A First Look at Laser-cooling Ions in an Ultra-cold Neutral Plasma

We discuss progress in laser-cooling ions in an ultra-cold neutral plasma. A major challenge to implementing a laser-cooling scheme is that the laser-cooling transition is not closed. A fraction of the ions decay into "dark" metastable states and are lost to the cooling process. We have built a system using two infrared diode lasers to optically pump atoms out of the metastable states. The optical pumping transitions form a $\lambda $-system. Avoiding atomic coherences associated with this configuration may be necessary to achieve maximum laser cooling. We describe our laser system and its use in cooling calcium ions in our ultra-cold neutral plasma. We report on the increased efficiency of ion-cooling through repumping into the cooling transition.