Optical Imaging of Ultracold Atomic Clouds

Azure Hansen, Stephan Albert, Rebekah Schiller, Daniel Pertot, David Sproles, Daniel Greif, Dominik Schneble

Department of Physics & Astronomy, Stony Brook University

Laser cooling and trapping techniques permit the creation of ultracold atomic clouds, degenerate Fermi gases and Bose-Einstein condensates (BEC). BEC's are unique, a purely quantum mechanical state of matter of great intrinsic interest. Ultracold atomic systems can also be applied to create idealized models of other physical systems (for example, in condensed matter physics) and perform fundamental measurements of unprecedented precision. The temperature of ultracold systems ranges from nanokelvin to milikelvin.

Important quantities of these systems include temperature, atom number and phase space density. These are calculated by imaging the atom cloud onto a CCD camera with a frequency-stabilized laser using a variety of techniques. In fluorescence imaging, the number of near-resonant photons scattered from the saturated atom cloud is measured. Accounting for the geometry of the system, this measurement is proportional to the atom number. Fluorescence imaging is insensitive to laser intensity fluctuations because the laser intensity is a few multiples of the saturation intensity. Absorption imaging gives a more accurate measure of the cold atom cloud parameters because the number of scattered photons is known directly. A resonant probe laser is scattered by the atom cloud and its shadow is imaged on the CCD camera. From the transmission of the cloud, the atom number is calculated. Absorption imaging works better for lower signal-to-noise, but is more sensitive to instabilities in the laser frequency.

Here we evaluate these two imaging schemes as implemented in our Rb-87 setup. The properties of the laser used in imaging effects the quality and consistency of the results; we discuss details of our improving commercial diode laser system.