Isotope-selective photoionization for calcium ion trapping

Abstract

We present studies of resonance-enhanced photoionization for isotope-selective loading of Ca ϩ into a Paul trap. The 4s 2 1 S 0 ↔4s4p 1 P 1 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 40 Ca ϩ . 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 170͑60͒ 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 43 Ca ϩ and 46 Ca ϩ , which have not previously been laser cooled, the loading is not fully isotope selective, but we show that pure crystals of 43 Ca ϩ may nevertheless be obtained. We find that for loading 40 Ca ϩ the 389 nm laser may be replaced by an incoherent source.

Two-color photoionization of calcium using SHG and LED light

“…It was observed that even if the 390nm laser is operated below its lasing threshold current, its weak incoherent emission was sufficient to load ions [4]. The efficiency of this process was observed to be independent of the linewidth, as a result of the broad autoionizing resonances in the vicinity of the first ionization limit [8,9,10].…”

“…The S-P dipole transition has the broadest linewidth of all transitions in Ca I (35.4MHz) and can easily be saturated with a laser tuned to resonance [4]. To excite the electron from the 1 P 1 -state into the continuum above the first ionization limit, a second photon of wavelength λ ≤ 389.89nm is necessary, as e.g.…”

“…⋆ Fax: +34-93-553-4000, E-mail: carsten.schuck@icfo.es and 390nm [3,4]. The idea of the scheme, as shown in Fig.…”

“…This is achieved with low-power photoionization light sources and low-flux atomic beams. Only for trapping of rare isotopes, slightly higher rates may be necessary to beat charge exchange processes with 40 Ca from the atomic beam [4].…”

“…The experience with a sub-threshold 390nm-laser [4] and light emitting diodes [11,12] also led us to implementing the second excitation step from the 1 P 1 -state into the continuum with a high-power LED and a custom designed optical system. The two systems will be described in more detail in the following sections.…”