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 S0 ↔ 4s4p P1 transition of neutral calcium is driven by a 423 nm laser and the atoms are photo-ionized by a second laser at 389 nm. Isotope-selectivity is achieved by using crossed atomic and laser beams to reduce the Doppler width significantly below the isotope shifts in the 423 nm transition. The loading rate of ions into the trap is studied under a range of experimental parameters for the abundant isotope Ca. Using the fluorescence of the atomic beam at 423 nm 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 Ca and Ca, which have not previously been laser-cooled, the loading is not fully isotope-selective but we show that pure crystals of Ca may nevertheless be obtained. We find that for loading Ca the 389 nm laser may be replaced by an incoherent source.