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 light. 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 much more light 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. The stage of research that we are currently involved in is to achieve frequency stabilization of our diode laser system, a key need in any laser cooling experiment. We use an external-cavity diode laser that takes optical feedback from a holographic 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.
Saturated Absorption Spectroscopy, or "Sat Spec" is an experimental technique that allows us to tune the laser to the hyperfine atomic transitions of Rb to a very high precision, by eliminating the Doppler broadening that results from the random thermal motion of atoms in the light field. The Doppler-free Sat Spec signal derived from the interaction of the laser beam with a gas cell containing Rb vapor is then fed back to the diode laser to achieve frequency stabilization. Thus far, we have achieved Doppler-free signals for the four Doppler-broadened peaks, corresponding to transitions from two different ground states of each of two rubidium isotopes. We are currently working to identify these transitions. The next step is to lock the laser frequency to one of the transitions by means of an electronic feedback loop that feeds an error signal generated by a lock-in amplifier back into the laser.
Some obstacles that we have encountered in the course of the project include power broadening, undesirable optical feedback from reflections back into the laser, and laser mode hops. These have been overcome by adding an optical isolator after the laser, analyzing the laser modes with a Fabry-Perot cavity, and by implementing a new design for the setup. We have gained an enormous amount of experience and knowledge from setting up, operating and attempting to frequency lock our first laser. We look forward to completing this phase of the project and moving on to the main goal, the implementation of a working Magneto-Optical Trap in the Laser Teaching Center.