Trapping Krypton and Argon Atoms With Laser Beams
Jonathan Shomsky and Prof. Matt Walhout, Calvin College
Supported by the National Science Foundation (Grant PHY-1068078)
Laser Frequency Lock
Research Goal
To detect weak molecular interactions
between krypton and argon atoms in a
Magneto-Optical Trap (MOT).
Magneto-Optical Trap (MOT)
A MOT captures and holds a cloud of
atoms in a very small volume (<1 mm3).
This type of atom trap uses magnetic
fields and laser beams to exert a force
which pushes each atom toward the
center of the trap. Atoms experience
slight “kicks” due to collisions with laser
photons; the local magnetic field
controls the direction of these “kicks.”
Once atoms are trapped, we probe and
monitor the trapped cloud in an attempt
to detect molecular interactions.
In our particular
case, we work with
krypton and argon
atoms. To create
overlapping traps,
we need two overlapping lasers to Schematic diagram of a MOT,
push on the two
showing laser beams and
[1]
magnetic
coils.
different types of
atoms.
By switching each laser on or off, we
can get a trap of either species by itself
or a combined trap of both.
0.2m
Why a laser frequency lock?
To interact with the atoms, the energy of
laser photons must precisely match the
energy between two atomic states. Since
laser frequencies drift, a restoring signal
must be used to maintain this precise
frequency.
Argon
We need an electrical “zero-crossing”
voltage which varies depending on
frequency. It pushes frequency up (+
voltage) when it is too low, and down (voltage) when it is too high. This idea
applies to many systems involving
stabilization at a fixed point.
Voltage
Laser
frequency
Molecular interactions are expected to
result in a loss of atoms in the trap.
Therefore, we need a way to detect the
relative number of atoms in the trap. We
do this using a camera and an ion
detector.
All these things together prepare us for
future photo-association experiments.
As we scan a photo-association laser to
find these the frequency of these
interactions, we must sync three signals:
Camera
The trapped cloud emits photons as
atoms continually scatters photons from
the MOT beams. Our camera converts
the number of photons which hit each
pixel to a integer number. This allows us
to calculate the number of atoms in the
trap using laser scattering theory. The
camera also allows us to measure the
size of the cloud and to monitor the
shape amount of overlap of the krypton
and argon clouds.
On these axes, zero
frequency identifies
the locking point.
Krypton MOT; from the
brightness of the trap, we
calculated that the trap
contains ~40,000 atoms.
An example “zero-crossing” signal used to lock the
frequency of our lasers.
• The frequency of the laser
• The fluorescence of the trap as
measured by the camera
• The ion production rate of the trap
As we scan the laser frequency, we look
for dips in the two trap detection signals
which indicate a loss of atoms in the
trap, caused by photo-association.
Conclusions
This summer, most of our work was spent
on setting up systems and preparing
techniques for future photo-association
experiments. Our next step is to perform
these experiments and use them to learn
about the quantum structure of Krypton
and Argon atoms.
~0.25mm
Photo-Association (PA)
The main phenomena studied is photoassociation which uses light to tie atoms
together into long-range molecules. Where
traditional chemical molecules bond by
sharing electrons, photo-association molecules
bond by sharing the absorbed photons.
Kr
Both
Pictures of the fluorescence of traps with one or both atom
species
Future Experiments
A Technical Requirement
Kr
Krypton
Detecting the trap
Photo-association ties atoms together with light.
References
Ion Detection
Collisions between metastable atoms
result in ion production. Thus, traps of
metastables produce ions at a rate which
depends sensitively on the number of
atoms on the trap. Our ion detector
counts the number of ions produced in a
certain bin of time. This allows us to see
how the number of atoms in the trap
changes through time.
[1] http://www.st-andrew.ac.uk/physics/pandaweb/
newtour/res/clasica.htm
[2] W. Vassen, C. Cohen-Tannoudji, M. Leduc, D. Boiron, C.
Westbrook, A. Truscott, K. Baldwin, G. Birkl, P. Cancio,
and M. Trippenbach, "Cold and trapped metastable noble
gases," Rev. Mod. Phys. 84 January-March (2012).
[3] E. van der Zwan, D. van Oosten, D. Nehari, P. van der
Straten, H.T.C. Stoof, "On the role of Penning ionization
in photoassociation spectroscopy," J. Phys. B: At. Mol.
Opt. Phys. 39 (2006) S825-S847.
[4] M.K. Shaffer, G. Ranjit, C.I. Sukenik, and M.
Walhout,"Photoassociative spectroscopy of ultracold
metastable argon," Phys. Rev. A 83, 052516 (2011).
[5] Z.S. Smith*, A. Harmon*, J. Banister*, R. Norman*, K.
Hoogeboom-Pot*, and M. Walhout, "Purely-long-range
krypton molecules in singly and doubly excited binding
potentials," Phys. Rev. A81, 013407 (January, 2010).