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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Dark region in fluorescence emitted by Rb atoms in a quadrupole magnetostatic field: observation and explanation

Q. Long, Shuyu Zhou, S.Y. Zhon, Yuewei Wang

Published Jan 20, 2003• 0 citations
Publisher

Abstract

The quadrupole magnetostatic field (QMF), together with 3 pairs of laser beams, known as the magneto-optical trap, has been widely used for cooling and trapping atoms. This report concerns itself with some new phenomena observed in fluorescence emitted by Rb atoms in QMF under the circumstances of a single /spl sigma/ laser beam. A circularly polarized laser traveling-wave tuned to D/sub 2/-line is passed through a vacuum chamber along the central axis of two anti-Helmholtz coils. The fluorescence emitted by Rb atoms excited by the circularly polarized light in the direction at right angle to the direction of the incident laser beam is collected and imaged with all polarized directions taken into account. From the camera picture we can see that, around the zero-field point, there exists a dark region, the size of which is inversely proportional to the field gradient and is not sensitive to the detuning. The dark region corresponds to a dip in the profile of the fluorescence intensity versus the space. An attempt is made to explain the origin of this dark region.

DOI
10.1109/cleopr.1999.814696

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Chinese Physics•Apr 29, 2004•5 citations
The quadrupole magnetostatic field (QMF), together with 3 pairs of laser beams, known as the magneto-optical trap, has been widely used for cooling and trapping atoms. This report concerns itself with some new phenomena observed in fluorescence emitted by Rb atoms in QMF under the circumstances of a single /spl sigma/ laser beam. A circularly polarized laser traveling-wave tuned to D/sub 2/-line is passed through a vacuum chamber along the central axis of two anti-Helmholtz coils. The fluorescence emitted by Rb atoms excited by the circularly polarized light in the direction at right angle to the direction of the incident laser beam is collected and imaged with all polarized directions taken into account. From the camera picture we can see that, around the zero-field point, there exists a dark region, the size of which is inversely proportional to the field gradient and is not sensitive to the detuning. The dark region corresponds to a dip in the profile of the fluorescence intensity versus the space. An attempt is made to explain the origin of this dark region.
We report an experimental observation of a dark region, around the zero-field point of a spheric quadrupole magnetostatic field, in spatial fluorescence emitted by dilute vapor atoms, which were excited by a circularly polarized traveling-wave laser tuned to a $\stackrel{\ensuremath{\rightarrow}}{F}F+1$ transition of 87Rb.{}^{87}\mathrm{Rb}.87Rb. By ruling out some possible sources, we show that the fluorescence decrease is associated with the three-dimensional nature of the magnetic field; it is a consequence of a precession of magnetic dipole moments induced by the radial magnetic field, which significantly cancels the orientation caused by optical pumping near the point of level crossing in the presence of the axial magnetic field. In an alternative way, the phenomenon can be understood as resulting from a destructive quantum interference, which is related to a coherent double-quantum coupling of atomic levels with two fields; one is the laser field, the other is the magnetic field. Analytic expressions for the half-width and contrast of the dark region have been obtained with a simplified model in the limit of low laser power. A qualitative agreement between the theory and experiment is found. The relevance of our study to laser cooling and trapping in a widely used magneto-optical trap based on such a quadrupole magnetic field is discussed. Finally, some applications of the phenomenon are also presented.
We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of (85)Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 10(15) m(-2). In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.
ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10­¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.
In our experiment, firstly, we carry out the loading of single atoms in the magneto-optical trap (MOT) by increasing the quadrupole magnetic field gradient, improving the background vacuum, and reducing the diameters of the cooling and trapping laser beams. Secondly, we get the single atomic fluorescence signal of a high signal-to-background ratio in the MOT by means of reducing the detuning of cooling light and increasing its intensity appropriately, and using the polarization spectroscopy locking technique to suppress the fluctuations of cooling laser. In addition, with the real-time feedback on quadrupole magnetic field gradient, we demonstrate a probability of loading single atoms in the MOT as high as 98%. We also measure the statistical properties of the single atomic fluorescence which is excited by continuous light in the MOT; the measured second-order correlation is g(2)( = 0) = 0.09.
Magneto-optical traps (MOTs) of Er and Dy have recently been shown to exhibit populationwide sub-Doppler cooling due to their near degeneracy of excited- and ground-state Land\'e ggg factors. We discuss here an additional, unusual intra-MOT sub-Doppler cooling mechanism that appears when the total Dy MOT cooling laser intensity and magnetic quadrupole gradient increase beyond critical values. Specifically, anisotropically sub-Doppler-cooled cores appear, and their orientation with respect to the quadrupole axis flips at a critical ratio of the MOT laser intensity along the quadrupole axis versus that in the plane of symmetry. This phenomenon can be traced to a loss of the velocity-selective resonance at zero velocity in the cooling force along directions in which the atomic polarization is oriented by the quadrupole field. We present data characterizing this anisotropic laser cooling phenomenon and discuss a qualitative model for its origin based on the extraordinarily large Dy magnetic moment and Dy's near degenerate ggg factors.