All-Optical Realization of an Atom Laser

We demonstrate an atom laser using all-optical techniques. A Bose-Einstein condensate of rubidium atoms is created by direct evaporative cooling in a quasistatic dipole trap realized with a single, tightly focused ${\mathrm{C}\mathrm{O}}_2$-laser beam. An applied magnetic field gradient allows the formation of the condensate in a field-insensitive $m_F=0$ spin projection only, which suppresses fluctuations of the chemical potential from stray magnetic fields. A collimated and monoenergetic beam of atoms is extracted from the Bose-Einstein condensate by continuously lowering the dipole trapping potential in a controlled way to form a novel type of atom laser.

Figure 1

False-color shadow images of the atomic cloud after 15 ms of free expansion (field of view: 0.33 mm square). (a) Stern-Gerlach magnetic field gradient applied throughout the experiment, so that a pure $m_F=0$ condensate is produced. (b) Field gradient activated only during free expansion phase. The three spin projections $m_F=-1$, $0$, and $1$ of a spinor condensate are visible as separate clouds.

Figure 2

Vertical cuts through shadow images recorded for pure $m_F=0$ ensembles for different final trap laser powers after 15 ms of free expansion. (a) 500 mW: yielding a thermal cloud with $T\simeq 350\mathrm{n}\mathrm{K}$, (b) 240 mW: partly condensed ($T\simeq 180\mathrm{n}\mathrm{K}$). The dashed line is to guide the eye in distinguishing the broad thermal background from the narrow condensate peak. (c) 200 mW, resulting in an almost pure Bose-Einstein condensate.

Figure 3

(a) False-color shadow image of the generated atom laser beam. The field of view comprises 0.28 mm by 0.5 mm. (b) Fitted transverse cut of the image averaged over a 0.19 mm beam length.