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Physical Review A

Stimulated focusing and deflection of an atomic beam using picosecond laser pulses

A. Goepfert, Immanuel Bloch, D. Haubrich, Frank Lison, R. Schütze, R. Wynands, Dieter Meschede

Published Nov 1, 1997• 35 citations
Publisher

Abstract

Using the stimulated force exerted by counterpropagating \ensuremath{\pi} pulses from a mode-locked Ti:sapphire laser we have focused a beam of laser-cooled cesium atoms along one dimension to about 57% of its original width in the detection zone. We determined the force profile outside and inside the overlap region of the pulses and found agreement with an earlier theoretical prediction. The scheme does not require an effective two-level system and is therefore suitable for a large variety of elements.

DOI
10.1103/physreva.56.r3354

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Using the stimulated force exerted by counterpropagating \ensuremath{\pi} pulses from a mode-locked Ti:sapphire laser we have focused a beam of laser-cooled cesium atoms along one dimension to about 57% of its original width in the detection zone. We determined the force profile outside and inside the overlap region of the pulses and found agreement with an earlier theoretical prediction. The scheme does not require an effective two-level system and is therefore suitable for a large variety of elements.
Using the stimulated force exerted by counterpropagating picosecond laser pulses from a mode-locked Ti:Sapphire laser we were able to focus a beam of laser-cooled cesium atoms along one dimension to about 57% of its original width in the detection zone. The force profile was measured outside and inside the overlap region of the pulses and found to be in agreement with an earlier theoretical prediction. A brief theoretical account of the interaction of atoms with pulsed laser light based on the optical Bloch equations is given.
We have measured the deflection of a thermal sodium beam by a force existing in a field of two counterpropagating short laser pulses. This field was created by a Π-pulse train of a mode-locked (mode) (spacing 82 MHz) dye laser beam, perpendicularly crossing a thermal sodium beam and retroreflected from a mirror behind it. Unlike the spontaneous force, this Π-pulse force is not limited by saturation effects and does not heat up the sodium beam transversally. Calculations yield a maximum Π-pulse force about three times larger than the spontaneous force, being in good agreement with our experimental results.
Recent advances in experimental laser cooling have shown the possibility of stopping an atomic beam using the light pressure force of a counter-propagating laser wave. As an application to laser cooling, it is proposed to build a single frequency cesium laser that has a narrow linewidth. Laser cooling techniques are used to cool an atomic beam of cesium to an average velocity of 5 m/s, corresponding to a temperature of 0.2°K. Expressions of the basic forces that a laser wave exerts on atoms are derived according to a semi-classical approach. The experimental problems and methods of avoiding these problems are treated in detail. A computer Monte-Carlo simulation is used to discuss the feasibility of building the proposed laser. This simulation was done for an ensemble of 10,000 atoms of cesium, and it included the effects of the gravitational force and the related experimental variables. The possibility of building single frequency lasers that use a cooled medium of noble gases, and many other applications of laser cooling are briefly discussed at the end of this work.
A picosecond laser is used to realize atomic beam splitters based on stimulated Raman transitions. With this approach, the interaction between the laser and the atom is localized in the overlap zone of pairs of counterpropagating picosecond pulses. This imposes constraints for implementing interferometers on free-falling atoms. We have developed a robust technique to control the pulses' overlap and ensure that it follows the atom's trajectory while minimizing the induced laser phase noise. We also demonstrate an atom interferometer where the atomic beam splitters are applied to one arm of the interferometer without interacting with the atomic wave packet propagating along the other arm.
Atomic cadmium ions are loaded into radiofrequency ion traps by photoionization of atoms in a cadmium vapor with ultrafast laser pulses. The photoionization is driven through an intermediate atomic resonance with a frequency-quadrupled mode-locked Ti:sapphire laser that produces pulses of either $100\text{\ensuremath{-}}\mathrm{fs}$ or $1\text{\ensuremath{-}}\mathrm{ps}$ duration at a central wavelength of 229nm229\phantom{\rule{0.3em}{0ex}}\mathrm{nm}229nm. The large bandwidth of the pulses photoionizes all velocity classes of the Cd vapor, resulting in a high loading efficiency compared to previous ion trap loading techniques. Measured loading rates are compared with a simple theoretical model, and we conclude that this technique can potentially ionize every atom traversing the laser beam within the trapping volume. This may allow the operation of ion traps with lower levels of background pressures and less trap electrode surface contamination. The technique and laser system reported here should be applicable to loading most laser-cooled ion species.
The spontaneous decay rate $\ensuremath{\gamma}$ of excited atomic states limits the radiative force on atoms to $\ensuremath{\Elzxh}k\ensuremath{\gamma}/2.$ This bottleneck in the dissipative exchange of momentum between atoms and light can be overcome by coherent control of the momentum exchange interactions. The bichromatic force provides such control by using stimulated instead of spontaneous emission to produce the force, while at the same time exploiting spontaneous emission to enable a dissipative interaction. It is implemented with two counterpropagating light beams, each containing two frequencies, whose phases, amplitudes, and frequency difference are carefully chosen. We have made precise measurements of the extremely large magnitude and velocity range of this bichromatic force, and have shown that its velocity dependence near the edge of its range is suitable for atomic beam slowing and laser cooling. Our measurements have corroborated various models and calculations of this bichromatic force.