Ph.D. Thesis - Physics

The base pressure of the vacuum chamber was 2×10 −9 torr, but rose to 3×10 −9 torr
when the ablation laser was fired 10 times in 10 seconds. This rise is less than the typical
value when one is first using an oven, but somewhat larger than the pressure rise due to an
oven after a long period of use.
A number of different ablation targets were tested. These include Sr metal (99% pure
random pieces from Sigma-Aldrich), Sr/Al alloy (10% Sr, 90% Al by mass from KB Alloys),
single crystal SrTiO3 (〈100〉 crystal orientation from Sigma-Aldrich), and SrTiO3 powder in
an epoxy resin (5 µm SrTiO3 powder from Sigma-Aldrich mixed with Loctite 5 min epoxy).
Of all the targets only Sr metal oxidizes in air, so although it’s the most obvious choice
of material, it may not be best. Results obtained with each of these targets are presented
below.
6.5.2 Experimental results
We now present data to answer each of the above research questions. We begin with
studying the effects of the composition of the target material. The goal is to measure both
the efficiency of the loading process and the number of times a single spot on the target
may be ablated before the efficiency begins to decrease. This is known to happen due to a
profile being formed in the material that modifies the ablation process [CH94]. This process
is not a fundamental limitation, however, since the spot being ablated can be varied from
shot to shot.
We study this question by ablating a given spot on each target a number of times,
and measuring the ion signal in each load. Since the electrons that short the trap during
ablation remove the ions already in the trap, we measure after each shot only the number
of ions loaded during that shot. We find that there is some variance in the number of ions
loaded per pulse, which is not ideal. More is said on this later. The data for each target is
presented in Fig. 6-14.
In all, we find that the SrTiO3 crystal is the “best” target choice. It produces, overall
much more consistent ion numbers than the other targets, and has the longest lifetime, as
well. The overall lower number of ions loaded is not a problem, since we are interested
mainly in loading small numbers of ions. It is somewhat odd that the ion loads from the
alloy peak after around 100 loads, but it is possible that impurities on the surface must be
removed before loading efficiency can reach its maximum potential.
Next, we turn to the question of loading into low trap depths. As in all our other traps,
the depth is calculated using CPO and the pseudopotential approximation. Here, ions are
loaded into the trap at a series of decreasing rf voltages which correspond to decreasing
trap depths. We use an ablation laser pulse energy of 1.1 mJ and a spot size of 680 µm
for this experiment; these values were chosen to maximize the ion signal at low trap depth.
Our results are presented in Fig. 6-15. We found that ions could be loaded into a minimum
trap depth of 40 meV, comparable to the shallowest depths loaded with photoionizaton of
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