Optical pumping of rare-gas metastable atom beams witha frequency-modulated multimode dye laser
J. G. Lynn, M. W. Hart, T. H. Jeys, and F. B. Dunning
Very efficient stable optical pumping of an argon metastable atom beam is achieved using a frequency-modulated multimode dye laser. Frequency modulation is accomplished by rapidly oscillating the laseroutput coupler using a piezoelectric translator. This technique appears attractive for use in opticallypumping a wide variety of atomic species.
In recent years there has been increasing interest inthe study of polarization dependences in atomic colli-sion processes. Beams of polarized atoms for use insuch studies are frequently produced by optical pump-ing.1-5 Repeated cycles, each consisting of the absorp-tion of circularly polarized light followed by spontane-ous emission, are used to transfer atoms to states forwhich the projection of the total angular momentumrelative to the direction of propagation of the radiationis maximal. A number of investigations of transverseoptical pumping of atomic beams using lasers havebeen reported. To achieve large polarizations severaloptical pumping cycles must occur during the time anatom is illuminated. Because illumination times aretypically rather short, the photoexcitation rate for thetransition involved in the optical pumping processmust be large. This photoexcitation rate decreasesrapidly as the separation between the laser and transi-tion frequencies increases. Thus, because the Dopplerwidth resulting from laser and atomic beam diver-gences is small, many early optical pumping experi-ments employed frequency-stabilized single-mode dyelasers.1 Atomic beams have also been successfullypumped using unstabilized multimode dye lasers.2 4Sizable output powers are, however, required so thatlarge photoexcitation rates are obtained even underthe worst case condition that the transition frequencylies midway between adjacent laser output modes.Earlier work in this laboratory5 showed that stable
When this work was done all authors were with Rice University,Houston, Texas 77251; T. H. Jeys is now with MIT Lincoln Labora-tory, P.O. Box 73, Lexington, Massachusetts 02173.
Received 10 February 1986.0003-6935/86/132154-04$02.00/0,
efficient optical pumping can be obtained using a low-power dye laser if its output frequency is modulated ona time scale that is short compared with that of atomicillumination. Frequency modulation results in a qua-si-continuous distribution of output frequencies.This means that laser tuning is less critical, and, evenat low output powers, the average photoexcitation rateis sufficient to obtain efficient pumping.5 In initialexperiments an intracavity phase/frequency modula-tor was employed which, despite use of Brewster cutsurfaces, introduced insertion losses that produced amarked decrease in the available laser output power.In the present work we demonstrate that an alternatemodulation technique, namely, changing the frequen-cies of the allowed cavity modes by rapidly oscillatingthe laser output coupler using a piezoelectric transla-tor (PZT), offers a number of advantages. This tech-nique is particularly simple and eliminates the lossesassociated with an intracavity modulator resulting inincreased laser output powers and higher beam polar-izations. The capabilities of this modulation tech-nique are illustrated by experimental results obtainedin a study of optical pumping of an Ar(3P2) metastableatom beam.
The present apparatus, which has been described indetail elsewhere,4 6 is shown schematically in Fig. 1. Afraction of the atoms contained in a ground-state argonatom beam is excited to 3Po,2 metastable levels bycoaxial electron impact. Charged particles are re-moved from the beam by a transverse electric field.The atoms then enter a weak magnetic field, perpen-dicular to the beam, that preserves a well-definedquantization axis. The 3P2 atoms are opticallypumped5 7 using circularly polarized 811.5-nm3p54s[3/2]2 (3P2 ) - 3p5 4p[5/2]3 (3D3 ) radiation, inci-dent parallel to the magnetic field, from a dye laser.The laser beam is reflected so as to make two passesthrough the metastable atom beam illuminating it fora distance of -1 cm. Because the mean reciprocalmetastable atom velocity is -2.8 X 10- s cm t 1,8 atoms
Fig. 1. Schematic diagram of the apparatus. The inset showsdetails of the piezoelectric translator.
will on average be illuminated for -28 As. The Dopp-ler width resulting from laser and atomic beam diver-gences is small, -25 MHz. Following optical pump-ing, the beam polarization is determined by spatialseparation of the Mj components using a Stern-Ger-lach (SG) analyzer.
