We report the first observation of translationally cold ( approximately 90 &mgr;K) Rb2 molecules. They are produced in a magneto-optical trap in their triplet ground state. The detection is performed by selective mass spectroscopy after two-photon ionization into Rb+2, resonantly enhanced through the intermediate a (3)Sigma(+)(u)-->2 (3)Pi(g) molecular band. The two rubidium isotopes present very different types of behavior that are interpreted in terms of their respective collisional properties.

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We demonstrate high-contrast Electromagnetically Induced Absorption (EIA) bright resonances on the D 1 line of 39 K with characteristics comparable to those of the Electromagnetically Induced Transparency (EIT) dark resonances observed in the same conditions. EIA is produced by the interaction of a weak probe beam with the atomic ground state driven in a degenerate coherent superposition by either a co-or counterpropagating pump beam. We have obtained an order of magnitude increase of the EIA's contrast with respect to previous similar experiments, performed with other alkalis, without compromising its linewidth. Furthermore, we show that the magneto-optic resonances can be continuously tuned from EIT to EIA by changing the relative handedness of circular polarizations of pump and probe beams, or depending on whether they co-or counter-propagate. This opens new perspectives in the use of EIA in a broad range of physical domains and in a large wealth of potential applications in optics and photonics. Contrary to Coherent Population Trapping and Electromagnetically Induced Transparency (EIT) with their ultra-narrow linewidths [1][2][3][4][5][6][7][8], Electromagnetically Induced Absorption (EIA) has found limited applications, mainly due to its broader linewidth and significantly smaller contrast. Nonetheless, an increasing interest in EIA was recently generated in view of potential applications, such as enhancement of the group velocity of light [9] and photonic metamaterials [10].EIA linewidths of several tens of kiloHertz have been reported, either with bichromatic excitation of Zeeman sublevels[11] or in Hanle configuration [12][13][14]. Bi-chromatic Hanle configuration has been explored by using Rb D 1 and D 2 lines [15]. Low-contrast EIA, resulting from EIT on D 2 line, was reported. Higher contrast EIA has been observed in [16], although overlapped to large background, which could be detrimental for highly-selective applications. More recently, sub-kHz EIA resonances were observed in Rb [17] and Cs [18,19]. In both cases, however, the contrast was of only a few percents. A pump/probe scheme in Hanle configuration for high-contrast EIA was proposed in a buffered vapor cell [20]. To date, however, no experimental demonstration has been reported. Highcontrast EIA has been observed on the 87 Rb D 2 line by using a complex excitation scheme with three separate photonic fields [21], and was theoretically investigated in [22]. EIA was also observed in the Paschen-Back regime in a purely non-degenerate three-level ladder system in 87 Rb [23]. Furthermore, while Rb and Cs have been widely investigated, this is the first observation of EIA in K, confirming our preliminary results [24].In this Letter, we report on an alternative and simpler approach for large contrast EIA in Hanle configuration using a pump/probe excitation scheme. We have observed EIA resonance contrast exceeding 50% in thermal K vapor. In this case, hyperfine optical pumping, which accumulates atoms to levels non interacting with the light [25,26], ...

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