Photoionisation Loading of Large {Sr}+ Ion Clouds with Ultrafast Pulses

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This paper reports on the use of ultrafast pulses for photoionisation loading of singly-ionised strontium ions in a linear Paul trap. We take advantage of an autoionising resonance of Sr neutral atoms to form Sr+ by two-photon absorption of femtosecond pulses at a wavelength of 431 nm. We compare this technique to electron-bombardment ionisation and observe several advantages of photoionisation. It actually allows for the loading of a pure Sr+ ion cloud in a low radio-frequency voltage amplitude regime. In these conditions, up to 4\texttimes 104 laser-cooled Sr+ ions were trapped.