A continuous beam of cold cesium atoms extracted from a two-dimensional magneto-optical trap

1 November 1997 OPTICS COMMUNICATIONS ELSEVIER

Optics Communications 143 (1997) 30-34

A continuous beam of cold cesium atoms extracted from a two-dimensional magneto-optical trap S. Weyers a,*, E. Aucouturier

b, C. Valentin b, N. Dimarcq b

a Physikalisch-Technische Bundesanstalt, Lab. 4.32, Bundesallee 100, D-38116 Braunschweig, Germany b Laboratoire de I’Horloge Atomique, C.N.R.S., BBt. 221, Uniuersite’ Paris-&d. 91405 Orsay Cedex, France

Received 25 February 1997; revised 23 May 1997; accepted 10 June 1997

Abstract A novel scheme has been developed, that produces a continuous beam of cold cesium atoms extracted from a two-dimensional Magneto-Optical Trap (2D-MOT) in a standard vapour cell. The continuous beam was generated by launching the atoms with lD moving optical molasses or with a static magnetic field. In the first case, mean drift velocities between 1.9 to 5 m/s and up to IO6 atoms/s at a temperature of (1 f 0.5) mK could be obtained. Launching with a static magnetic field was similarly efficient but exhibited a double-peaked structure in the velocity distribution of the launched atoms. 0 1997 Elsevier Science B.V. PACS: 32.8O.Pj Keywords: Atomic beams; Laser cooling

Atomic beams are the basis of many precision spectroscopic experiments. As the time of flight through the apparatus is often the limiting factor for precision, a beam of atoms, which are slow compared to thermal velocities, would be advantageous. Usually, a continuous beam of cold atoms is obtained by decelerating a thermal atomic beam. For this purpose, a large variety of techniques has been developed and thermal beams have been cooled to a few hundredth of a Kelvin [l]. As the usual direct deceleration of atoms in a beam suffers from the fact that the transversal velocity becomes comparable to the longitudinal velocity, several successful approaches were made to overcome this difficulty by two-dimensional magneto-optical cooling techniques providing an atomic funnel (see e.g. Refs. [2-41). Either the atoms are loaded from a longitudinally cooled thermal

* Corresponding author. E-mail: [email protected].

atomic beam into a 2D-MOT and extracted by the moving molasses technique [5] (see also Refs. [2,4]) or the longitudinally cooled beam undergoes velocity selective laser deflection and is subsequently compressed in the transverse direction in a 2D-MOT (see Ref. [3]). In Ref. [2], the beam has to be pulsed in order to prevent the atoms from travelling in the light-field of the cooling lasers, which for many experiments entails the problem of ac-Stark shifts of the atomic energy levels. What is common to these experiments is the use of relatively complex techniques for decelerating a thermal beam. Recently Lu et al. succeeded in extracting a continuous beam of rubidium atoms from a MOT [6], where the atoms are loaded from a vapour cell and extracted by an intensity imbalance. Two disadvantages of this technique are the relatively high longitudinal velocity spread of about 2.7 m/s FWHM and the fact that the atoms travel in one of the laser beams, which again entails the problem of ac-Stark shifts. We report here on an experiment in which we succeeded in continuously extracting atoms from an appropri-

0030-4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved, PII SOO30-4018(97)003 12-X

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Communications

crossed cooling

laser beams atomic beam Fig. 1. View of the 7D-MOT. The atoms are cooled and trapped by two pairs of horizontal counterpropagating laser beams in the 0.q plane. The crossed cooling and launching laser beams (cp = 14”) provide 1D optical molasses to cool the atoms along OZ. The magnetic field gradients in this plane are generated by two pairs of octagonal coils in an anti-Helmholtz configuration.

