115 research outputs found
A conjugate gradient minimisation approach to generating holographic traps for ultracold atoms
Direct minimisation of a cost function can in principle provide a versatile
and highly controllable route to computational hologram generation. However, to
date iterative Fourier transform algorithms have been predominantly used. Here
we show that the careful design of cost functions, combined with numerically
efficient conjugate gradient minimisation, establishes a practical method for
the generation of holograms for a wide range of target light distributions.
This results in a guided optimisation process, with a crucial advantage
illustrated by the ability to circumvent optical vortex formation during
hologram calculation. We demonstrate the implementation of the conjugate
gradient method for both discrete and continuous intensity distributions and
discuss its applicability to optical trapping of ultracold atoms.Comment: 11 pages, 4 figure
Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments
The authors acknowledge funding from the Leverhulme Trust Research Project Grant RPG-2013-074 and from the EPSRC grant GR/T08272/01.We demonstrate a method to independently and arbitrarily tailor the spatial profile of light of multiple wavelengths and we show possible applications to ultracold atoms experiments. A single spatial light modulator is programmed to create a pattern containing multiple spatially separated structures in the Fourier plane when illuminated with a single wavelength. When the modulator is illuminated with overlapped laser beams of different wavelengths, the position of the structures is wavelength-dependent. Hence, by designing their separations appropriately, a desired overlap of different structures at different wavelengths is obtained. We employ regional phase calculation algorithms and demonstrate several possible experimental scenarios by generating light patterns with 670 nm, 780 nm and 1064 nm laser light which are accurate to the level of a few percent. This technique is easily integrated into cold atom experiments, requiring little optical access.PostprintPeer reviewe
Light-induced atomic desorption in a compact system for ultracold atoms
In recent years, light-induced atomic desorption (LIAD) of alkali atoms from
the inner surface of a vacuum chamber has been employed in cold atom
experiments for the purpose of modulating the alkali background vapour. This is
beneficial because larger trapped atom samples can be loaded from vapour at
higher pressure, after which the pressure is reduced to increase the lifetime
of the sample. We present an analysis, based on the case of rubidium atoms
adsorbed on pyrex, of various aspects of LIAD that are useful for this
application. Firstly, we study the intensity dependence of LIAD by fitting the
experimental data with a rate-equation model, from which we extract a correct
prediction for the increase in trapped atom number. Following this, we quantify
a figure of merit for the utility of LIAD in cold atom experiments and we show
how it can be optimised for realistic experimental parameters.Comment: 10 pages, 8 figure
Feedback-enhanced algorithm for aberration correction of holographic atom traps
We show that a phase-only spatial light modulator can be used to generate
non-trivial light distributions suitable for trapping ultracold atoms, when the
hologram calculation is included within a simple and robust feedback loop that
corrects for imperfect device response and optical aberrations. This correction
reduces the discrepancy between target and experimental light distribution to
the level of a few percent (RMS error). We prove the generality of this
algorithm by applying it to a variety of target light distributions of
relevance for cold atomic physics.Comment: 5 pages, 4 figure
Axially open nonradiative structures: an example of single-mode resonator based on the sample holder
The concept of nonradiative dielectric resonator is generalized in order to
include axially open configurations having rotational invariance. The resulting
additional nonradiative conditions are established for the different resonance
modes on the basis of their azimuthal modal index. An approximate chart of the
allowed dielectric and geometrical parameters for the TE011 mode is given. A
practical realization of the proposed device based on commercial fused quartz
tubes is demonstrated at millimeter wavelengths, together with simple
excitation and tuning mechanisms. The observed resonances are characterized in
their basic parameters, as well as in the field distribution by means of a
finite element method. The predictions of the theoretical analysis are well
confirmed, both in the general behaviour and in the expected quality factors.
The resulting device, in which the sample holder acts itself as single-mode
resonating element, combines an extreme ease of realization with
state-of-the-art performances. The general benefits of the proposed open
single-mode resonators are finally discussed.Comment: 18 pages, 10 figure
Reflection of Channel-Guided Solitons at Junctions in Two-Dimensional Nonlinear Schroedinger Equation
Solitons confined in channels are studied in the two-dimensional nonlinear
Schr\"odinger equation. We study the dynamics of two channel-guided solitons
near the junction where two channels are merged. The two solitons merge into
one soliton, when there is no phase shift. If a phase difference is given to
the two solitons, the Josephson oscillation is induced. The Josephson
oscillation is amplified near the junction. The two solitons are reflected when
the initial velocity is below a critical value.Comment: 3 pages, 2 figure
Measurement of Vacuum Pressure with a Magneto-Optical Trap: a Pressure-Rise Method
The lifetime of an atom trap is often limited by the presence of residual
background gases in the vacuum chamber. This leads to the lifetime being
inversely proportional to the pressure. Here we use this dependence to estimate
the pressure and to obtain pressure rate-of-rise curves, which are commonly
used in vacuum science to evaluate the performance of a system. We observe
different rates of pressure increase in response to different levels of
outgassing in our system. Therefore we suggest that this is a sensitive method
which will be useful in applications of cold atom systems, in particular where
the inclusion of a standard vacuum gauge is impractical.Comment: 5 pages, 6 figure
Collisional relaxation of Feshbach molecules and three-body recombination in 87Rb Bose-Einstein condensates
We predict the resonance enhanced magnetic field dependence of atom-dimer
relaxation and three-body recombination rates in a Rb Bose-Einstein
condensate (BEC) close to 1007 G. Our exact treatments of three-particle
scattering explicitly include the dependence of the interactions on the atomic
Zeeman levels. The Feshbach resonance distorts the entire diatomic energy
spectrum causing interferences in both loss phenomena. Our two independent
experiments confirm the predicted recombination loss over a range of rate
constants that spans four orders of magnitude.Comment: 4 pages, 3 eps figures (updated references
Wave function recombination instability in cold atom interferometers
Cold atom interferometers use guiding potentials that split the wave function
of the Bose-Einstein condensate and then recombine it. We present theoretical
analysis of the wave function recombination instability that is due to the weak
nonlinearity of the condensate. It is most pronounced when the accumulated
phase difference between the arms of the interferometer is close to an odd
multiple of PI and consists in exponential amplification of the weak ground
state mode by the strong first excited mode. The instability exists for both
trapped-atom and beam interferometers.Comment: 4 pages, 5 figure
Trapped-Atom-Interferometer in a Magnetic Microtrap
We propose a configuration of a magnetic microtrap which can be used as an
interferometer for three-dimensionally trapped atoms. The interferometer is
realized via a dynamic splitting potential that transforms from a single well
into two separate wells and back. The ports of the interferometer are
neighboring vibrational states in the single well potential. We present a
one-dimensional model of this interferometer and compute the probability of
unwanted vibrational excitations for a realistic magnetic potential. We
optimize the speed of the splitting process in order suppress these excitations
and conclude that such interferometer device should be feasible with currently
available microtrap technique.Comment: 6 pages, 6 figures, submitted to PR
- …