Two-dimensional magneto-optical trap as a source for cold strontium atoms

We report on the realization of a transversely loaded two-dimensional magneto-optical trap serving as a source for cold strontium atoms. We analyze the dependence of the source's properties on various parameters, in particular the intensity of a pushing beam accelerating the atoms out of the source. An atomic flux exceeding ${10}^9\mathrm{atoms}/\mathrm{s}$ at a rather moderate oven temperature of $500{}^{\circ }\mathrm{C}$ is achieved. The longitudinal velocity of the atomic beam can be tuned over several tens of m/s by adjusting the power of the pushing laser beam. The beam divergence is around 60 mrad, determined by the transverse velocity distribution of the cold atoms. The slow atom source is used to load a three-dimensional magneto-optical trap realizing loading rates up to ${10}^9\mathrm{atoms}/\mathrm{s}$ without indication of saturation of the loading rate for increasing oven temperature. The compact setup avoids undesired effects found in alternative sources like, e.g., Zeeman slowers, such as vacuum contamination and black-body radiation due to the hot strontium oven.

Figure 1

Schematic of the experimental setup for the transversely loaded 2D-MOT. Left: Experimental setup. Hot strontium atoms effusing from the oven in the bottom of the setup are cooled and trapped in the center of the multiway cross vacuum chamber by a perpendicular pair of retroreflected cooling beams. Atoms in the 2D-MOT are pushed to a UHV chamber (not shown) where they are further loaded into a 3D-MOT. A Zeeman slower beam (optional) is used to decelerate hot strontium atoms through the top viewport of the setup. Heated parts of the setup are highlighted in red. The vacuum of the setup is maintained by an ion getter pump. Center: Zoom-in of the highlighted rectangle in the left part. Four stacks of permanent magnets are used to generate the 2D quadrupolar magnetic field. A differential pumping tube is located between the 2D-MOT vacuum chamber and the UHV chamber for vacuum isolation. Right: Magnetic field along the $z$ direction. The red shaded area depicts the strontium oven, while the blue shaded area represents the 2D cooling beams. For Zeeman deceleration, either region $①$ or $②$ can be used.

Figure 2

Energy-level scheme and schematic of the laser setup for the strontium 2D-MOT. (a) Energy diagram of the bosonic strontium. $5s5p{}^3{P}_2$ is a dark state in the laser cooling and trapping. It is also a reservoir state for the magnetic-field trap. Spectroscopic parameters are taken from Ref. [40]; decay rates involving $5s4d{}^1{D}_2$ can be found in Ref. [34]. (b) Schematic of the laser setup. A frequency-doubled Ti:sapphire laser system at a wavelength of 461 nm is used to provide all laser beams for the experiment. The Ti:sapphire laser frequency is stabilized by a high-finesse wavemeter with a servo loop. QWP: quarter-wave plate; HWP: half-wave plate; WLM: wavemeter.

Figure 3

The most probable longitudinal velocity, divergence, and the total flux of the atomic beam versus the pushing beam light intensity, respectively. The insets in (a) and (b) show the longitudinal velocity distribution and the transverse distribution of the atomic beam at $I_{\mathrm{p}}=0.33I_{\mathrm{sat}}$. The field gradient is chosen to $50\mathrm{G}/\mathrm{cm}$, the oven temperature is $465{}^{\circ }\mathrm{C}$, and Zeeman slower loading was applied (see Sec. 5).

Figure 4

3D-MOT loading rate as a function of the pushing beam light intensity, the 2D-MOT cooling beam light intensity, and the oven temperature. For each panel, the unvaried parameters are set to $0.2I_{\mathrm{sat}}$, $4I_{\mathrm{sat}}$ (four beams combined), and $465{}^{\circ }\mathrm{C}$, respectively. (a) Loading rate versus the pushing beam light intensity. The triangles indicate results of a model calculation. (b) Loading rate versus the 2D-MOT cooling beam light intensity. The solid line represents a fit with a model. (c) Loading rate versus the oven temperature (using flux enhancement of the 2D-MOT by the decreasing field Zeeman slower for this set of measurements). The solid line represents a model calculating the flux from the strontium vapor pressure and the 2D-MOT capture efficiency. Details on all model calculations can be found in the text.