Laser ablation loading of a surface-electrode ion trap

We demonstrate loading of ${}^{88}\mathrm{Sr}^{+}$ ions by laser ablation into a mm-scale surface-electrode ion trap. The laser used for ablation is a pulsed, frequency-tripled Nd:YAG with pulse energies of $1–10\mathrm{mJ}$ and durations of $4\mathrm{ns}$. An additional laser is not required to photoionize the ablated material. The efficiency and lifetime of several candidate materials for the laser ablation target are characterized by measuring the trapped ion fluorescence signal for a number of consecutive loads. Additionally, laser ablation is used to load traps with a trap depth $\left(40\mathrm{meV}\right)$ below where electron impact ionization loading is typically successful $(\gtrsim 500\mathrm{meV})$.

Figure 1

(Color online) The surface-electrode ion trap used for testing ablation loading. The rf electrodes are spaced by $2\mathrm{mm}$ center-to-center, leading to an ion height above the trap of $0.8\mathrm{mm}$. The long center electrode is held at rf ground, but may have a dc offset applied to it. The segmented electrodes on the sides carry dc potentials for confinement along the long axis of the trap, as well as elimination of stray electric fields.

Figure 2

(Color online) A diagram of the setup showing the position and orientation of the ablation target relative to the ion trap. The surface of the ablation target is approximately $25\mathrm{mm}$ from the trap center and is orthogonal to the direction to the ion trap. Not to scale.

Figure 3

(Color online) A plot of the trapped ion signal as a function of the number of ablation pulses fired on a single spot of the target for several ablation target materials. Each point represents the signal due to a single ablation pulse of energy $8\mathrm{mJ}$. The ions from the previous pulse are lost when the electrons in the ablation plume short the trap, so the trapped ion signal is roughly proportional to the number of ions loaded by a single pulse of the ablation laser. For this experiment, the ablation laser was focused to a spot size of $300\mu \mathrm{m}$. For reference, a single ion scatters roughly 0.2 photons/ms into the PMT in this setup.

Figure 4

(Color online) A plot of the trapped ion signal as a function of the computed trap depth for both ablation and electron impact ionization loading. An ablation pulse energy of $1.1\mathrm{mJ}$ was used with a spot size of $680\mu \mathrm{m}$. Each point is the ion signal obtained either from a single pulse of the ablation laser or from loading using electron impact ionization until the ion signal stops increasing.

Figure 5

(Color online) Probability distribution of the number of ions loaded with a single ablation laser pulse. The circles are experimental data and the line is a Poisson fit with a mean ion number of 0.16. The experiment was performed in a smaller ion trap than the one used for the other experiments in this paper with an ablation pulse energy of $2\mathrm{mJ}$ and a spot size of $500\mu \mathrm{m}$.