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arXiv:1006.4526 · Optics Letters

Laser cooling with a single laser beam and a planar diffractor

Matthieu Vangeleyn, Paul F. Griffin, Erling Riis, Aidan S. Arnold

Published Oct 11, 2010• 72 citations
arXivHTMLPDF

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.
DOI
10.1364/ol.35.003453

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The grating magneto-optical trap (GMOT) requires only one beam and three planar diffraction gratings to form a cloud of cold atoms above the plane of the diffractors. Despite the complicated polarization arrangement, we demonstrate sub-Doppler cooling of Rb87 atoms to a temperature of 7.6(0.6) μK through a multistage, far-detuned GMOT in conjunction with optical molasses. A decomposition of the electric field into polarization components for this geometry does not yield a mapping onto standard sub-Doppler laser-cooling configurations. With numerical simulations, we find that the polarization composition of the GMOT optical field, which includes σ and π polarized light, does produce sub-Doppler temperatures.
We have realized a 4-beam pyramidal magneto-optical trap ideally suited for future microfabrication. Three mirrors split and steer a single incoming beam into a tripod of reflected beams, allowing trapping in the four-beam overlap volume. We discuss the influence of mirror angle on cooling and trapping, finding optimum efficiency in a tetrahedral configuration. We demonstrate the technique using an ex-vacuo mirror system to illustrate the previously inaccessible supra-plane pyramid MOT configuration. Unlike standard pyramidal MOTs both the pyramid apex and its mirror angle are non-critical and our MOT offers improved molasses free from atomic shadows in the laser beams. The MOT scheme naturally extends to a 2-beam refractive version with high optical access. For quantum gas experiments, the mirror system could also be used for a stable 3D tetrahedral optical lattice.
Shimizu et al. demonstrated an alternative of the usual six beam magneto-optical trap (MOT), using four independent beams in a tetrahedral configuration. This paper demonstrates a novel version of the four beam MOT using a triplet of mirrors to split and steer an incoming beam into three parts such that all four beams cross in the correct configuration. In the usual pyramid configuration, the MOT forms below the top surface of the pyramid and the absence of optical access strongly reduces any manipulation of the cold atomic cloud. In addition, a micro-fabricated tetrahedral configuration offers an ideal tool for a high phase stability optical lattice, with the benefit of fixed lattice geometry. In the demonstration setup, the triplet is positioned underneath the quartz vacuum chamber and its thickness had to be considered. Therefore, the angle of the mirrors is slightly more acute than in the ideal situation, resulting in 45Adeg beams, in order to form the MOT above the plate. Moreover, irect modulation of the diode current by a frequency corresponding to the alkali atom energy splitting in order to generate the repumping light is used. The setup was used to cool down87Rb atoms via the F = 2 and F' = 3 energy levels of the D2 transition line. With the traditional cooling/repump lasers, 1.3 x 106atoms were trapped. With a 10mW microwave source, we generated enough repump light into the laser to get 1.1 x 106atoms, that is 85% compared to the two laser setup.
The design, fabrication, and demonstration of a planar two-dimensional-crossed reflective diffractive grating are proposed to construct a novel optical configuration, to the best of our knowledge, potentially applied for atom cooling and trapping in a magneto-optical trap. Based on the proposed single-beam single-exposure scheme by means of an orthogonal two-axis Lloyd’s mirrors interferometer, we rapidly patterned a ∼ 1 µ m period grating capable of providing a uniform intensity of the diffracted beams. The key structural parameters of the grating including the array square hole’s width and depth were determined, aiming at providing a high energy of the diffracted beams to perform the atom cooling and trapping. To guarantee the diffracted beams to be overlapped possibly, we adopted a polarized beam splitter to guide the optical path of the incident and zero-order diffracted beams. Therefore, one zero-order diffracted beam with a retroreflected mode and four first-order diffracted beams with appropriate optical path constructed a three-dimensional optical configuration of three orthogonal pairs of counterpropagating beams. Finally, three pairs of the counterpropagating cooling laser beams with 9 mm diameter and > 10 % diffraction efficiencies were achieved, and the circular polarization chirality, purity, and compensation of the desired diffracted beams are further evaluated, which preliminarily validated a high applicability for the magneto-optical trap system.
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We present detailed instructions for the construction of a pyramidal-style laser cooling and trapping apparatus. This scheme requires only a single beam, rather than the three pairs of orthogonal beams of the standard magneto-optical trap, which greatly simplifies the geometry and substantially reduces the cost. The trap is based largely on low-cost commercially available items and is simple to construct. It is remarkably insensitive to alignment and reliable to operate. Using a single laser beam with an intensity of 1.3 mW/cm2 we cool and trap more than 4 million rubidium atoms.