82 research outputs found
Doppler cooling with coherent trains of laser pulses and tunable "velocity comb"
We explore the possibility of decelerating and Doppler cooling of an ensemble
of two-level atoms by a coherent train of short, non-overlapping laser pulses.
We develop a simple analytical model for dynamics of a two-level system driven
by the resulting frequency comb field. We find that the effective scattering
force mimics the underlying frequency comb structure. The force pattern depends
strongly on the ratio of the atomic lifetime to the repetition time and pulse
area. For example, in the limit of short lifetimes, the frequency peaks of the
optical force wash out. We show that laser cooling with pulse trains results in
a "velocity comb", a series of narrow peaks in the velocity space
Trapping of a microsphere pendulum resonator in an optical potential
We propose a method to spatially confine or corral the movements of a
micropendulum via the optical forces produced by two simultaneously excited
optical modes of a photonic molecule comprising two microspherical cavities. We
discuss how the cavity enhanced optical force generated in the photonic
molecule can create an optomechanical potential of about 10 eV deep and 30 pm
wide, which can be used to trap the pendulum at any given equilibrium position
by a simple choice of laser frequencies. This result presents opportunities for
very precise all-optical self-alignment of microsystems.Comment: 13 pages, 3 figure
Theory of an optical dipole trap for cold atoms
The theory of an atom dipole trap composed of a focused, far red-detuned, trapping laser beam, and a pair of red-detuned, counterpropagating, cooling beams is developed for the simplest realistic multilevel dipole interaction scheme based on a model of a (3+5)-level atom. The description of atomic motion in the trap is based on the quantum kinetic equations for the atomic density matrix and the reduced quasiclassical kinetic equation for atomic distribution function. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with low saturation, the one-photon absorption (emission) processes responsible for the trapping potential can be well separated from the two-photon processes responsible for sub-Doppler cooling atoms in the trap. Two conditions are derived that are necessary and sufficient for stable atomic trapping. The conditions show that stable atomic trapping in the optical dipole trap can be achieved when the trapping field has no effect on the two-photon cooling process and when the cooling field does not change the structure of the trapping potential but changes only the numerical value of the trapping potential well. It is concluded that the separation of the trapping and cooling processes in a pure optical dipole trap allows one to cool trapped atoms down to a minimum temperature close to the recoil temperature, keeping simultaneously a deep potential well
Doppler cooling of three-level -systems by coherent pulse trains
We explore the possibility of decelerating and Doppler cooling an ensemble of
tree-level -type atoms by a coherent train of short, non-overlapping
laser pulses. We show that -atoms can be Doppler cooled without
additional repumping of the population from the intermediate ground state. We
derive analytical expression for the scattering force in the quasi-steady-state
regime and analyze its dependence on pulse train parameters. Based on this
analysis we propose a method of choosing pulse train parameters to optimize the
cooling process.Comment: 22 pages, 6 figure
Dissipative light field as a way to create strongly localized structures for atom lithography
Generally, the conditions for deep sub-Doppler laser cooling do not match the
conditions for the strong atomic localization that takes a place in deeper
optical potential and, in consequence, leads to larger temperature. Moreover,
for a given detuning in a deep optical potential the secular approximation
which is usually used for quantum description of laser cooling becomes no more
valid. Here we perform an analysis of atomic localization in optical potential
based on a full quantum approach for atomic density matrix. We also show that
the laser cooling in a deep far-off detuned optical potential, created by a
light field with a polarization gradient, can be used as an alternative method
for forming high contrast spatially localized structures of atoms for the
purposes of atom lithography and atomic nanofabrication. Finally, we perform an
analysis of the possible limits for the width and the contrast of localized
atomic structures that can in principle be reached by this type of the light
mask.Comment: 4 figure
Atomic micromotion and geometric forces in a triaxial magnetic trap
Non-adiabatic motion of Bose-Einstein condensates of rubidium atoms arising
from the dynamical nature of a time-orbiting-potential (TOP) trap was observed
experimentally. The orbital micromotion of the condensate in velocity space at
the frequency of the rotating bias field of the TOP was detected by a
time-of-flight method. A dependence of the equilibrium position of the atoms on
the sense of rotation of the bias field was observed. We have compared our
experimental findings with numerical simulations. The nonadiabatic following of
the atomic spin in the trap rotating magnetic field produces geometric forces
acting on the trapped atoms.Comment: 4 pages, 4 figure
Sub-Doppler cooling of three-level A Atoms in space-shifted standing light waves
We present an investigation of an alternative mechanism for sub-Doppler cooling of atoms, based on coherent population transfer in three-level LAMBDA systems. The mechanism considered is that of a LAMBDA atom interacting with two standing light waves with a mutual spatial phase shift phi not-equal 0. The spatial dependence of the level populations of the LAMBDA atom for different values of phi is presented. For phi not-equal 0, this clearly demonstrates coherent population transfer in an atom with transverse motion along the space-shifted nodes and antinodes of the two standing waves. We show that this allows translational temperatures well below the Doppler limit T(D) = hgammaBAR/k(B) to be achieved
Steady state of atoms in a resonant field with elliptical polarization
We present a complete set of analytical and invariant expressions for the
steady-state density matrix of atoms in a resonant radiation field with
arbitrary intensity and polarization. The field drives the closed dipole
transition with arbitrary values of the angular momenta and of
the ground and excited state. The steady-state density matrix is expressed in
terms of spherical harmonics of a complex direction given by the field
polarization vector. The generalization to the case of broad-band radiation is
given. We indicate various applications of these results.Comment: revtex, 26 pages, including 3 eps figures; PRA accepted for
publication;v2 three typos are fixe
Atoms in axially shifted tightly focused counter-propagating beams : The role of the Gouy and curvature phases
We consider the interaction of atoms with two tightly focused and axially shifted counter-propagating optical beams. At sub-wavelength focusing, we find that the scattering force potential in the three-dimensional space between the shifted focal planes changes from a feature with a saddle-point to a three-dimensional trapping potential. Further analysis shows that due to the tight focusing, the trapping depends on significant contributions arising from the Gouy and curvature phase gradients of the interfering beams. The physics and its effects are illustrated with reference to the sub-wavelength trapping of sodium atoms
Collisions of Ultracold Trapped Cesium Feshbach Molecules
We study collisions in an optically trapped, pure sample of ultracold Cs
molecules in various internal states. The molecular gas is created by Feshbach
association from a near-degenerate atomic gas, with adjustable temperatures in
the nanokelvin range. We identify several narrow loss resonances, which point
to the coupling to more complex molecular states and may be interpreted as
Feshbach resonances in dimer-dimer interactions. Moreover, in some molecular
states we observe a surprising temperature dependence in collisional loss. This
shows that the situation cannot be understood in terms of the usual simple
threshold behavior for inelastic two-body collisions. We interpret this
observation as further evidence for a more complex molecular structure beyond
the well-understood dimer physics.Comment: To appear in Laser Physics, special issue in memoriam Prof. Vladilen
S. Letokho
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