Two-tone Doppler cooling of radial two-dimensional crystals in a radio-frequency ion trap
Alexander Kato, A. B. GOEL, Raymond Lee, Zeyu Ye, Samip Karki, Jian Jun Liu, A. Nomerotski, B. B. Blinov
Abstract
We study the Doppler-cooling of radial two-dimensional (2D) Coulomb crystals of trapped barium ions in a radiofrequency trap. Ions in radial 2D crystals experience micromotion of an amplitude that increases linearly with the distance from the trap center, leading to a position-dependent frequency modulation of laser light in each ion's rest frame. We use two tones of Doppler-cooling laser light separated by approximately 100 MHz to efficiently cool distinct regions in the crystals with differing amplitudes of micromotion. This technique allows us to trap and cool more than 50 ions populating four shells in a radial 2D crystal, where with a single tone of Doppler cooling light we are limited to 30 ions in three shells. We also individually characterize the micromotion of all ions within the crystals and use this information to locate the center of the trap and to determine the Matthieu parameters ${q}_{x}$ and ${q}_{y}$.