Controlled photoionization loading of 88Sr+ for precision ion-trap experiments

We report on a photoionization scheme for controlled loading of Sr-88(+) ions into a radio-frequency (rf) ion trap. The two-step doubly-resonant process drives the Sr-88 atom on the 5s(2 1)S(0) - 5s5p P-1(1) transition at 461 nm using a frequency-doubled diode laser, and then excites the atom to the autoionizing (4d(2) + 5p(2)) D-1(2) state with 405 nm light from a free-running diode laser. This method of trap loading is quantitatively compared to electron bombardment loading, and shown to reduce the Sr vapour pressure required to load by four orders of magnitude. It also provides more than an order of magnitude reduction in the day-to-day variation of the voltages required to compensate micromotion. We additionally introduce a two-step atomic source consisting of an oven and hotplate, which reduces the number of impurities evaporated towards the trap. This is shown to significantly reduce fluctuations in required micromotion compensation voltages. The demonstrated reduction in flux and increased source purity are expected to improve the reliability of trap operation for precision ion-trap experiments.