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I was reading Stamper-Kurn's article Experimental Methods in Ultracold Atomic Physics (link). In the imaging section (page 13), he mentions:
Cold atoms are conventionally probed by optical imaging. Probe light at a well defined optical frequency is sent through the atomic gas and imaged onto a camera.
What determines this "well defined optical frequency"? For example, why would one choose 780 nm, which seems to be a common wavelength?
The most common type of imaging in these systems is absorption imaging- you just shine light on the atoms that matches a strong electronic transition, so the atoms scatter as much light as possible, and then get an image from seeing how much light is scattered. Essentially, you are looking at the shadow cast by the atoms.
So this frequency is, naturally, dependent on the specific atoms that you are using. For Rubidium it would indeed usually be at 780 nm, which corresponds to the so-called "D2 transition" between the S state and one of the P states of the one valence electron. Light near this frequency is also used for laser cooling, so basically any Rb experiment will need to have this laser anyway.
A good reference for choosing appropriate laser beam for a given type of atom, plz read Michal J Martin's PhD thesis at jila, Prof. Ye Jun's group site. Some moderator would not be happy with links so I delete it. Chapter 2 & 4 all provides condensed materials about laser wavelength.
