All-optical production and trapping of metastable noble gas atoms

All-optical production and trapping ofmetastable noble gas atoms down to

the single atom regime

M. Kohler1, H. Daerr1, P. Sahling1, C. Sieveke1, N. Jerschabek1,M.B. Kalinowski1, C. Becker2 and K. Sengstock2

1Carl Friedrich von Weizsäcker Centre for Science and Peace Research,University of Hamburg, 20144 Hamburg, Germany

2Institut für Laser-Physik, University of Hamburg, 22761 Hamburg, Germany

Abstract

The determination of isotope ratios of noble gas atoms has many applications e.g. in physics, nucleararms control, and earth sciences. For several applications, the concentration of specific noble gas isotopes(e.g. Kr and Ar) is so low that single atom detection is highly desirable for a precise determination of theconcentration. As an important step in this direction, we demonstrate operation of a krypton Atom TrapTrace Analysis (ATTA) setup based on a magneto-optical trap (MOT) for metastable Kr atoms excitedby all-optical means. Compared to other state-of-the-art techniques for preparing metastable noble gasatoms, all-optical production is capable of overcoming limitations regarding minimal probe volume andavoiding cross-contamination of the samples. In addition, it allows for a compact and reliable setup. Weidentify optimal parameters of our experimental setup by employing the most abundant isotope 84Kr, anddemonstrate single atom detection within a 3D MOT.

I. Introduction

Tracers and their analysis are essential in earthsciences, physics, and nuclear arms control.Natural and anthropogenic radioactive noblegas isotopes are ideally suited for use as trac-ers since they are chemically inert. Applica-tions include Comprehensive Nuclear-Test-BanTreaty verification based on xenon monitor-ing [7], groundwater dating [2, 15], and discov-ery of plutonium production (Kr) [8]. However,the sensitivity of the aforementioned applica-tions is limited by the analysis technique used.Reduction of cross-contamination and samplesize opens up many applications in the fieldsof research mentioned above.

One of the most profound and sensi-tive techniques is Atom Trap Trace Analysis(ATTA) [4]. The basic idea is to count singleatoms of the isotopes of interest within a mag-neto-optical trap (MOT), which enables mea-

surements with a sensitivity down to 10−12 for81Kr [15].

Since the optical transition for laser coolingand trapping from the ground state lies in thedeep vacuum ultra violet (VUV) range, no suit-able continuous wave laser source exists. Toovercome this problem, the noble gas atomshave to be prepared in a metastable triplet-statefirst (He, Ne, Ar, Kr, Xe,[1, 14, 9, 16]). For thispurpose, state-of-the-art experiments use colli-sions between electrons and noble gas atoms inRF-driven plasmas, followed by a combinationof Zeeman slower and MOT.

In these experiments the initial produc-tion of metastable atoms is the limiting fac-tor since a minimum gas density for sustain-ing the plasma discharge is required. Furtherproblems arise, for example, due to cross-con-tamination of the sample of interest causedby krypton atoms implanted in the walls of

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All-optical production and trapping of metastable noble gas atoms

the vaccum chamber during previous measure-ments [11]. In subsequent measurements theseatoms are released through ion and electronbombardment in the discharge plasma and con-taminate the current sample. Therefore state-of-the-art ATTA setups have to be cleaned for 36hours using a xenon discharge to push the pre-implanted krypton out of the walls. This limitsthe capacity per ATTA apparatus to about 120samples per year [11]. Furthermore, analyticalcorrections depending on previous measure-ments have to be done to account for residualcross-contamination [10]. All-optical produc-tion of metastable noble gas has the potentialto overcome these limitations [10].

Here we report on the successful opera-tion of a 84Kr and 83Kr 2D–3D MOT apparatusbased on all-optical production of metastableatoms. In addition, we demonstrate the sin-gle atom detection capability of our apparatususing 84Kr and 83Kr.