The optical pumping radiation is provided by a mod-ified Spectra-Physics model 375B dye laser, which hasa cavity mode spacing of -400 MHz. The laser istuned by a three-element birefringent filter and a sin-gle 3-mm uncoated etalon; the dye employed is LD700.The laser is pumped by a krypton-ion laser and pro-vides an output power of -250 mW at 811.5 nm, result-ing in an intensity of -400 mW cm-2 in the opticalpumping region. The laser output consists of radia-tion from several cavity modes that together span -4GHz.
The frequencies of the individual cavity modes aremodulated by rapidly oscillating the output couplerusing a PZT,9 and details of the PZT assembly areshown in the inset in Fig. 1. The output coupler isclamped to an aluminum disk, which is affixed to oneface of a PZT by conductive epoxy. The other face ofthe PZT is glued, also using conductive epoxy, to astainless steel disk that is screwed onto a large brassmass. The sides of the PZT are sealed with RTVsealant to exclude moisture. Electrical contact to thePZT is made via the attached disks. The PZT can bedriven sinusoidally at frequencies up to -200 kHzusing a transformer-coupled power amplifier. To ob-tain the required unipolar PZT drive voltage, a diode iswired across the PZT, and the PZT is connected to thesecondary of the output transformer via a series capac-itor. The power amplifier can provide drive voltageswith peak-to-peak amplitudes of up to -800 V.
The operating characteristics of the PZT were inves-tigated in a series of subsidiary experiments in whichthe PZT was used to mount the driven element of ascanning Fabry-Perot etalon of -40-GHz free spectralrange. The output beam from the (unmodulated) LD700 dye laser was directed through this etalon and theintensity of the transmitted beam monitored by a fastphotodiode. The mirror translations resulting fromdifferent drive voltage amplitudes and frequencieswere determined by monitoring, using an oscilloscope,the output of the photodiode as a function of appliedvoltage, and observing the number of free spectral
w
CUH
CHQNNELTRON POSITION
Figure 2. (a) SG profile for a mixed Ar(3PO,2) beam with no opticalpumping. The 3PO contribution is indicated by the shaded region.(b) SG profile obtained following optical pumping with RHCP811.5-nm radiation from the frequency-modulated laser. The ar-rows indicate the expected peak positions for atoms with different
values of MJ.
ranges through which the etalon scanned. Thesestudies revealed a number of mechanical resonances atwhich the mirror translation was maximal. A pro-nounced resonance was observed at -70 kHz, and thePZT was routinely operated at this frequency, which issufficient to allow several modulation cycles during thetime an atom is illuminated. A drive voltage of -180-V peak-to-peak amplitude was sufficient to scan theetalon by one free spectral range, corresponding to amirror translation of -400 nm. This translation in-creased approximately linearly with drive voltage upto 800 V. During operation with large amplitudedrive voltages, however, power dissipation in the pie-zoelement is sufficient to cause significant heating,and for extended operation the PZT assembly must becooled by a fan.
The spatial profile at the output of the SG analyzerin the absence of optical pumping is shown in Fig. 2(a).Electron impact excites both 3PO and 3P2 metastableatoms, and each of these contributes to the central MJ= 0 peak. The 3PO contribution, which can be deter-mined as described elsewhere,4,5,10 is indicated by theshading. The 3P2 M = +1, +2, and MJ = -1, -2,features are not resolved because the velocity distribu-tion of the metastable atoms in the beam leads to arange of deflection angles for each MJ. Figure 2(b)shows the SG profile for a mixed 3PO,2 beam followingoptical pumping with right-hand circularly polarized(RHCP) 811.5-nm radiation from the frequency-mod-ulated laser. The laser intensity in the optical pump-ing region is -400 mW CM-2, and the amplitude of thePZT drive voltage is -750 V. The 3PO contribution isagain indicated by shading, and it is apparent thatoptical pumping leads to very efficient transfer of 3P2
Fig. 3. Percentage of 3P2 atoms that remains in states of negative Mjfollowing optical pumping with RHCP radiation as a function of thepeak-to-peak amplitude of the voltage applied to the PZT. Thevertical extent of each data point indicates the change in this per-
centage observed at each amplitude over a period of -15 min.
