Femtosecond Laser Cooling of Trapped Ions

We present work toward laser cooling of trapped ions with femtosecond pulse trains. Our scheme is applicable to hydrogen and other elements that are not currently laser cooled. In this proof-of-principle experiment, we confine approximately $10^5 \: {\rm Yb}^+$ ions in a linear Paul trap and probe them on the strong ${\rm S}_{1/2} - {\rm P}_{1/2}$ transition at 370 nm. A high-repetition-rate mode-locked laser drives the two-photon ${\rm S}_{1/2} - {\rm D}_{3/2}$ transition at 871 nm for laser cooling. An optical resonator enhances the mode-locked laser intensity applied to the ions, greatly increasing the two-photon transition rate.