ate configuration of a compact 2D-MOT, which is loaded continuously from a standard vapour cell. We studied the extraction of the atoms both by ID moving optical molasses and by a static magnetic field. In order to avoid ac-Stark shifts, the atoms are extracted in a direction which does not coincide with any of the laser beam directions. In a usual MOT, trapping and cooling of atoms from a vapour cell result from the application of three pairs of counterpropagating light waves emitted by a laser locked a few MHz below a “cycling” atomic transition (cesium: F’ = 5) and of magnetic field F=4+6’q,,,, 6 ‘S/z, gradients in the region of intersection of the laser beams, which have to be appropriately polarized co* - (T-configuration) [7]. In general, the three-dimensional magnetic field gradient along the three axes of the cooling laser beams is generated by one pair of symmetric magnetic field coils supplied with opposite currents (antiHelmholtz configuration). Deviating from this set-up, in our scheme (Fig. 11, the trapping process in one direction (0~) of the MOT is suppressed in order to facilitate the extraction of atoms. We designed a 2D-MOT in which the magnetic field gradients in the intersection region are produced by two pairs of anti-Helmholtz coils. The field gradients created by each pair of coils add in the two horizontal directions (Ox and Oy) and compensate in the Oz-direction, so that the magnetic field is quasi zero on the 0~ axis. Finally the atoms are only cooled but not trapped along the 0~ axis of the 2D-MOT and the cold atoms are allowed to leak from the intersection region of the laser beams. The cooling along the 0~ axis is brought about by two pairs of counterpropagating laser beams (“crossed cooling

143 (I 997130-34

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and launching beams”) 2.5 mm in diameter, which is limited by the mountings of the magnetic field gradient coils. These beams form an angle of cp= 14” with the vertical axis and intersect with the horizontal beams (12 mm diameter) so that the atoms can be cooled and launched with no laser beam being directed along OZ (Fig. Il. A repumping laser tuned to the 6 ‘S,,?, F = 3 + 6 ‘Ps,?, F’ = 4 transition is superposed to the horizontal beams to avoid atoms leaking to the state 6 2S,,2, F = 3. The total volume of our source of cold atoms amounts to 0.6 dm3 (defined by the magnetic gradient field coils). A surrounding p-metal chamber shields external magnetic fields to less than 2 mG. The cesium pressure in the trap region amounts to lo-’ Torr. This region is separated from the detection region by a graphite tube in order to further reduce the background pressure and thus to minimize collisions which expel the launched atoms from the beam. We use two different high-power laser diodes, injection-locked to the same master Extended Cavity Laser (ECL). The first one serves to provide the horizontal laser beams, whose frequency is adjusted with the aid of acousto-optical modulators (AOMS) to be - 3 r (r- ’ is the excited-state lifetime) below the F = 4 -+ F’ = 5 transition. The second slave laser is split into two beams whose frequencies can be independently adjusted by means of AOMs around the frequency of the horizontal beams. These two beams provide the mounting and descending crossed launching beams, respectively. The emission linewidth of the laser diodes is of the order of 100 kHz. Our first method to launch the atoms consists in subjecting the atoms to a moving laser wave generated by the mounting and descending beams (circularly polarized of and u’-, respectively), whose frequencies are symmetrically shifted around the frequency of the horizontal beams by *Af. The atoms are thus cooled in a frame which moves at a velocity & = hA,f/cos

cp,

(1)

where A is the laser wavelength [5]. The second method consists in using a stationary laser wave, whose counterpropagating progressive components are circularly polarized o+ and o-, respectively, and applying a static homogeneous magnetic field along the 0: axis. As mentioned in Ref. [8], in this case, the magnetic field along the of ((r-) beam has the same effect on the evolution of an atom as lowering (raising) the optical frequency by the Larmor frequency. In our experiment, the stationary laser wave is provided by the crossed cooling beams tuned to the same frequency as the horizontal beams. The magnetic field is produced by a pair of symmetrical magnetic field coils (not shown in Fig. 11 supplied with equal currents (Helmholtz configuration), so that a magnetic field amplitude B, of up to 7 G could be obtained. After the trapping and cooling lasers have been switched

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143 f 1997) 30-34

Fizi! 1 MHz

0

50

100

150

t,(ms)

Fig. 2. Fluorescence signal measured in the detection region 30 cm below the 2D-MOT. tf is the time of fall after the pushing laser beam has been tuned out of resonance. The atoms are extracted in 1D moving optical molasses at Af = 3 MHz. The signal corresponds to a continuous beam of 7 X 10’ atoms/s.

