3,187 research outputs found
Four wave mixing with self-phase matching due to collective atomic recoil
We describe a method for non-degenerate four-wave mixing in a cold sample of
4-level atoms. An integral part of the four-wave mixing process is a
collective instability which spontaneously generates a periodic density
modulation in the cold atomic sample with a period equal to half of the
wavelength of the generated high-frequency optical field. Due to the generation
of this density modulation, phase-matching between the pump and scattered
fields is not a necessary initial condition for this wave-mixing process to
occur, rather the density modulation acts to "self phase-match" the fields
during the course of the wave-mixing process. We describe a one-dimensional
model of this process, and suggest a proof-of-principle experiment which would
involve pumping a sample of cold Cs atoms with three infra-red pump fields to
produce blue light.Comment: to appear in Physical Review Letter
Inducing strong density modulation with small energy dispersion in particle beams and the harmonic amplifier free electron laser
We present a possible method of inducing a periodic density modulation in a particle beam with little increase in the energy dispersion of the particles. The flow of particles in phase space does not obey Liouville's Theorem. The method relies upon the Kuramoto-like model of collective synchronism found in free electron generators of radiation, such as Cyclotron Resonance Masers and the Free Electron Laser. For the case of an FEL interaction, electrons initially begin to bunch and emit radiation energy with a correlated energy dispersion which is periodic with the FEL ponderomotive potential. The relative phase between potential and particles is then changed by approximately 180 degrees. The particles continue to bunch, however, there is now a correlated re-absorption of energy from the field. We show that, by repeating this relative phase change many times, a significant density modulation of the particles may be achieved with only relatively small energy dispersion. A similar method of repeated relative electron/radiation phase changes is used to demonstrate supression of the fundamental growth in a high gain FEL so that the FEL lases at the harmonic only
The Semiclassical and Quantum Regimes of Superradiant Light Scattering from a Bose-Einstein Condensate
We show that many features of the recent experiments of Schneble et al. [D.
Schneble, Y. Torii, M. Boyd, E.W. Streed, D.E. Pritchard and W. Ketterle,
Science vol. 300, p. 475 (2003)], which demonstrate two different regimes of
light scattering by a Bose-Einstein condensate, can be described using a
one-dimensional mean-field quantum CARL model, where optical amplification
occurs simultaneously with the production of a periodic density modulation in
the atomic medium. The two regimes of light scattering observed in these
experiments, originally described as ``Kapiza-Dirac scattering'' and
``Superradiant Rayleigh scattering'', can be interpreted as the semiclassical
and quantum limits respectively of CARL lasing.Comment: 10 pages, 5 figures - to appear in Journal of Optics
Mode-locked Bloch oscillations in a ring cavity
We present a new technique for stabilizing and monitoring Bloch oscillations
of ultracold atoms in an optical lattice under the action of a constant
external force. In the proposed scheme, the atoms also interact with a
unidirectionally pumped optical ring cavity whose one arm is collinear with the
optical lattice. For weak collective coupling, Bloch oscillations dominate over
the collective atomic recoil lasing instability and develop a synchronized
regime in which the atoms periodically exchange momentum with the cavity field.Comment: 7 pages, 5 figure
Chaotic resonances of a Bose-Einstein condensate in a cavity pumped by a modulated optical field
We present a theoretical analysis of a Bose-Einstein condensate (BEC) enclosed in an optical cavity driven by a modulated external laser beam where the cavity-field variable is adiabatically eliminated. The modulation of the amplitude of the pump laser induces nonlinear resonances and the widespread presence of chaotic oscillations even when repulsive atom-atom interactions are negligible. Close to resonance, varying the modulation amplitude by just a few percent causes abrupt and erratic changes to the output laser intensity with peak power increasing by almost an order of magnitude. We also use a simplified model of the BEC-cavity system that considers only a small number of spatial modes of the BEC to show that, despite the disruptive presence of a modulation in the pump beam, the system can still be considered to be low-dimensional
Ground-state degeneracies leave recognizable topological scars in the one-particle density
In Kohn-Sham density functional theory (KS-DFT) a fictitious system of
non-interacting particles is constructed having the same ground-state (GS)
density as the physical system of interest. A fundamental open question in DFT
concerns the ability of an exact KS calculation to spot and characterize the GS
degeneracies in the physical system. In this article we provide theoretical
evidence suggesting that the GS density, as a function of position on a 2D
manifold of parameters affecting the external potential, is "topologically
scarred" in a distinct way by degeneracies. These scars are sufficiently
detailed to enable determination of the positions of degeneracies and even the
associated Berry phases. We conclude that an exact KS calculation can spot and
characterize the degeneracies of the physical system
The role of quantum fluctuations in the optomechanical properties of a Bose-Einstein condensate in a ring cavity
We analyze a detailed model of a Bose-Einstein condensate trapped in a ring
optical resonator and contrast its classical and quantum properties to those of
a Fabry-P{\'e}rot geometry. The inclusion of two counter-propagating light
fields and three matter field modes leads to important differences between the
two situations. Specifically, we identify an experimentally realizable region
where the system's behavior differs strongly from that of a BEC in a
Fabry-P\'{e}rot cavity, and also where quantum corrections become significant.
The classical dynamics are rich, and near bifurcation points in the mean-field
classical system, the quantum fluctuations have a major impact on the system's
dynamics.Comment: 11 pages, 11 figures, submitted to PR
Optical pattern formation with a 2-level nonlinearity
We present an experimental and theoretical investigation of spontaneous
pattern formation in the transverse section of a single retro-reflected laser
beam passing through a cloud of cold Rubidium atoms. In contrast to previously
investigated systems, the nonlinearity at work here is that of a 2-level atom,
which realizes the paradigmatic situation considered in many theoretical
studies of optical pattern formation. In particular, we are able to observe the
disappearance of the patterns at high intensity due to the intrinsic saturable
character of 2-level atomic transitions.Comment: 5 pages, 4 figure
Attosecond electronic and nuclear quantum photodynamics of ozone: time-dependent Dyson orbitals and dipole
A nonadiabatic scheme for the description of the coupled electron and nuclear
motions in the ozone molecule was proposed recently. An initial coherent
nonstationary state was prepared as a superposition of the ground state and the
excited Hartley band. In this situation neither the electrons nor the nuclei
are in a stationary state. The multiconfiguration time dependent Hartree method
was used to solve the coupled nuclear quantum dynamics in the framework of the
adiabatic separation of the time-dependent Schr\"odinger equation. The
resulting wave packet shows an oscillation of the electron density between the
two chemical bonds. As a first step for probing the electronic motion we
computed the time-dependent molecular dipole and the Dyson orbitals. The latter
play an important role in the explanation of the photoelectron angular
distribution. Calculations of the Dyson orbitals are presented both for the
time-independent as well as the time-dependent situations. We limited our
description of the electronic motion to the Franck-Condon region only due to
the localization of the nuclear wave packets around this point during the first
5-6 fs
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