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Laser cooling with a single laser beam and a planar diffractor

Abstract : A planar triplet of diffraction gratings is used to transform a single laser beam into a four-beam tetrahedral magneto-optical trap. This "flat" pyramid diffractor geometry is ideal for future microfabrication. We demonstrate the technique by trapping and subsequently sub-Doppler cooling 87 Rb atoms to 30 μK. A magneto-optical trap (MOT) [1] is the starting point for the vast majority of cold and ultracold atomic physics ex-periments. Atoms are trapped and cooled to submillikel-vin temperatures using light scattering modified by the Zeeman and Doppler effects, respectively. MOTs are ty-pically formed at the center of a spherical quadrupole magnetic field, in the overlap region of six (or less com-monly four [2]) appropriately polarized red-detuned laser beams. The original pyramid MOT (PMOT) [3], utilizing a square-based pyramidal reflector with 90° apex angle be-tween opposite sides, was devised as a means to turn a single laser beam into the six appropriately polarized beams required for an MOT. The PMOT simplifies optical alignment, saves a large number of optical components, and can also be modified to produce a beam source of cold atoms [4]. The original PMOT has since been used to make a compact gravimeter [5] and a millimeter-scale chip trap [6]. Recently, we demonstrated a new kind of pyramid MOT, based on a four-beam tetrahedral geometry [7] ori-ginating from a single beam interacting with a triangular pyramid reflector. This geometry has many advantages over the original design: MOT formation outside the pyr-amid is possible, which simplifies optical access to the atoms, and the apex region of the pyramid is noncritical (the latter feature is also present in the PMOT design in [8]). The apex and mirror edges in the original PMOT will generate diffraction, and an incorrect apex angle will also generate intensity irregularities in the doubly reflected beams counterpropagating with the input beam. As these irregularities pass directly through the MOT, they can hinder further cooling in optical molasses. Moreover, although sub-Doppler cooling is possible with a small atom number [5], for larger PMOTs the counterpropagat-ing beam will contain a shadow of the atoms from the input beam, creating an intensity imbalance that will hinder molasses [9]. This problem is obviated in the tetra-hedral PMOT. In this Letter, we have experimentally realized our proposal to extend the tetrahedral PMOT [7] to a "flat" geometry using diffraction gratings. The grating magneto-optical trap (GMOT) has a very similar working principle to the tetrahedral PMOT, and its properties are again largely the result of intensity balance and polarization de-composition [7]. One major difference is due to the fact that gratings spatially compress beams with a corre-sponding intensity increase [Fig. 1(a)]. The relationship between the intensities I i and I 1 of the vertical incident beam and the first-order diffracted beam, respectively, is determined by the corresponding beam widths w i and w 1 and the first-order diffraction efficiency R 1 :
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Matthieu Vangeleyn, Paul F. Griffin, Erling Riis, Aidan S. Arnold. Laser cooling with a single laser beam and a planar diffractor. Optics Letters, Optical Society of America - OSA Publishing, 2010, 35 (20), pp.3. ⟨10.1364/OL.35.003453⟩. ⟨hal-01095913⟩

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