The optical production of metastable kryp-ton in the electronic configuration 4p55s[3/2]2(see Figure 1) is based on the absorption oftwo resonant photons (123.6 nm & 819 nm),followed by spontaneous emission of a pho-ton (760 nm) [17]. In our setup, the radiationat 123.6 nm is provided by an array of homebuilt VUV lamps [3] directly connected to the2D MOT vacuum chamber (see Figure 2 & 3),while the infrared light is delivered by a 1.4 Wlaser system. The final decay to the desiredmetastable state occurs with a probability of75%.

Figure 1: Optical excitation scheme of krypton for trans-fer to the metastable state 4p55s[3/2]2.

Figure 2: Sketch of the VUV lamp setup attached to thetop of the 2D MOT chamber.

Figure 3: Illustration of the beam guidance used in ourATTA setup.

II. Design and optical excitation

Our new concept consists of a three chambervacuum system; a reservoir for storing andrecapturing noble gas samples, and a 2D–3DMOT setup allowing for single atom detection.

The 2D MOT serves to achieve two impor-tant steps towards isotope selective single atomdetection in our apparatus. First metastable84Kr is produced by all optical means usingan array of self-made microwave driven VUVlamps [3] and a transfer laser. Subsequent lasercooling and trapping provides the required iso-tope selectivity. The resulting beam of coldatoms is directed through a differential pump-ing stage (cone shaped type; smallest diameter1 mm) into a separate 3D MOT chamber, wherethe atoms are cooled and trapped. We employ

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All-optical production and trapping of metastable noble gas atoms

lock-in techniques (130 Hz) to detect the fluo-rescence of numerous trapped atoms using acommon photodiode; however for single atomdetection an avalanche diode with continuouscooling of trapped atoms is implemented.

For experimentally achievable VUV in-tensity, the transition 4p6 → 4p55s[3/2]1 isfar from saturation. Hence, the number ofmetastable krypton atoms increases linearlywith VUV intensity. Since the optical powerof an individual lamp is limited and smallerthan 1 mW (5.5 · 1014 Photons/s [12]) the onlyway to increase the VUV intensity is to use sev-eral lamps. We implemented an array of eightVUV lamps attached to the top of the 2D MOTchamber as depicted in Figure 2 & 3.

For an effective production of metastablekrypton, the whole transfer volume under thelamps must be further illuminated with suf-ficient 819 nm radiation. This is done by amultiply saturated elliptic laser beam with di-ameters of 10 mm and 25 mm and a power ofup to 1.4 W, which is multi reflected by prismsunder total reflection.

The 2D MOT, which is superimposed withthe transfer volume, consists of a cylindricalquadrupole magnetic field and two retro re-flected cooling beams to form a trapping vol-ume that is as large as possible. The geometryof the 2D cooling beams has an elliptic shapewith diameters of 3 cm and 7 cm. The powerof the beam on the symmetry axis correspondsto approximately 3 ISat. Four rectangular coilswith 405 turns each are used to generate a cylin-dric quadrupole field with a magnetic gradientof up to 1.45 mT/cm.

To minimize the loss of trapped Kr atomsthrough collisions with background atoms, the3D MOT is realized in a separate vacuum cham-ber connected to the 2D MOT via a differen-tial pumping stage allowing for a hundredfoldlower pressure in the 3D MOT chamber. Tomaximize the loading rate the distance between2D and 3D MOT is kept as short as possible(∼18 cm).

Six laser beams are used for 3D cooling andtrapping, each having a diameter of 30 mmand about 20 mW of power. The nearly spheri-

cal quadrupole magnetic field (0.8 mT/cm and1.75 mT/cm) is generated by two coils with 395turns operated at 2.5 A.

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M i c r o w a v e P o w e r ( W )Figure 4: Fluorescence of trapped atoms as a function of

microwave power per lamp. The experimentalsettings are 6 · 10−6 mbar 2D MOT pressure,0.7 mT/cm 2D MOT magnetic field gradient,∼ 1 W transfer laser power and detunings of−2.5 γ (2D), −1.5 γ (3D).

