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Journal of Physics Conference Series

Excited-state imaging of cold atoms

David V. Sheludko, Simon C Bell, Edgar J. D. Vredenbregt, Robert E Scholten

Published Sep 1, 2007• 5 citations
Publisher

Abstract

We have investigated state-selective diffraction contrast imaging (DCI) of cold 85 Rb atoms in the first excited (5 2 P 3/2 ) state. Excited-state DCI requires knowledge of the complex refractive index of the atom cloud, which was calculated numerically using a semi-classical model. The Autler-Townes splitting predicted by the model was verified experimentally, showing excellent agreement. 780 nm lasers were used to cool and excite atoms within a magneto-optical trap, and the atoms were then illuminated by a 776 nm imaging laser. Several excited-state imaging techniques, including blue cascade fluorescence, on-resonance absorption, and DCI have been demonstrated. Initial results show that improved signal-to-noise ratio (SNR) will be required to accurately determine the excited state fraction. We have demonstrated magnetic field gradient compression of the cold atom cloud, and expect that further progress on compression and additional cooling will achieve sufficient diffraction contrast for quantitative state-selective imaging.

DOI
10.1088/1742-6596/80/1/012040

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We have investigated state-selective diffraction contrast imaging (DCI) of cold 85 Rb atoms in the first excited (5 2 P 3/2 ) state. Excited-state DCI requires knowledge of the complex refractive index of the atom cloud, which was calculated numerically using a semi-classical model. The Autler-Townes splitting predicted by the model was verified experimentally, showing excellent agreement. 780 nm lasers were used to cool and excite atoms within a magneto-optical trap, and the atoms were then illuminated by a 776 nm imaging laser. Several excited-state imaging techniques, including blue cascade fluorescence, on-resonance absorption, and DCI have been demonstrated. Initial results show that improved signal-to-noise ratio (SNR) will be required to accurately determine the excited state fraction. We have demonstrated magnetic field gradient compression of the cold atom cloud, and expect that further progress on compression and additional cooling will achieve sufficient diffraction contrast for quantitative state-selective imaging.
Atomic coherence phenomena are usually investigated using single beam techniques without spatial resolution. Here we demonstrate state-selective imaging of cold 85Rb^{85}\text{R}\text{b}85Rb atoms in a three-level ladder system, where the atomic refractive index is sensitive to the quantum coherence state of the atoms. We use a phase-sensitive diffraction contrast imaging (DCI) technique which depends on the complex refractive index of the atom cloud. A semiclassical model allows us to analytically calculate the detuning-dependent refractive index of the system. The predicted Autler-Townes splitting and our experimental measurements are in excellent agreement. DCI provided a quantitative image of the distribution of the excited-state fraction, and compared with on-resonance absorption and blue cascade fluorescence techniques, was found to be experimentally simple and robust.
Imaging cold atom samples provides interesting opportunities for studying not only the external atomic distribution, but also the internal atomic state, particularly for atoms prepared in a coherent state. We have previously developed techniques for off-resonant imaging of a cold atom cloud, without optical elements, computationally extracting the atomic column density map from a diffraction pattern (diffraction contrast imaging or DCI). We are now combining two lasers and three atomic states in a Λ-system to explore atomic coherence via two-dimensional imaging of electromagnetically induced transparency.
We demonstrate a new imaging technique for cold atom clouds based on phase retrieval from a single diffraction measurement. Most single-shot diffractive imaging methods for cold atoms assume a monomorphic object to extract the column density. The method described here allows quantitative imaging of an inhomogeneous cloud, enabling recovery of either the atomic density or the refractive index, provided the other is known. Using ideas borrowed from density functional theory, we calculate the approximate paraxial diffracted intensity derivative from the measured diffracted intensity distribution and use it to solve the Transport of Intensity Equation (TIE) for the phase of the wave at the detector plane. Back-propagation to the object plane yields the object exit surface wave and then provides a quantitative measurement of either the atomic column density or refractive index. Images of homogeneous clouds showed good quantitative agreement with conventional techniques. An inhomogeneous cloud was created using a cascade electromagnetically induced transparency scheme and images of both phase and amplitude parts of refractive index across the cloud were separately retrieved, showing good agreement with theoretical results.
Abstract We propose a method to characterize the features of a cold strontium cloud in a magneto-optical trap (MOT) through the photoionization of cold Sr atoms in a custom-designed reaction microscope. Sr atoms in the dark state of 5s5p3P2 populated via the cascade transition 5s5p1P1 → 5s4d1D2 → 5s5p3P2 accumulate a significant fraction, giving a long lifetime of 520 s. These atoms in the dark state are subsequently trapped by the gradient magnetic field of the MOT. By scanning the Sr+ momentum distributions ionized with an 800 nm infrared femtosecond laser, we are able to outline the size of ∼0.55 mm in radius and the temperature of ∼0.40 mK for the dark-state atoms, which is significantly cooler than the MOT temperature of 3.3 mK trapped in the 461 nm. The size of MOT exhibits an oblate spheroidal distribution with a radius of approximately 0.35 mm and 0.55 mm, extracted with momenta of photoion and absorption imaging, respectively. The results using the photoion momenta are consistent with the expected results from absorption imaging, which confirms the method’s reliability. The advantage of this method is the ability to simultaneously characterize the distribution information of atoms in different initial states within the cold atomic cloud.
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 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.