Efficient photo-ionization for barium ion trapping using a dipole-allowed resonant two-photon transition

Two efficient and isotope-selective resonant two-photon ionization techniques for loading barium ions into radio-frequency (RF)-traps are demonstrated. The scheme of using the strong dipole-allowed transition 6s ²∈ ¹S ₀→6s6p∈ ¹P ₁ at λ=553 nm as a first step towards ionization is compared to the established technique of using a weak inter-combination line (6s ²∈ ¹S ₀→5d6p∈ ³D ₁,λ=413 nm). An increase of two orders of magnitude in the ionization efficiency is found favoring the transition at 553 nm. This technique can be implemented using commercial all-solid-state laser systems and is expected to be advantageous compared to other narrowband photo-ionization schemes of barium in cases where highest efficiency and isotope-selectivity are required.