III. Characterization

Various measurements have been carried outto characterize the experimental setup, bothwith 84Kr and 83Kr. The following parameteroptimization was performed with 84Kr. We em-ployed the fluorescence of the trapped atomswithin the 3D MOT as a measure for the qual-ity of the loading parameters of our apparatus.In this way, we studied the effect of variationsof microwave power, power of the 819 nm ra-diation, pressure within the 2D MOT, and thecurrent of the 2D MOT coils.

Variation of microwave power One of themost crucial issues of all-optical production isthe limited lifetime of the VUV lamps, mainlycaused by color centers developing on the win-dows. This process mostly depends on the mi-crowave power used to operate the lamps [3].Therefore, it is important to find the optimalpoint of operation in terms of balancing sta-bility and microwave power. For the exper-iments presented here, we typically use fivelamps together with three ohmic loads. Wehave measured the fluorescence of the 3D MOT

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as a function of microwave power per lamp be-tween 3.75 W and 16.25 W and observed a steepincrease between 5 W and 6.25 W (see Figure 4).This is the regime where the individual lampsstart to sustain a stable plasma. From this pointon, the atomic fluorescence is constant up to ap-proximately 10 W. Microwave power over 10 Wshows only a slight increase in fluorescence [3].

Under continuous operation we use apower of about 11 W per lamp as a compro-mise between a sufficiently high fluorescence,stable operation, and an acceptable lifetime ofthe lamps.

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N I R P o w e r ( W )Figure 5: Fluorescence of trapped atoms as a function

of transfer laser power. Experimental param-eters are ∼ 11 W microwave power per lamp,0.7 mT/cm 2D MOT magnetic field gradi-ent, 6.2 · 10−6 mbar 2D MOT pressure anddetunings of −2.5 γ (2D), −1.5 γ (3D).

Power of the IR radiation In contrast tothe power of the VUV lamps, there is enoughpower to saturate the transition 4p55s[3/2]1 →4p55p[3/2]2 with 819 nm radiation. The effi-ciency of the all-optical excitation as a functionof the 819 nm radiation power is depicted inFigure 5. Note that we used a combination oftwo separate laser sources for measurementsexceeding 1.0 W. As a consequence, the fluo-rescence exhibits a discontinuity as a result ofa marginal mismatch of these two laser beams.In accordance with these measurements, wetypically work with 1.4 W at 819 nm.

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P r e s s u r e ( m b a r )Figure 6: Fluorescence of trapped atoms as a function

of pressure within the 2D MOT chamber. Ex-perimental parameters are ∼ 11 W microwavepower per lamp, 0.7 mT/cm 2D MOT mag-netic field gradient, 1.4 W transfer laser powerand detunings of −2.5 γ (2D), −1.5 γ (3D).

Krypton partial pressure of the 2D MOTOptimization of the Kr partial pressure in the2D MOT allows us to achieve the highest fluxof cold atoms being fed to the 3D MOT, en-abling reasonably fast measurements. Two con-trary effects dominate the pressure dependenceof the fluorescence in the 3D MOT. First, theproduction rate of metastable Kr atoms, whichis proportional to the Kr partial pressure, ben-efits from increased pressure. Second, thereare various loss mechanisms, such as collisionswith background atoms, as well as collisionsamong meta-stable slow Kr atoms and groundstate Kr atoms in the 2D MOT. Furthermore,in the 3D MOT, background gas collisions, in-elastic two-body collisions among metastableKr atoms, as well as radiative decay of themetastable state and the non-unity duty cy-cle of the lock-in detection lead to additionalloss. In general, the different loss rates de-pend on the Kr partial pressure in the 2D MOTeither linearly or quadratically, hence givingrise to an optimum in 3D MOT atom numberas a function of the pressure. Note that thebackground pressures of the 2D and 3D MOTare linearly coupled through the differentialpumping stage. Our measurements shown inFigure 6 reveal a behavior that confirms thesequalitative considerations. We observed thehighest fluorescence at about 8 · 10−6 mbar .

