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Journal of Modern Optics

Imaging velocity selective resonances in a magnetic field

Fredrik K. Fatemi, Matthew L. Terraciano, Zachary Dutton, Mark Bashkansky

Published Sep 14, 2009• 0 citations
Publisher

Abstract

We demonstrate techniques for imaging both magnetic fields and magnetic sublevel distributions using velocity-selective Raman transitions in cold 87Rb and 85Rb ensembles. When an expanding atom cloud is exposed to counterpropagating laser beams, resonant atom trajectories undergoing Raman transitions are perturbed and result in distinctive features in images of the expanded cloud. If the counterpropagating laser fields couple magnetic sublevels in different hyperfine manifolds, the images provide a spectrograph of sublevel occupation. The techniques are simple to implement and provide a useful, rapid diagnostic tool for cold atom experiments.

DOI
10.1080/09500340903225510

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We demonstrate techniques for imaging both magnetic fields and magnetic sublevel distributions using velocity-selective Raman transitions in cold 87Rb and 85Rb ensembles. When an expanding atom cloud is exposed to counterpropagating laser beams, resonant atom trajectories undergoing Raman transitions are perturbed and result in distinctive features in images of the expanded cloud. If the counterpropagating laser fields couple magnetic sublevels in different hyperfine manifolds, the images provide a spectrograph of sublevel occupation. The techniques are simple to implement and provide a useful, rapid diagnostic tool for cold atom experiments.
We have studied velocity-selective resonances in the presence of a uniform magnetic field and shown how they can be used for rapid, single-shot assessment of the ground state magnetic sublevel spectrum in a cold atomic vapor. Cold atoms are released from a magneto-optical trap in the presence of a small bias magnetic field ( approximately 300 mG) and exposed to a laser field comprised of two phase-locked counterpropagating beams connecting the two ground state hyperfine manifolds. An image of the expanded cloud shows the velocity-selected resonances as distinct features, each corresponding to specific magnetic sublevel, in a direct, intuitive manner. We demonstrate the technique with both 87Rb and 85Rb, and show the utility of the technique by optically pumping into particular magnetic sublevels. The results are shown to agree with a theoretical model, and are compared to traditional Raman spectroscopy.
We demonstrate a technique for imaging magnetic fields using velocity-selective two-photon resonances in a cold atom cloud. Freely expanding (85)Rb atoms released from a magneto-optical trap are exposed to a brief (approximately 1 ms), off-resonant, retro-reflected laser pulse in a lin-perp-lin configuration. Two-photon resonance between magnetic sublevels occurs only for atoms in narrow velocity classes dependent on the magnetic field strength. The momentum of resonant atoms is altered by the pulse, and this two-photon momentum change is easily visible after further ballistic expansion. When the momentum pulse is applied to an atom cloud with finite size, magnetic field variations across the sample result in position-dependent features in images of the expanded cloud. We demonstrate the technique by imaging magnetic field variations over approximately 5 mm with approximately 250 microm spatial resolution.
We describe a simple time-of-flight technique for measuring the magnetic moment of an optically pumped magneto-optical trap. The technique relies on free-expansion imaging of a cold atom cloud in a small magnetic field gradient without the need to detect spatial separation between magnetic sublevels. We find that the effective acceleration of the cloud can be used to characterize extreme state optical pumping. In the general case, we show that the integrated displacement of the falling cloud can be accurately modeled using rate equation simulations of magnetic sublevel populations, and knowledge of local magnetic fields, field gradients, and light intensities. The agreement between the model and the data allows the reconstruction of magnetic moments and suggests that this technique may be suitable for the measurement of population distributions over a range of optical pumping conditions.
We report on the use of a stimulated Raman transition in a slowed atomic beam to produce a narrow velocity distribution of atoms in a selected electronic state, which could be used for atomic collisions studies in the low temperature regime(1). The velocity selection is part of the deceleration process and in this sense it is unique. The atomic beam is decelerated by the radiation pressure force exerted on the atoms by a counter- propagating laser beam. The resonance condition is maintained along the deceleration path because the changing Doppler shift is compensated by Zeeman tuning the electronic sublevels(2) with a spatialy inhomogneous magnetic field. At the end of the slowing process, the initial Maxwell-Boltzman distribution is compressed to a narrow velocity distribution (Δv ~ 50 m/s), centered close to v = 0. This velocity bunching(3) increases considerably the number of atoms in each velocity class, allowing the use of velocity selection techniques as a feasible way of studying low velocity collisions.
A method for detecting the internal characteristic (including density, velocity and temperature) distribution of cold atom clouds is reported in this paper. During the detection process, the spatial resolution of the cold atom cloud is achieved at sub-mm by precisely adjusting the spatial position of the Raman laser beam. Meanwhile, to minimize the impact of frequency and intensity fluctuations in the Raman laser on measurement results, we propose to utilize a differential atomic interferometer to suppress common-mode noise, thereby enhancing measurement accuracy. This method enables precise detection of the internal structure of cold atom clouds, providing a reference for in-depth studies of atom cloud properties.