A low-velocity beam of rubidium atoms is produced from a two-dimensional magneto-optic trap or atomic funnel. Atoms from a thermal beam are slowed by chirped laser cooling and then loaded into the funnel. The cold atoms are ejected by moving molasses formed with frequency-shifted laser beams. The resultant atomic beam has a controllable velocity in the range of 3 to 10 m/s, a temperature of 500 μK, and a flux of 1010 atoms/s

Maki, J. J.

Mayer, Shannon K.

McIntyre, D. H.

Silva, N. J.

Swanson, T. B.

English

University of Portland

University of PortlandPilot ScholarsPhysics Faculty Publications and Presentations Physics9-1996Rubidium atomic funnelT. B. SwansonN. J. SilvaShannon K. MayerUniversity of Portland, mayers@up.eduJ. J. MakiD. H. McIntyreFollow this and additional works at: http://pilotscholars.up.edu/phy_facpubsPart of the Plasma and Beam Physics CommonsThis Journal Article is brought to you for free and open access by the Physics at Pilot Scholars. It has been accepted for inclusion in Physics FacultyPublications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact library@up.edu.Citation: Pilot Scholars Version (Modified MLA Style)Swanson, T. B.; Silva, N. J.; Mayer, Shannon K.; Maki, J. J.; and McIntyre, D. H., "Rubidium atomic funnel" (1996). Physics FacultyPublications and Presentations. 30.http://pilotscholars.up.edu/phy_facpubs/30Swllilson et al. Vol. 13, No. 9/ September 1996/J. Opt. Soc. Am. B 1833 Rubidium atomic funnel T. B. Swanson,* N. J. Silva, S. K. Mayer, J. J. Maki,+ and D. H. Mcintyre Department of Physics, Oregon State University, Corvallis, Oregon 97331 Received November 1, 1995 A low-velocity beam of rubidium atoms is produced from a two-dimensional magneto-optic trap or atomic fun-nel. Atoms from a thermal beam are slowed by chirped laser cooling and then loaded into the funnel. The cold atoms are ejected by moving molasses formed with frequency-shifted laser beams. The resultant atomic beam has a controllable velocity in the range of 3 to 10 mis, a temperature of 500 µK, and a flux of 1010 atoms/s. © 1996 Optical Society of America. 1. INTRODUCTION The recent development of laser-cooling techniques has opened the way to experiments with very slow atoms.1 This is particularly valuable to atomic-beam research, which has played an important role in the development of physics in this century. Slow atoms allow for longer in-teraction times, hence, better frequency resolution, and also reduce Doppler effects that normally broaden or shift resonance lines. Unfortunately, an atomic beam that is slowed in one dimension will suffer large transverse di-vergence as the longitudinal velocity is reduced to a value of the order of the transverse velocity. To alleviate this problem, one can use position-dependent trapping forces to compress the beam spatially. One promising tech-nique to achieve this is based on magneto-optic trapping, which permits spatial and velocity compression of atoms in appropriately designed magnetic and laser fields. 2 In this paper we report on the use of a two-dimensional magneto-optic trap or atomic funnel to produce a low-velocity atomic rubidium beam. An atomic funnel for so-dium was first demonstrated by Riis et al. 3 Related work has been reported by Nellessen et al.4 with sodium and by Yu et al. 5 with cesium. In the standard three-dimensional magneto-optic trap, a spherical quadrupole magnetic field is used, wherein the magnetic field is zero at the origin and increases lin-early along any direction.2 Three pairs of counterpropa-gating laser beams with appropriate circular polarization are directed along the three coordinate axes. The lasers are detuned to the red of the resonance to provide optical molasses damping of the velocity. The Zeeman shift in-troduces a differential absorption rate between oppositely directed a-+ and <T- laser beams, forcing atoms within the intersecting laser beams to the origin. Such a three-dimensional trap has proved useful for a wide variety of experiments. The atomic funnel is the two-dimensional version of such a trap, with no spatial confinement along the third dimension or axis of the funnel. A slow atomic beam can be generated along this axis by adjustment of the laser-beam frequencies. Such a beam, with a control-lable velocity and a narrow velocity distribution, will prove useful in new experiments including atom interfer-ometry, high-resolution spectroscopy, time st andards, 07 40-3224/96/0901833-04$10.00 and low-velocity collisions. It will also permit old experi-ments to be revisited with increased precision. The atomic funnel reported on here is loaded with at-oms from a thermal beam that is slowed by chirped laser cooling.6 Slow atoms that drift into the trap are com-pressed in the two spatial dimensions transverse to the funnel axis to a size of the order of 500 µm. The atomic velocity distribution is compressed in all three dimen-sions to yield a temperature of approximately 500 µK. By adjustment of the laser-beam frequencies, atoms are ejected from the funnel with controllable mean velocities in the range of 3-10 mis. In contrast to the experiment of Riis et al. 3 we have three-dimensional velocity com-pression without using a laser along the funnel axis, which allows unlimited downstream access to the beam. The other funnel experiments used velocity compression in only the two transverse directions. 