Spatially resolved photoionization of ultracold atoms on an atom chip

We report on photoionization of ultracold magnetically trapped Rb atoms on an atom chip. The atoms are trapped at $5\mu \mathrm{K}$ in a strongly anisotropic trap. Through a hole in the chip with a diameter of $150\mu \mathrm{m}$, two laser beams are focused onto a fraction of the atomic cloud. A first laser beam with a wavelength of $778\mathrm{nm}$ excites the atoms via a two-photon transition to the $5D$ level. With a fiber laser at $1080\mathrm{nm}$ the excited atoms are photoionized. Ionization leads to depletion of the atomic density distribution observed by absorption imaging. The resonant ionization spectrum is reported. The setup used in this experiment is suitable not only to investigate mixtures of Bose-Einstein condensates and ions but also for single-atom detection on an atom chip.

Figure 1

(Color online) Ionization scheme. ${}^{87}\mathrm{Rb}$ atoms in the $5S$ ground state are resonantly excited by two-photon absorption into the $5D$ level $(2\times 778\mathrm{nm})$. Before they decay back into the ground state the excited atoms are rapidly ionized with a second laser $\left(1080\mathrm{nm}\right)$.

Figure 2

Simulation of the ionization. The population probability of the different states is plotted for different positions (a) and times (b) as the atom is dragged into the laser beams. The beam radius of the two lasers is set to be $w_0=30\mu \mathrm{m}$ (solid lines). Increasing the waist of the fiber laser to $60\mu \mathrm{m}$ (dashed lines) results in an earlier and faster ionization. The dotted line shows a simulation with the intensities used in Sec. 4.

Figure 3

(Color online) Experimental setup. The laser beams are injected from outside the vacuum chamber and reflected by a mirror. A lens with a focal length of $4\mathrm{cm}$ focuses the light through a hole with a diameter of $150\mu \mathrm{m}$ in the atom chip onto the trapping region of the atoms. Behind the trap the light passes an aperture to minimize stray light. A mirror and a second collimation lens guide the beams out of the chamber.

Figure 4

Absorption image of the atomic cloud. The cloud is exposed (a) to the diode laser at a wavelength of $778\mathrm{nm}$ and power $8\mathrm{mW}$; (b) to the fiber laser at $1080\mathrm{nm}$ and power $2\mathrm{W}$. The dashed circle indicates the area of exposure.

Figure 5

Integrated density profiles. In (a) and (b) only one laser was turned on. As in Fig. 4 the dipole forces lead to an increase of the local density. In (c) both lasers are turned on. With the diode laser tuned out of resonance both dipole forces cancel (dashed line). In resonance additional losses due to ionization appear (solid line). The integrated density is normalized to the unperturbed density distribution.

Figure 6

Resonant ionization spectrum. Tuning the diode laser across the resonances of the $5S\to 5D$ 5D transition leads to losses proportional to the number of ionized atoms.