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.

James Scholtz
Summer 2003
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