Efficient photoionization loading of trapped ions with ultrafast pulses

Atomic cadmium ions are loaded into radiofrequency ion traps by photoionization of atoms in a cadmium vapor with ultrafast laser pulses. The photoionization is driven through an intermediate atomic resonance with a frequency-quadrupled mode-locked Ti:sapphire laser that produces pulses of either $100\text{−}\mathrm{fs}$ or $1\text{−}\mathrm{ps}$ duration at a central wavelength of $229\mathrm{nm}$. The large bandwidth of the pulses photoionizes all velocity classes of the Cd vapor, resulting in a high loading efficiency compared to previous ion trap loading techniques. Measured loading rates are compared with a simple theoretical model, and we conclude that this technique can potentially ionize every atom traversing the laser beam within the trapping volume. This may allow the operation of ion traps with lower levels of background pressures and less trap electrode surface contamination. The technique and laser system reported here should be applicable to loading most laser-cooled ion species.

Figure 1

(a) Relevant energy levels for the neutral Cd atom. From the two-electron ${}^{1}S_0$ ground state, the intermediate excited $5s5p{}^{1}P_1$ state (radiative linewidth $\gamma ∕2\pi =91\mathrm{MHz}$) is populated with a pulsed laser near $\lambda =228.9\mathrm{nm}$. This state is well above the midpoint of the energy difference between the $5s^2{}^{1}S_0$ ground state and the continuum, so the same laser can subsequently ionize the atom. (b) Relevant energy levels for even isotopes of the ${\mathrm{Cd}}^{+}$ ion. A continuous wave laser tuned red of the $5s{}^{2}S_{1∕2}\to 5p{}^{2}P_{3∕2}$ cycling transition near $\lambda =214.5\mathrm{nm}$ (radiative linewidth of ${\gamma }_o∕2\pi =60\mathrm{MHz}$) provides Doppler cooling of the motion for the newly formed ions and localizes them to the center of the rf trap. For odd isotopes, there is also hyperfine structure.

Figure 2

(Color online) Ion traps used in this experiment: (a) ring-fork quadrupole trap [18, 20, 21], (b) three-layer linear trap [18, 22], (c) GaAs chip linear trap [23], (d) two-needle quadrupole trap [24], and (e) four-rod linear trap [21].

Figure 3

Fluorescence spectrum of a single trapped ${}^{111}\mathrm{Cd}^{+}$ ion as the center frequency of the psec laser is scanned [25]. The observed $0.07\text{−}\mathrm{nm}$ $\left(400\mathrm{GHz}\right)$ bandwidth is consistent with a transform-limited pulse of duration $\tau \approx 1\mathrm{ps}$. The dashed line indicates the expected resonance position of the ${}^{2}S_{1∕2}\to {}^{2}P_{1∕2}$ transition in ${}^{111}\mathrm{Cd}^{+}$ at $226.57\mathrm{nm}$.

Figure 4

Measured spectrum of the fsec laser at a central wavelength of $915\mathrm{nm}$. The full width at half maximum bandwidth of approximately $6$ nm is consistent with the production of $\tau \approx 200\text{−}\mathrm{fs}$ pulses in the infrared. When quadrupled to the ultraviolet, we expect pulses of duration around $100\mathrm{fs}$.

Figure 5

Schematic for the layout of the experiment, with the GaAs chip trap of Fig. 2c.

Figure 6

Schematic of photoionization laser focused through ion trap volume. The laser has waist $\rho$ ($1∕e$ electric-field radius) and overlaps with the ion trap volume over an axial distance $L$. This gives a cylindrical ion trap loading volume of $\pi {\rho }^{2}L$, assuming the Rayleigh range $\pi {\rho }^2∕\lambda$ of the laser beam to be much larger than $L$.

Figure 7

Loading rate of ${\mathrm{Cd}}^{+}$ ions in the ring-fork quadrupole trap of Fig. 2a vs detuning of the psec photoionization laser from the neutral Cd ${}^{1}S_0\to {}^{1}P_1$ transition at $228.9\mathrm{nm}$. The laser repetition rate is $80\mathrm{MHz}$, and each pulse has approximate duration $1\mathrm{ps}$ and energy $60\mathrm{pJ}$. The loading rate tracks the spectrum of the psec laser, with a bandwidth of approximately $400\mathrm{GHz}$. The dashed line indicates the expected resonance position of the ${}^{1}S_0\to {}^{1}P_1$ transition in neutral Cd at $228.9\mathrm{nm}$.

Figure 8

Observed loading rate vs pulse energy in the fsec photoionization laser in the linear trap of Fig. 2b. The pulse energy is varied with attenuators, and the waist of approximately $\rho \approx 25\mu \mathrm{m}$ is held constant. At the highest energy of $60\mathrm{pJ}$, we estimate that $\theta \simeq 0.3\pi$. The line is the best-fit quadratic dependence of loading rate on energy as expected from theory. The observed absolute loading rate is within an order of magnitude of theory, which is reasonable considering the many other variables that impact successful loading and the uncertainty in the photoionization cross section. The considerable spread in the data may be attributed to varying positions of the pulsed laser within the ion trap volume or varying trap characteristics, as the data were taken over the course of several weeks.

Figure 9

Histogram of the number of ions loaded in a $0.5\text{−}\mathrm{s}$ exposure interval to the fsec pulsed laser in the large four-rod linear trap of Fig. 2e. The two sets of bars correspond to the presence (206 trials) and absence (152 trials) of the cw Doppler cooling laser beam during the loading process. The average loading rate is observed to be $0.7{\mathrm{s}}^{-1}$ $\left(0.9{\mathrm{s}}^{-1}\right)$ when the cw Doppler laser beam is present (absent), with the discrepancy not statistically significant.

Figure 10

(Color online) Schematic of the cavity for the long-wavelength mode-locked Ti:sapphire pulsed laser. The general configuration is identical to the cavity design of Ref. 26.

Figure 11

Simplified energy-level diagram for photoionization from the ground $S$ state through the intermediate $P$ state (radiative decay rate $\gamma$) and into the continuum. Laser pulses of duration $\tau$ and period $T⪢1∕\gamma$ are resonant with the $S\text{−}P$ electric dipole transition, with Rabi frequency $g$ per pulse. The same laser pulse can ionize the atom from the $P$ state with rate $\Gamma$, or alternatively, a second continuous-wave (cw) laser can ionize the atom from the $P$ state with rate ${\Gamma }_{\mathrm{cw}}$.

Figure 12

Section of cylindrical loading volume of radius $\rho$ and length $L⪢\rho$. An atom moving with speed $v$ enters the surface of the cylindrical volume at polar (azimuthal) angle ${\theta }^{\prime }$ $\left({\varphi }^{\prime }\right)$. For ${\varphi }^{\prime }=0$, the atom crosses the center of the laser beam.