Development of a Magneto-Optical Trap (MOT) in an Undergraduate
Laboratory
James Scholtz
Laser Teaching Center, Department of Physics and Astronomy,
University of New York at Stony Brook
Advisors: Dr. John Noe and Prof. Hal Metcalf
Laser Cooling of an atom makes use of radiation pressure, a force
exerted by the
repeated absorption and re-emission of photons. An atom moving towards
a laser
beam encounters a higher frequency than an atom moving away from the
same beam
(the Doppler shift). In cooling, the frequency of the beam is adjusted
so that
an atom moving into the beam scatters many more photons than an atom
moving away
from the beam. The net effect is to reduce the speed of the atom and
thus cool
it. By using six intersecting laser beams in an inhomogeneous magnetic
field, a
Magneto-Optical Trap (MOT) can be created in which the cooled atoms are
confined to a small volume, where they can be used for a variety of
experiments.
MOT systems necessarily include a number of quite complex elements,
including
ultra-high vacuum chambers, frequency-stabilized lasers, magnetic coils
and
multiple optical elements. Despite this complexity, there are a number
of
examples of MOT systems which have been successfully developed in small
undergraduate laboratories, for example at Holy Cross College, Davidson
College,
and Bryn Mawr. The long-term goal of our project, which is still at a
very early
stage, is to implement such a device in an undergraduate laboratory at
Stony
Brook, the Laser Teaching Center.
The key component in any laser cooling experiment is the
frequency-stabilized
laser system. We will be using an existing diode laser that uses
optical
feedback from a diffraction grating to generate a wavelength of 780.0
nm that
precisely matches the D2 transition in rubidium atoms. The frequency
(or
equivalently, the wavelength) of the emitted light is dependent upon
the
injection current, the temperature of the diode, and the cavity
length. By
manipulating each of these variables we can control what wavelength is
produced.
Frequency stabilization is achieved by deriving a feedback signal from
the
interaction of a portion of the laser beam with a cell containing Rb
vapor.
The initial phase of the project will consist of gaining experience
with setting
up, operating and frequency locking this laser. At the same time
information
will be collected on the designs of existing MOT's in small
laboratories, and
these designs will be carefully compared and evaluated.
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