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Journal of the Optical Society of America B

Optical collimation of an atomic beam using a white light molasses

R. D. Glover, Thierry Bastin

Published Apr 1, 2015• 7 citations
Publisher

Abstract

We investigate optical collimation of an atomic beam using a spectrally broadened white light molasses. We compare this technique with the standard technique using an atomic collimator and find that, under certain conditions, the white light molasses is preferable. Specifically, a white light molasses is suitable for atomic beams with relatively low mean velocities and broad velocity distributions, for example, a beam effusing from a cryogenically cooled reservoir. This result is demonstrated by numerically modeling the atomic trajectories of argon atoms effusing from a cryogenically cooled reservoir. We find that, given sufficient power, a white light molasses captures a larger fraction of atoms when compared to an atomic collimator, and the velocity distribution of the resulting beam is shifted toward slower velocities. Further, an optical molasses is geometrically relatively simple compared to an atomic collimator; therefore, the complexity of the in-vacuum optical arrangement can be significantly reduced.

DOI
10.1364/josab.32.0000b1

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We investigate optical collimation of an atomic beam using a spectrally broadened white light molasses. We compare this technique with the standard technique using an atomic collimator and find that, under certain conditions, the white light molasses is preferable. Specifically, a white light molasses is suitable for atomic beams with relatively low mean velocities and broad velocity distributions, for example, a beam effusing from a cryogenically cooled reservoir. This result is demonstrated by numerically modeling the atomic trajectories of argon atoms effusing from a cryogenically cooled reservoir. We find that, given sufficient power, a white light molasses captures a larger fraction of atoms when compared to an atomic collimator, and the velocity distribution of the resulting beam is shifted toward slower velocities. Further, an optical molasses is geometrically relatively simple compared to an atomic collimator; therefore, the complexity of the in-vacuum optical arrangement can be significantly reduced.
A thermal atomic beam from a cesium oven was slowed down by use of the Hoffnagle modified white-light cooling technique. In addition, the atomic beam was collimated by use of a two-dimensional optical molasses that was installed transverse to the atomic-beam direction. The flux of the atomic beam was 2×1010 atoms/s, an increase of a factor of 16 as a result of the collimation. The mean longitudinal velocity was ∼24.4 m/s, and the rms velocity spread of the slowed atomic beam was ∼1 m/s. Compared with other methods, we found that the Hoffnagle method is suitable for the generation of slow atomic beams to be used in an atomic clock, which requires an ultralow magnetic field environment. This atomic beam was deflected by an angle of 30° by a one-dimensional optical molasses to separate it from laser light and high-velocity atoms.
Engineering continuous, high-flux, and collimated atomic beams is a useful resource for metrology and material deposition. Developments in this area have been essential for the evolution of cold-atom based quantum experiments, yet the ubiquitous balanced-force methods such as transverse molasses cooling degrade at high atomic flux due to absorption-induced force imbalance. We introduce a collimation scheme that combines the use of a broadband transition for velocity shifting with a narrowband transition for velocity selection, enabling velocity-selective beam collimation without relying on balanced-power counter-propagating beams. Collimated atoms are shelved in a long-lived internal state, reducing the total light scattering and providing a state-heralded collimated beam. Simulations using strontium as a model system show a highly effective collimation process that does not suffer from absorption-induced force-imbalance and experimental results agree well with these predictions.
The number of laser cooled and trapped thulium atoms in a magneto-optical trap is increased by a factor of 3 using a two-dimensional optical molasses which collimated the atomic beam before entering a Zeeman slower. A diode laser operating at 410.6 nm was employed to form optical molasses: The laser was heated to 70 Degree-Sign C by a two-step temperature stabilisation system. The laser system consisting of a master oscillator and an injection-locked amplifier emitted more than 100 mW at 410 nm and had a spectral linewidth of 0.6 MHz. (extreme light fields and their applications)
In the early 1970s, various researchers showed that the force exerted by light on an atomic beam could deflect the beam. In particular, traveling wave light perpendicular to the beam could be used to impart velocity along the wave vector k of the light.1,2 In these experiments, the angular deflection of an atom depended on the interaction time between the atom and field and thus on the longitudinal velocity of the atoms; consequently, the resulting atomic beam had an increased angular divergence. Other experiments have demonstrated that light could be used to collimate an atomic beam.3 In this experiment we show that a 1-D spontaneous optical molasses nearly perpendicular to an atomic beam can be used to both collimate and angularly deflect an atomic beam, where the deflection is insensitive to longitudinal velocity. In fact, this technique decreases rather than increases the angular divergence of the initial beam. In our experiment we started with a 300°C thermal sodium atomic beam with an angular divergence of ~10 mrad. The light was detuned by Δ ≈ − 20 MHz from the 32S1/2 → 32P3/2 transition in sodium, which corresponds to twice the natural linewidth Γ. Using 1-D molasses formed from counterpropagating light beams of the 1/e waist size of the order of 3 mm, we were able to produce an atomic beam with an ~l-mrad divergence and an angular deflection of >10 mrad. The maximum possible angular deflection θ is of the order.
We have discovered a new cooling process that uses an applied magnetic field to mix differentially light shifted atomic ground state sublevels. In related experiments cooling is achieved by mixing caused by polarization gradients. Because the energy loss mechanism stems from the light shift and not the momentum transfer of a scattering event, the accessible temperature is much lower than the limit of Doppler cooling. We used this method for optical collimation of an 85Rb atomic beam to a transverse rms speed of 3 cm/s, well below the v = ( 7 ℏ γ / 20 M ) ≅ 10 cm / s of the Doppler limit for two-level atoms in this 1-D optical molasses. This low velocity, limited only by experimental geometry, is achieved only when a magnetic field of ~20 µT is applied perpendicular to the axis of the circularly polarized diode laser beam. We use a thermal beam of natural Rb produced by an oven at T ~ 150°C with an ~0.33-mm diam aperture and a defining aperture of ~0.33-mm diam about 24 cm away. The atoms are optically collimated by laser beams transverse to their motion that excite the transition at λ = 780 nm. The atomic beam profile is measured with a vertically oriented scanning hot tungsten wire, 25 µm in diameter, 1.3 m away.
We present a three-dimensional theoretical comparison between the radiation-pressure forces exerted on an atom in an isotropic light cooling scheme and in a six-beam molasses. We demonstrate that, in the case of a background vapor where all the space directions of the atomic motion have to be considered, the mean cooling rate is equal in both configurations. Nevertheless, we also point out what mainly differentiates the two cooling techniques: the force component orthogonal to the atomic motion. If this transverse force is always null in the isotropic light case, it can exceed the radiation-pressure-force longitudinal component in the six-beam molasses configuration for high atomic velocities, hence reducing the velocity capture range.