205 research outputs found
Collapses and revivals of stored orbital angular momentum of light in a cold atomic ensemble
We report on the storage of orbital angular momentum of light in a cold
ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored
optical information impressed into the atomic coherence by the incident light
fields. The stored information can be manipulated by an applied magnetic field
and we were able to observe collapses and revivals due to the rotation of the
stored atomic Zeeman coherence for times longer than 15 .Comment: Submitted to Physical Review
Dynamics of a stored Zeeman coherence grating in an external magnetic field
We investigate the evolution of a Zeeman coherence grating induced in a cold
atomic cesium sample in the presence of an external magnetic field. The
gratings are created in a three-beam light storage configuration using two
quasi-collinear writing laser pulses and reading with a counterpropagating
pulse after a variable time delay. The phase conjugated pulse arising from the
atomic sample is monitored. Collapses and revivals of the retrieved pulse are
observed for different polarizations of the laser beams and for different
directions of the applied magnetic field. While magnetic field inhomogeneities
are responsible for the decay of the coherent atomic response, a five-fold
increase in the coherence decay time, with respect to no applied magnetic
field, is obtained for an appropriate choice of the direction of the applied
magnetic field. A simplified theoretical model illustrates the role of the
magnetic field mean and its inhomogeneity on the collective atomic response.Comment: To appear in J. Phys.
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
Polarization dependence of four-wave mixing in a degenerate two-level system
Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate
two-level atomic transition is theoretically and experimentally examined. Using
the model presented by A. Lezama et al [Phys. Rev. A 61, 013801 (2000)] the
NDFWM spectra corresponding to different pump and probe polarization cases are
calculated and discussed. The calculated spectra are compared to the
observation of NDFWM within the transition of
cesium in a phase conjugation experiment using magneto optically cooled atomsComment: 10 pages, 13 figures; submitted to Phys. Rev.
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
Dynamics of saturated Bragg diffraction in a stored light grating in cold atoms
We report on a detailed investigation of the dynamics and the saturation of a
light grating stored in a sample of cold cesium atoms. We employ Bragg
diffraction to retrieve the stored optical information impressed into the
atomic coherence by the incident light fields. The diffracted efficiency is
studied as a function of the intensities of both writing and reading laser
beams. A theoretical model is developed to predict the temporal pulse shape of
the retrieved signal and compares reasonably well with the observed results.Comment: Submitted to Phys. Rev.
Spectroscopic observation of the rotational Doppler effect
We report on the first spectroscopic observation of the rotational Doppler
shift associated with light beams carrying orbital angular momentum. The effect
is evidenced as the broadening of a Hanle/EIT coherence resonance on Rb vapor
when the two incident Laguerre-Gaussian laser beams have opposite topological
charges. The observations closely agree with theoretical predictions.Comment: Submited to Physical Review Lette
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