Isotope-selective photo-ionization for calcium ion trapping

We present studies of resonance-enhanced photo-ionization for isotope-selective loading of Ca+ into a Paul trap. The 4s^2 ^1S_0 <-> 4s4p ^1P_1 transition of neutral calcium is driven by a 423nm laser and the atoms are photo-ionized by a second laser at 389nm. Isotope-selectivity is achieved by using crossed atomic and laser beams to reduce the Doppler width significantly below the isotope shifts in the 423nm transition. The loading rate of ions into the trap is studied under a range of experimental parameters for the abundant isotope 40Ca+. Using the fluorescence of the atomic beam at 423nm as a measure of the Ca number density, we estimate a lower limit for the absolute photo-ionization cross-section. We achieve loading and laser-cooling of all the naturally occurring isotopes, without the need for enriched sources. Laser-heating/cooling is observed to enhance the isotope-selectivity. In the case of the rare species 43Ca+ and 46Ca+, which have not previously been laser-cooled, the loading is not fully isotope-selective but we show that pure crystals of 43Ca+ may nevertheless be obtained. We find that for loading 40Ca+ the 389nm laser may be replaced by an incoherent source.