4•5 2. EXPERIMENT A schematic diagram of the experiment is shown in Fig. 1. The thermal rubidium beam is produced by an effusive oven source with an aperture of 1-mm diameter. The oven is operated at a temperature of 235 °C and has a re-circulation system to increase the time interval between oven reloadings. 7 The atomic beam propagates along the positive x direction 1 m to the funnel region and is slowed by a counterpropagating laser beam with a frequency chirp to compensate for the changing Doppler shift of the decelerating atoms. 6 Atoms with initial velocities less than 350 mis are slowed to approximately 20 mis before the slowing laser frequency is quickly changed to turn off the slowing mechanism. This value of the final velocity is chosen to maximize the number of atoms loaded into the trap; atoms with larger velocities cannot be trapped, and atoms with smaller velocities tend to miss the trap because of their transverse velocity acquired during the one-dimensional slowing. The slowing chirp is repeated with a frequency of 24 Hz, producing a pulsed source of atoms to load into the funnel. The slowing and the trap-ping are done with the 85Rb 58112 F = 3 ~ 5P312 F' = 4 transition. Secondary lasers are used to repump atoms lost to t he other hyperfine level of the ground state. The funnel is defined by the two-di~ensional magnetic © 1996 Optical Society of America 1834 J. Opt. Soc. Am. B/Vol. 13, No. 9/September 1996 Hot Atomic Beam (b) r'v/') Trapping/Molasses ~ Lasers(4) Funnel / -----~~----'F~u~nn~e~I A~x=is _ _. Wires~ ,,.;gz;:m11 __ ,_,......., Cooling Laser Fig. 1. (a) Funnel magnetic-field wires in a hairpin geometry. (b) Schematic of funnel showing lasers in the horizontal (xy) plane. The origin of the coordinate system (shown displaced for clarity) is at the intersection of the funnel axis and the hot atomic beam. The striped area represents the region of trap-ping. quadrupole field, which has its axis along they axis. The magnetic field is formed by four parallel wires (copper tubing) arranged at the corners of a 2.5-cm square, with alternating current directions. Each wire has a return path in a hairpin geometry,3 as shown in Fig. 1. The re-turn paths are located at the corners of a 6.3-cm square and decrease the field by 20%. The wires carry a current of 70 A and are water cooled. The maximum field gradi-ent is 13 G/cm in the xz plane and decreases to 15% of this value 1 cm outside the end of the funnel. The three orthogonal pairs of laser beams overlap at the intersection of the thermal beam and the funnel axis . The intensity in each beam is 8 mW/cm2, or five times the saturation intensity. One counterpropagating u+ -u-pair is along the z axis, while the other two pairs are in the xy plane and make angles of 45° with respect to the x and the y axes. The beams along the z axis are from a single laser at a frequency f. The beams with a propaga-tion component along the positive y axis come from a sec-ond laser tuned to a frequency f + t::.f. The beams with a propagation component along the negative y axis come from a third laser tuned to a frequency f - t::.f. In the presence of the frequency shift t::.f, atoms at rest will ex-perience a force in the positive y direction because of the imbalance in the scattering rates from the beams in the xy plane. This moving molasses configuration8 will re-sult in a drift velocity v = fix.t::.f [1.1 (m/s)/MHz for Rb] of the transversely trapped atoms. Atoms moving at the drift velocity see all six laser beams at the frequency f,3 which permits the possibility of orientational cooling.9 The lasers used in this experiment are grating--Swanson et al stabilized diode lasers operating near 780 nm.10 Optical feedback from the Littrow mounted diffraction grating re-duces the laser linewidth from 50 MHz to 150 kHz. The trapping lasers used in the funnel are each frequency off-set locked to a master laser that is stabilized to a polar-ization spectroscopy signal from a rubidium vapor ceu.10 The frequency differences among the three