i-J4m 5 MHz

2 MHz

t 3 MHz

I h -10-8

on, we trap up to 4 X IO6 atoms (loading time constant: typically 1 s), whose fluorescence is detected by a calibrated photodiode. With a CCD camera we observe an atomic cloud extended by I cm in the vertical direction and by 0.2 cm in the horizontal direction. The atoms are then extracted from the 2D-MOT with a drift velocity L:~ and fall due to gravity. For measuring the continuous beam properties, the falling atoms cross a transverse “pushing” laser beam (I = 10 mW/cm’) 0.7 cm in diameter. Initially, the pushing laser is injection-locked to the laser diode which provides the horizontal cooling and trapping beams ( - 3 r below the F = 4 --) F’ = 5 transition) and it efficiently pushes the atoms away. At fr = 0 we quickly (i< 1 ms) detune the pushing laser far off resonance, so that the atoms pass this laser beam and arrive in a detection zone, situated 30 cm below the trap center, at the time ff. Here the atoms cross a transverse standing laser wave tuned to the F = 4 + F’ = 5 transition and generated by a second ECL. The atomic fluorescence is collected on a low-noise photodiode. Fig. 2 shows a typical signal for launching with 1D moving molasses with Af = 3 MHz. The fluorescence is assumed to be proportional to the number of atoms N arriving in the detection zone at a certain arrival time If, which depends on their initial velocity distribution in the 2D-MOT along the 0: axis. The analysis of the launching process gives for the velocity distribution f(c) at the level of the pushing beam:

(2) where g is the acceleration due to gravity, H the distance between pushing and detection beam and N(t,) the number of detected atoms at a certain instant 1,. In order to get the initial velocity distribution fly,> at the trap level, ~1 has to be replaced by u,, = -(u2 - 2hgl’/‘, taking into account the distance of fall h = 7 cm between the trap and the pushing beam. In Figs. 3 and 4 the velocity distributions thus derived are shown for the two launching methods.

1

I

t

t

-6 -4 vd (m/s)

1

“I -2

i 0

m 8 MHz

Fig. 3. Velocity distributions f(~:~) of the atoms extracted from the 2D-MOT by a 1D moving optical molasses for different detunings Af of the mounting and descending crossed laser beams. The distributions are derived from the fluorescence signal in the detection region as described in the text. The effect of taking the derivative for deriving the velocity distributions is to make them noisy. This effect is even more marked for small velocity values due to the factor in front of the derivative in Eq. (2). The fact that for low velocities the velocity distributions sometimes seem to drop below the level for 10 m/s is an artifact which is due to this noise and is not reproducible.

f&J

q 5.5 G

m qm 8.2 G

4.7 G

-10-8

-6 -4 -2 vd (m/s)

7.0 0

0

Fig. 4. Velocity distributions f(~~) of the atoms extracted from the ZD-MOT by a static magnetic field for different magnetic field amplitudes B.. The distributions are derived from the fluorescence signal in the detection region as described in the text. (For noise, see caption of Fig. 3.)

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For launching with 1D moving optical molasses the initial velocities are well described by a Gaussian distribution for detunings Af ranging from 1 to 4 MHz. In this range, the temperatures derived from the velocity distributions are similar within the error limits and are of the order of (1 & 0.5) mK. As an example, in Fig. 5, the measured data at a detuning of 3 MHz is shown with its Gaussian fit. For larger detunings (Af> 4 MHz), there seems to be heating and the distributions become non-Gaussian. We attribute the heating to an increasing misalignment of the crossed launching beams due to the deflection by the AOMs. in this case. the 1D moving optical molasses does not cool the atoms properly. To explain the non-Gaussian shapes further investigations of the heating are necessary. As we measure a temperature of 45 FK for the atomic cloud in the MOT using a conventional time-of-flight technique [9], a heating process during the extraction of the atoms is obvious. The atoms, when leaving the trap, possibly cross a region, where they only interact with the horizontal cooling beams. This would lead to heating of the atoms in the vertical direction. Further experimental investigations to solve this problem are in progress. The measured mean drift velocities L‘dare shown in Fig. 6 as a function of the laser detuning Af. The error bars are cautiously estimated by taking into account the noise of the derived velocity distributions. The fact that the measured drift velocities seem to exhibit a smaller slope than the calculated ones (straight line) and a non-zero drift velocity at zero detuning may be due to the effect of a residual magnetic field along the 0: axis and a finite damping time in the 2D-MOT, as pointed out in Ref. [4]. By launching with a static magnetic field, the velocity distributions (Fig. 4) reveal a double-peaked structure, which suggests the existence of two different mean velocities for the atoms. As the two peaks are nor completely resolved. it is difficult to determine the corresponding mean velocities. However, the positions of the two peaks

