAM instead of SAM is not observed
in chiral matter.
Isotropic materials can show CD when subjected to a magnetic field (MCD). In
Ref. ~\onlinecite{Mathevet2012} we report a set of experiments, under well
defined conditions, searching for magnetic orbital dichroism (MOD),
differential absorption of light as a function of the sign of its OAM. We
experimentally demonstrate that this effect, if any, is smaller than a few
of MCD for the Nd:YAG transition.
This transition is essentially of electric dipole nature. We give an intuitive
argument suggesting that the lowest order of light matter interaction leading
to MOD is the electric quadrupole term.
We give here more experimental details and extra measurements.Comment: 6 pages, 7 figures. Supplemental material for a publication in Optics
Expres
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