trapping la-sers were measured to have stabilities of the order of 20 kHz. This ensures that the mean velocity of the emerg-ing atomic beam is stable to 2 emfs. Fluorescence from atoms in the funnel is collected with a photomultiplier tube and a CCD camera. The photomultiplier-tube signal indicates that approximately 4 x 108 atoms are loaded into the funnel with each chirp, yielding a funnel output of 1010 atoms/s. The atoms leave the funnel in a pulse with approximately the same width (-6 ms) as the input pulse from the chirped cool-ing. The CCD image is used to optimize laser-beam alignment in the funnel and to estimate the size of the atomic beam. To characterize the motion of the atoms, we place a standing-wave probe downstream of the fun-nel. The probe beam is 1 mm wide (along the atomic-beam direction) and 3 mm high and is placed 10 mm downstream from the end of the funnel. Fluorescence from this probe region is collected with a second photo-multiplier tube and can also be viewed with a CCD cam-era. To measure the velocity of the atomic beam, we use a time-of-flight technique.3 Atoms near the exit end of the funnel are deflected with a resonant laser beam. The fre-quency of this laser is shifted off resonance for 2-3 ms to allow a short pulse of atoms to travel to the probe region. The transit time and the spreading of this pulse as mea-sured with the downstream probe fluorescence yield the mean beam velocity and the longitudinal temperature. The measured signals are fit to a function that models the funnel as a line of point sources, each with the same mean drift velocity and velocity spread. 3. RESULTS AND DISCUSSION A sample fluorescence signal and the resultant fit from the model are shown in Fig. 2. In this experirrn=mt the 0.5 0.3 . . . ·-· . 0.2 2 3 4 . . . .... 5 Time (ms) ..· .· .. · 6 7 8 Fig. 2. Fluorescent signal from the downstream probe laser used in time-of-flight analysis (filled circles). The curve is ~ ~ to the signal and yields the mean beam velocity and longitudlII temperature. 3wanson et al. 12 10 ;n 8 ~ ~ 6 0 Q; 4 > 2 .. · ....... ~ 0 .. 0 2 4 6 8 10 Frequency Offset {MHz) [<lg. 3. Measured velocity of atoms leaving the funnel as a func-tion of the frequency offset t:.f of the horizontal lasers. The X ;ymbols ( + symbols) correspond to a mean laser detuning of 2r '.30 from resonance. The dotted line is the expected velocity of a moving molasses. The solid curve represents a more detailed model t hat includes an axial magnetic field (4G) and the finite damping time (1.2 ms) of the atoms in the molasses. pwse of atoms is long enough that the mean beam veloc-ity is determined primarily by the delay of the leading edge of the pulse with respect to the gate, and the longi-tudinal temperature is determined primarily by the slopes of the leading and the trailing edges of the pulse. Data were taken for a range of values of the detuning 6.f of the horizontal lasers from the vertical laser. The mean detuning of the trapping lasers was 2f (3f) below resonance for values of 6.f below (above) 5 MHz. Figure 3 shows the results of the measured beam velocity. The expected straight line v = VJ.A. 6.f is shown as a dotted line. The data show consistently higher velocities over the complete range of detuning. Intensity imbalances in our system are small and cannot explain the large devia-tions seen. Displacement of the trapped atoms from the ax.is of the trap would subject them to axial magnetic fields that could shift the expected drift velocity of the beam. This effect is estimated to be less than 0.3 mis for our experiment. The deviation of the data from the expected moving mo-lasses result is most likely caused by a combination of two effects. The atoms experience an axial magnetic field caused by the earth's field and other magnetic equipment in the vicinity of the vacuum chamber. In the case of Doppler cooling, a magnetic field B along the axis of u+ -u- molasses leads to a drift velocity of the atoms of u = -g,µ,8 B!hk, whereg, is the excited-state Lande fac-tor, µ,8 is the Bohr magneton, and k is the laser-beam wave vector.11 This effect would result in a change in the offset of the dotted line shown in Fig. 3 but would not af-fect the slope. A change in slope can be explained by ex-amining the dynamics of atoms in the trap. Atoms are damped to their final velocity with a characteristic time r. If the transit time of the atoms through the funnel is of the order of or less than the damping time, then the at-oms will not have enough time to accelerate to the final expected velocity. If rubidium were a simple two-level atom, then the damping time in the funnel would be of the order of 600 µ,s (800 µ,s) for the detuning of 2f (3f).12 