I”,

-4.5

’ 1

-4.0

”

”

1

”

”

-3.5

I

”

-3.0 vd

”

I

-2.5

”

1

-2.0

”

“I

143 (1997130-34

5

33

-

0

1

2

3 Af

4

5

6

7

(MHz)

Fig. 6. (0) Measured mean drift velocities P, for atoms extracted by 1D moving optical molasses as a function of the detuning Af of the mounting and descending crossed laser beams. The straight line gives the mean drift velocities calculated from Eq. (1).

seem to depend linearly on the static magnetic field ampiitude B, with a slope of 0.41 f 0.08 (m/s)/G. Theoretical studies predict three different ranges for launching by a static magnetic field (1D models) [lo-121. For small static magnetic field amplitudes ( < 5.5 G), there should be a single velocity peak due to launching by a sub-Doppler mechanism. The expected drift velocity is proportional to the Larmor frequency of the ground state and the slope is 0.30 (m/s)/G. In an intermediate range (5.5 G < B; < 10 G). a double-peaked structure is expected to appear, the second peak of which is due to launching by a Doppler mechanism. Finally, for large B; (> 10 G), only the latter peak remains. for which a slope of 0.48 (m/s)/G is calculated [ lo,1 I]. In this case, the drift velocity is proportional to the Larmor frequency of the excited state. The existence of a double-peaked structure has also been experimentally proven in 1D optical molasses [ 13,141. For small values of B; ( < 3.1 G) our signal was so weak that the derivation of a velocity distribution was not possible because of noise. Even the velocity distribution at 3.1 G is still too noisy to show clearly if there are one or two peaks (see Fig. 4). In any case, our results prove the existence of a double-peaked structure by launching with 3.5 G < B, < 7.0 G. We attribute the disagreement between the predictions of the ID models and our measurements to the fact that in our 3D experiment contributions of Sisyphus type cooling due to polarization gradients exist. which are not taken into account in the theoretical models. Furthermore, the true level scheme of cesium has not been taken into account in the models ‘. The reason for the unexpectedly large velocity distributions may be the same as for the high temperatures obtained by

-1.5

(m/s)

Fig. 5. Velocity distribution fC L‘~1of the atoms extracted from the ZD-MOT by ID moving optical molasses at a detuning Af = 3 MHz of the mounting and descending crossed laser beams. The data points are shown with a Gaussian fit.

’ The limits indicated for the three ranges are calculated for an F = 1 -+ F’ = 2 transition (laser intensity I = 2.2 mW/cm’, detuning S = - 5r 1. but with the Land6 factors of the F = 4 + F’ = 5 transition [ 131.

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launching with ID moving optical molasses (see above). A better resolution of the two peaks (i.e. lower temperatures) would make it possible to determine the slopes of the two peaks more precisely. To conclude, we succeeded in generating a continuous beam of cold atoms using a 2D-MOT in a standard vapour cell and two different extraction mechanisms. At launching velocities of a few m/s, we obtained up to lo6 atoms/s in our beam. As a preliminary result, launching by the mechanism of ID moving optical molasses seems to be more promising than launching with a static magnetic field, where the involved processes are more complex and lead to larger velocity distributions. However, for a final conclusion, the problem of heating the atoms during their extraction has to be solved. Our source of a continuous beam of cold atoms may be extremely valuable in atomic frequency standards, atom optics, atomic interferometry, collision studies, and high resolution-spectroscopy in general. As a continuous source, it could overcome, e.g., two inherent drawbacks of an atomic fountain frequency standard [15]. Such a fountain is operated in a pulsed mode, which consists in loading optical molasses and launching the atoms, free ballistic flight and microwave transition and finally the detection. At the same atomic flux, a continuous beam offers the advantage of lower density, so that the collisional shift of the microwave transition frequency [16,17] is strongly reduced. Moreover, in an atomic fountain, the short-term frequency stability is limited by the intermittent generation of the error signal in the servoloop that controls the local oscillator [ 181, which would be avoided by using a continuous beam as atomic source [ 191.

Acknowledgements We are very grateful to P. Petit for the careful construction of the apparatus and his important advice. We acknowledge the assistance of G. Granger during part of the measurements. This’work was financially supported by the Bureau National de Metrologie (Paris) and the Direction des Recherches, Etudes et Techniques (Paris). One of us (SW.) acknowledges financial support from the European HCM Program.

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