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All-optical production and trapping of metastable noble gas atoms

0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00 . 00 . 20 . 40 . 60 . 81 . 0

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2 D F i e l d G r a d i e n t ( m T / c m )Figure 7: Fluorescence of trapped atoms as a function of

2D MOT magnetic field gradient. Experimen-tal parameters are ∼ 11 W microwave powerper lamp, 6.2 · 10−6 mbar 2D MOT pressure,1.4 W transfer laser power and detunings of−2.5 γ (2D), −1.5 γ (3D).

Magnetic field gradient of the 2D MOTcoils The characterization of the 2D MOT in-cludes determining the dependency on themagnetic field gradient.

At about 0.7 mT/cm 2D MOT coil current,the positions of the coils were optimized tomaximize the fluorescence signal of trappedatoms. Subsequently, the dependency of thefluorescence on the magnetic field gradient ofthe 2D MOT was determined. In Figure 7the fluorescence of trapped atoms up to amagnetic field gradient of about 1.0 mT/cmshows a steep increase between 0.3 mT/cm and0.8 mT/cm. The diameter of the atomic beamfeeding the 3D MOT decreases for steepermagnetic field gradients and results in a bet-ter match to the small hole of the differentialpumping stage.

Single atom detection The detection of sin-gle atoms captured within the 3D MOT isthe feature which fully exploits the benefit ofATTA. Single atom detection for neutral atomswas first demonstrated [5, 13] and results ex-hibit precise determination of atom numbersin mesoscopic samples [6]. To demonstrate thesingle atom detection capability of our setup,we reduced the flux of metastable atoms origi-nating from the 2D MOT by lowering the pres-sure within the 2D MOT, the power of the

819 nm radiation, and the microwave powergiven to the lamps until single atoms were ob-served in the 3D MOT. We could demonstratesingle atom detection for both isotopes, 84Krand 83Kr. In Figure 8 we observe very fewatoms within the 3D MOT, ranging from oneto four atoms or one to eight atoms. For largenumbers of atoms, the measured mean fluores-cence fn̄ becomes smaller than the intuitivelyexpected value n× f1̄, since the probability offinding the corresponding number of atoms nover the whole integration time of 1 s is smallerthan 1. To be more precise, the probability ofcollisions between trapped atoms increases asa function of n as can be determined from Fig-ure 8.

IV. Conclusion

We demonstrate the first neutral noble gas 2D–3D MOT apparatus with combined all-opticalproduction of metastable atoms and cooling.This feature relies on the first successful im-plementation of a 2D MOT with VUV lampsthat have a long lifetime. Our results, based onoptical production and single atom detectioncapability, pave the way to overcome the tech-nical problems of present RF-driven noble gastrace analysis techniques.

Future ATTA setups using our new technol-ogy will allow higher sample throughput, and,equally as important, smaller sample sizes, asit avoids cross-contamination and does not re-quire large samples for sustaining a plasma.Our new approach will further increase the ap-plicability of noble gas trace analysis in physics,earth sciences and nuclear arms control.

Acknowledgments

We gratefully acknowledge the technical sup-port of the staff members of the mechanic work-shop of the department of chemistry at the Uni-versity of Hamburg in developing our threechamber ultra high vacuum system includingeight VUV lamps.

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Figure 8: Fluorescence of single trapped 84Kr or 83Kratoms as a function of time. The experi-mental parameters were adjusted to a fluxof metastable atoms consisting of only a fewatoms per minute. In all of the figures, thefluorescence levels corresponding to differentatom numbers are plotted. The dotted linesare multiples of one atom fluorescence level,while the full lines correspond to the measuredmean of the fitted fluorescence distribution ofindividual atom numbers. Integration timeamounts to 1 s.

We thank Niko Lehmkuhl for a novel lasersetup, and we thank Simon Hebel for trailblaz-ing discussions about future applications. Wethank Franziska Herrmann for designing theimaging optics.

We would like to thank the DeutscheForschungsgemeinschaft (grant KA 821/1-1)and the German Foundation for Peace Re-search for funding.

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