We expect it to be somewhat larger than this because ru-Vol. 13, No. 9/September 1996/J. Opt. Soc. Am. B 1835 bidium is not a two-level system, nor is there a single la-ser polarization in the funnel. We combined these two effects in a simple model that accounts for a finite damping time in the funnel and an axial magnetic field. The solid curve in Fig. 3 shows the result of this model for an axial magnetic field of 4G and a damping time of 1.2 ms. The agreement between the data and this model is quite good, suggesting that these effects explain the observed discrepancies from the simple moving molasses values. The time-of-flight signals yield a longitudinal tempera-ture of 500 ~~gg µ,K. The transverse temperature is deter-mined by CCD images of the atoms in the probe region. The expansion of the atoms at this point indicates a transverse temperature of 380 ~~8 µ,K. Both of these val-ues are consistent with the Doppler cooling limit for ru-bidium of 300 µ,K (for a detuning of 2f), suggesting that orientational cooling is not present in our funnel. 4. CONCLUSION In summary, we have produced a source of slow, cold ru-bidium atoms. The low velocities and the high flux should make this an attractive source for new experi-ments. Possible improvements include loading the fun-nel with a continuous slow beam, as from a Zeeman slower.13 This would increase the atomic-beam flux since chirped cooling is duty cycle limited. In addition, the transverse temperature of the atomic beam could be re-duced with downstream orientational cooling.9 Finally, it may be possible to simplify the experiment by perform-ing it in a vapor cell, 14 which would make for a very com-pact and inexpensive cold atomic beam. ACKNOWLEDGEMENT Support for this work was provided in part by the Office of Naval Research . *Present address: Department of Chemistry, Simon Fraser University, c/o Tri-University Meson Facility, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T2A3. +Present address: University of Leuven, Molecular-Electronic and Photonics Research Center , Laboratory of Chemical and Biological Dynamics, Celestijnenlaan 200 D, B-3001 Heverlee, Belgium. REFERENCES 1. See, e.g., the following special issues: J. Opt. Soc. Am. B 2, 11 (1985); J. Opt. Soc. Am. B 6, 11 (1989). 2. E. L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Prit-chard, "Trapping of neutral sodium atoms with radiation pressure," Phys. Rev. Lett. 59, 2631- 2634 (1987). 3. E. Riis, D. S. Weiss, K A. Moler, and S. Chu, "Atom funnel for the production of a slow, high-density atomic beam," Phys. Rev. Lett. 64, 1658-1661 (1990). 4. J. Nellessen, J. Werner, and W. Ertmer, ''Magneto-optical compression of a monoenergetic sodium atomic beam," Opt. Commun. 78, 300-308 (1990). 5. J. Yu, J. Djemaa, P. Nosbaum, and P. Pillet, "Funnel with oriented Cs atoms," Opt. Commun. 112, 136- 140 (1994). .836 J. Opt. Soc. Am. B/ Vol. 13, No. 9/September 1996 6. W. Ertmer, R. Blatt, J. L. Hall, and M. Zhu, "Laser manipu-lation of atomic beam velocities: demonstration of stopped atoms and velocity reversal," Phys. Rev. Lett. 54, 996- 999 (1985). 7. M. Lambropoulos and S. E. Moody, "Design ofa three-stage alkali beam source," Rev. Sci. Instrum. 48, 131-134 (1977). 8. M. Kasevich, D. S. Weiss, E. Riis, K. Moler, S. Kasapi, and S. Chu, "Atomic velocity selection using stimulated Raman transitions," Phys. Rev. Lett. 66, 2297- 2300 (1991). 9. J. Dalibard and C. Cohen-Tannoudji, "Laser cooling below the Doppler limit by polarization gradients: simple theo-retical model," J. Opt. Soc. Am. B 6, 2023- 2045 (1989); P. J. Ungar, D. S. Weiss, E. Riis, and S. Chu, "Optical molasses and multilevel atoms: theory," J. Opt. Soc. Am. B 6, 2058-2071 (1989). Swanson et al. 10. J. J. Maki, N. S. Cam~b.ell, C._ M. Grande, R. P. Knorpp, and D. H. Mcintyre, "Stabilized diode-laser system with gratin feedback and frequency-offset locking," Opt. Commun. io2g 251- 256 (1993). ' 11. !:'1· Walhou~, J. Dalibard, .s. L. Rol~ton~ and W. D. Phillips, u +- u - optical molasses m a longitudmal magnetic field " J. Opt. Soc. Am. B 9, 1997-2007 (1992). ' 12. P. D. Lett, W. D. Phillips, S. L. Rolston, C. E. Tanner R. N Watts, and C. I. Westbrook, "Optical molasses," J. Opt. Soc. Am. B 6, 2084- 2107 (1989). . 13. W. D. Phillips and H. Metcalf, "Laser deceleration of an atomic beam," Phys. Rev. Lett. 48, 596-599 (1982). 14. C. Monroe, W. Swann, H. Robinson, and C. Wieman, "Very cold trapped atoms in a vapor cell," Phys. Rev. Lett. 65 1571-1574 (1990). . 

Rubidium atomic funnel

https://pilotscholars.up.edu/cgi/viewcontent.cgi?article=1030&amp;context=phy_facpubs

Rubidium atomic funnel

Abstract

Similar works

Full text

Available Versions

University of Portland