Isotope-selective photoionization for calcium ion trapping

We present studies of resonance-enhanced photoionization for isotope-selective loading of Ca⁺ into a Paul trap. The 4s²¹S₀↔4s4p¹P₁ transition of neutral calcium is driven by a 423 nm laser and the atoms are photoionized 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 ⁴⁰Ca⁺. 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 photoionization cross section of 170(60) Mb. 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 389 nm laser may be replaced by an incoherent source.