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Journal of the Optical Society of Korea

A Magneto-optical Trap Below a Dielectric Coated Mirror Surface

Hoon Yu, Lim Lee, Kyung Hyun Lee, Jung Bog Kim

Published Jun 25, 2009• 9 citations
Publisher

Abstract

A Magneto-Optical Trap (MOT) for $^{87}Rb$ atoms near the surface of a dielectric coated mirror at the top of a small $20{\times}25{\times}40\;mm^3$ cell has been observed. Two beams of $3.3\;mW/cm^2$ were used for optical cooling and an anti-Helmholtz magnetic field with a spatial gradient of 9.1 G/cm was used for magnetic trapping. The thickness of the mirror coated on a cover glass was less than $100{\mu}m$ . The mirror covered the top of a cell and the atom-chip was located outside the vacuum in order to exploit the long life time of the mirror and easy operation of the chip. The trapping position was found 5 mm beneath the mirror surface. The number of trapped atoms was roughly $3{\times}10^7$ atoms and the temperature was approximately a few tens mK. In this paper, we describe the construction of the mirror-MOT in detail.

DOI
10.3807/josk.2009.13.2.223

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A Magneto-Optical Trap (MOT) for 87Rb^{87}Rb87Rb atoms near the surface of a dielectric coated mirror at the top of a small 20×25×40  mm320{\times}25{\times}40\;mm^320×25×40mm3 cell has been observed. Two beams of 3.3  mW/cm23.3\;mW/cm^23.3mW/cm2 were used for optical cooling and an anti-Helmholtz magnetic field with a spatial gradient of 9.1 G/cm was used for magnetic trapping. The thickness of the mirror coated on a cover glass was less than 100μm100{\mu}m100μm . The mirror covered the top of a cell and the atom-chip was located outside the vacuum in order to exploit the long life time of the mirror and easy operation of the chip. The trapping position was found 5 mm beneath the mirror surface. The number of trapped atoms was roughly 3×1073{\times}10^73×107 atoms and the temperature was approximately a few tens mK. In this paper, we describe the construction of the mirror-MOT in detail.
We describe a dynamic magneto-optical trap (MOT) suitable for the use with vacuum systems in which optical access is limited to a single window. This technique facilitates the long-standing desire of producing integrated atom chips, many of which are likely to have severely restricted optical access compared with conventional vacuum chambers. This 'switching-MOT' relies on the synchronized pulsing of optical and magnetic fields at audio frequencies. The trap's beam geometry is obtained using a planar mirror surface, and does not require a patterned substrate or bulky optics inside the vacuum chamber. Central to the design is a novel magnetic field geometry that requires no external quadrupole or bias coils which leads toward a very compact system. We have implemented the trap for 85 Rb and shown that it is capable of capturing 2 million atoms and directly cooling below the Doppler temperature.
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.
A mirror magneto-optical trap (MOT) that exploits a hexapole-compensated magnetic field was developed and used in the experimental surface trapping of neutral atoms. A pair of subsidiary wires, which was placed near the main current-carrying wire, was designed to improve the uniformity of the quadrupole magnetic field and thus increased the effective capture volume of our mirror-MOT. In the experiment, the number of 87Rb^{87}\mathrm{Rb}87Rb atoms captured with our mirror-MOT was approximately twice that captured with a conventional mirror-MOT.
This thesis describes the design and construction of a new magneto optical trap that is suitable for use in integrated atom chips and other vacuum systems in which optical access is limited to a single window. The trap design relies on the switching of optical and magnetic fields and can operate at frequencies at least within the region of 1 kHz to 60 kHz. The design does not need patterned surfaces in order to generate the necessary beam geometry, requiring only the use of a single, standard mirror. Early temperature measurements have indicated that the trap may be capable of sub-Doppler cooling, and that it is able to capture on the order of 1.7 x 106 atoms in a capture volume of 0.18 cm3.