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Journal of the Optical Society of America B

Diode-laser deceleration and collimation of a rubidium beam

B. Sheehy, S-Q. Shang, Richard N. Watts, S. Hatamian, H. Metcalf

Published Nov 1, 1989• 51 citations
Publisher

Abstract

We report the use of frequency-chirped diode lasers to decelerate a thermal atomic beam of Rb and, in a separate experiment, the use of frequency-stabilized diode lasers to collimate the beam with optical molasses. For this we used a 100-mW cw laser-diode array injection locked to light from a low-power, frequency-narrowed, single-stripe diode laser. Also, a novel scheme for imaging the neutral-atom beam profile has been developed for monitoring the collimation experiments in two dimensions. We have increased both beam brightness and intensity by a factor of more than 20.

DOI
10.1364/josab.6.002165

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We report the use of frequency-chirped diode lasers to decelerate a thermal atomic beam of Rb and, in a separate experiment, the use of frequency-stabilized diode lasers to collimate the beam with optical molasses. For this we used a 100-mW cw laser-diode array injection locked to light from a low-power, frequency-narrowed, single-stripe diode laser. Also, a novel scheme for imaging the neutral-atom beam profile has been developed for monitoring the collimation experiments in two dimensions. We have increased both beam brightness and intensity by a factor of more than 20.
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 have laser cooled an atomic beam of rubidium with diode lasers and have used these slow atoms to load a magneto-optical trap. The cooling is performed with the frequency-chirp method,1,2 in which the laser's frequency is ramped linearly (by the injection currant) to maintain resonance with the decelerating atoms. Two synchronously chirped lasers are required to prevent optical hyperfine pumping of the ground state. The chirp typically covers 500 MHz in 10 ms. At the end of each ramp, the resulting velocity distribution is measured with a pulsed probe laser. By varying the end point of the chirp, we are able to obtain slow atoms with average velocities ranging from 200 to −50 m/s. The widths of the velocity distributions are measured to be approximately 40 m/s. This relatively large width results from the inherent linewidth (~30 MHz) of the free-running diode lasers used for both cooling and probing.
We describe the results of three sets of experiments to investigate producing cold slow collimated potassium atoms for applications with no access available for the introduction of counterpropagating laser light along the exiting atomic beam's axis. Velocity profiles are obtained via fluorescence from D1 excitation at perpendicular and nearly parallel incidence by a probe laser. Spatial profiles of both the cold- and residual hot-beam components are measured with a position-scanned hot-wire detector. Velocity-component holes and decelerated cold components are observed, along with direct evidence of perpendicular heating of the cold components. A first set of experiments uses a variant of conventional chirp cooling and/or hole burning. Light is introduced by a heated inclined mirror, while atoms exit through a slit in the mirror. Loss of intensity at low velocity is observed and discussed. A second set of experiments investigates velocity-selective optical-pumping crossover resonances that occur near the mirror, where the incident and reflected beams overlap for several alternative geometries. A third set of experiments investigates cold-beam production using laser-induced velocity selection and deflection. It provides a higher cold-atom intensity, along with the means to perform high-resolution spectroscopy.
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)