16 research outputs found
Finite temperature theory of the scissors mode in a Bose gas using the moment method
We use a generalized Gross-Pitaevskii equation for the condensate and a
semi-classical kinetic equation for the noncondensate atoms to discuss the
scissors mode in a trapped Bose-condensed gas at finite temperatures. Both
equations include the effect of collisions between the condensate and
noncondensate atoms. We solve the coupled moment equations describing
oscillations of the quadrupole moments of the condensate and noncondensate
components to find the collective mode frequencies and collisional damping
rates as a function of temperature. Our calculations extend those of
Gu\'ery-Odelin and Stringari at T=0 and in the normal phase. They complement
the numerical results of Jackson and Zaremba, although Landau damping is left
out of our approach. Our results are also used to calculate the quadrupole
response function, which is related to the moment of inertia. It is shown
explicitly that the moment of inertia of a trapped Bose gas at finite
temperatures involves a sum of an irrotational component from the condensate
and a rotational component from the thermal cloud atoms.Comment: 18 pages, 8 figure
Enhancement of the scissors mode of an expanding Bose-Einstein condensate
We study the time-evolution of the scissors mode of a Bose-Einstein
condensate during the ballistic expansion after release from the magnetic trap.
We show that despite the nontrivial character of the superfluid expansion, the
sinusoidal behavior of the scissor oscillations is recovered after an
asymptotic expansion, with an enhancement of the final amplitude. We
investigate this phenomenon with a condensate held in an elongated
magnetostatic potential, whose particular shape allows for the excitation of
the scissors mode.Comment: RevTeX, 5 figure
Optical trapping and manipulation of nanostructures
Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate-nanoscale-range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors
Optical feedback radiation forces: Intracavity optical trapping with feedback-locked diode lasers
We demonstrate a novel mechanism for optical tweezing, where a trapped particle dynamically alters an external cavity quality factor, reduceing the average intensity and photodamage, even employing low-numerical aperture lenses and wide fields-of-view. © OSA 2012
Calculation of mode coupling for quadrupole excitations in a Bose-Einstein condensate
In this paper we give a theoretical description of resonant coupling between
two collective excitations of a Bose condensed gas (BEC) on, or close, to a
second harmonic resonance. Using analytic expressions for the quasi-particle
wavefunctions we show that the coupling between quadrupole modes is strong,
leading to a coupling time of a few milliseconds (for a TOP trap with radial
frequency 100 Hz and 10^4 atoms). Using the hydrodynamic approximation, we
derive analytic expression for the coupling matrix element. These can be used
with an effective Hamiltonian (that we also derive) to describe the dynamics of
the coupling process and the associated squeezing effects.Comment: 12 pages, 3 figure
Collective excitations of trapped Bose condensates in the energy and time domains
A time-dependent method for calculating the collective excitation frequencies
and densities of a trapped, inhomogeneous Bose-Einstein condensate with
circulation is presented. The results are compared with time-independent
solutions of the Bogoliubov-deGennes equations. The method is based on
time-dependent linear-response theory combined with spectral analysis of
moments of the excitation modes of interest. The technique is straightforward
to apply, is extremely efficient in our implementation with parallel FFT
methods, and produces highly accurate results. The method is suitable for
general trap geometries, condensate flows and condensates permeated with vortex
structures.Comment: 6 pages, 3 figures small typos fixe
Anomalous rotational properties of Bose-Einstein condensates in asymmetric traps
We study the rotational properties of a Bose-Einstein condensate confined in
a rotating harmonic trap for different trap anisotropies. Using simple
arguments, we derive expressions for the velocity field of the quantum fluid
for condensates with or without vortices. While the condensed gas describes
open spiraling trajectories, on the frame of reference of the rotating trap the
motion of the fluid is against the trap rotation. We also find explicit
formulae for the angular momentum and a linear and Thomas-Fermi solutions for
the state without vortices. In these two limits we also find an analytic
relation between the shape of the cloud and the rotation speed. The predictions
are supported by numerical simulations of the mean field Gross-Pitaevskii
model.Comment: 4 RevTeX pages, 2 EPS figures; typos fixed, reference adde
Spectral method for the time-dependent Gross-Pitaevskii equation with a harmonic trap
We study the numerical resolution of the time-dependent Gross-Pitaevskii
equation, a non-linear Schroedinger equation used to simulate the dynamics of
Bose-Einstein condensates. Considering condensates trapped in harmonic
potentials, we present an efficient algorithm by making use of a spectral
Galerkin method, using a basis set of harmonic oscillator functions, and the
Gauss-Hermite quadrature. We apply this algorithm to the simulation of
condensate breathing and scissors modes.Comment: 23 pages, 5 figure
Self-similar expansion of the density profile in a turbulent Bose-Einstein condensate
In a recent study we demonstrated the emergence of turbulence in a trapped
Bose-Einstein condensate of Rb-87 atoms. An intriguing observation in such a
system is the behavior of the turbulent cloud during free expansion.The aspect
ratio of the cloud size does not change in the way one would expect for an
ordinary non-rotating (vortex-free) condensate. Here we show that the anomalous
expansion can be understood, at least qualitatively, in terms of the presence
of vorticity distributed throughout the cloud, effectively counteracting the
usual reversal of the aspect ratio seen in free time-of-flight expansion of
non-rotating condensates.Comment: 8 pages, 4 figure
Collisionless and hydrodynamic excitations of trapped boson-fermion mixtures
Within a scaling ansatz formalism plus Thomas-Fermi approximation, we
investigate the collective excitations of a harmonically trapped boson-fermion
mixture in the collisionless and hydrodynamic limit at low temperature. Both
the monopole and quadrupole modes are considered in the presence of spherical
as well as cylindrically symmetric traps. In the spherical traps, the frequency
of monopole mode coincides in the collisionless and hydrodynamic regime,
suggesting that it might be undamped in all collisional regimes. In contrast,
for the quadrupole mode, the frequency differs largely in these two limits. In
particular, we find that in the hydrodynamic regime the quadrupole oscillations
with equal bosonic and fermionic amplitudes generate an exact eigenstate of the
system, regardless of the boson-fermion interaction. This resembles the Kohn
mode for the dipole excitation. We discuss in some detail the behavior of
monopole and quadrupole modes as a function of boson-fermion coupling at
different boson-boson interaction strength. Analytic solutions valid at weak
and medium fermion-boson coupling are also derived and discussed.Comment: 29 pages + 7 figures, resubmitted to Physical Review