results in a similarly efficient transfer to the Mj = -2state. No difficulties were experienced in tuning thelaser to the 3P2 - 3D3 transition, and efficient pumpingwas routinely obtained for periods of several hourswithout further adjustment of the laser.
The improvements in optical pumping efficiencyand stability that result from use of the PZT modula-tor are illustrated in Figs. 3 and 4. In Fig. 3 is shownthe percentage of 3P2 atoms that remain in states ofnegative Mj following optical pumping with RHCPradiation at an intensity of --'400 mW cm-2 as a func-tion of the amplitude of the drive voltage applied to thePZT. At each amplitude the laser was initially tunedto minimize the percentage of Mj = -1, -2 atoms.This percentage was monitored for a period of -15min, and the vertical extent of each data point indi-cates the range of values observed and thus provides ameasure of the temporal stability of optical pumping.With no voltage applied to the PZT laser tuning wasvery critical and efficient stable pumping could not beachieved. Both the efficiency and stability improvedas the drive voltage and hence frequency deviationwere increased. A voltage amplitude of -200 V, suffi-cient to cause a deviation in the frequencies of theindividual cavity modes equal to the cavity mode spac-ing, results in stable efficient optical pumping, andfurther increases in voltage amplitude led to only mi-nor improvement. This suggests that, on the timescale for which the atoms are illuminated, the lasermust operate on all adjacent cavity modes.
The improved stability afforded by frequency mod-ulation is further demonstrated in Fig. 4. This showsfor RHCP pumping radiation the percentage of 3P2atoms that remains in states of negative Mj followingoptical pumping as a function of the intensity in theoptical pumping region, both with no drive voltageapplied to the PZT and with a drive-voltage amplitudeof -700 V. To ensure that the spectral characteristicsof the pumping radiation did not change during thesemeasurements, the laser operating characteristics
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5 10 20 50 100 200 500INTENSITY (mWCm-2)
Fig. 4. Percentage of 3 P2 atoms that remains in states of negativeMJ following optical pumping with RHCP radiation, both with andwithout a drive voltage applied to the PZT, as a function of theintensity in the optical pumping region. The vertical extent of eachdata point indicates the change in this percentage observed at each
intensity over a period of -15 min.
were maintained constant and the intensity varied byuse of neutral density filters. The vertical extent ofeach data point again provides a measure of the tempo-ral stability over a period of -15 min. Although re-ductions in intensity lead to lower optical pumpingefficiencies, the improved optical pumping stabilityand efficiency afforded by frequency modulation areclearly evident.
The present laser was also used to optically pump aNe(3P2) metastable atom beam. The dye employed toobtain the required 640.2-nm 3P2 - 3D3 radiation wasDCM, and an argon-ion pump laser was used. Withfrequency modulation, optical pumping efficienciesand stabilities comparable with those observed forAr(3P2) were obtained, and the technique thus appearssuitable for use in optically pumping a variety of atom-ic species. The resultant polarized beams will permitstudy of polarization dependences in a wide range ofatomic collision processes.
It is a pleasure to acknowledge valuable discussionswith G. K. Walters during the course of this work andthe assistance of L. K. Johnson and M. S. Hammond indata acquisition. This research was supported by theDivision of Materials Sciences, Office of Basic EnergyScience, U. S. Department of Energy; The Robert A.Welch Foundation; and the Donors of the PetroleumResearch Fund administered by the American Chemi-cal Society.
References1. J. J. McClelland and M. H. Kelley, "Detailed Look at Aspects of
Optical Pumping in Sodium," Phys. Rev. A 31, 3704 (1985) andreferences therein.
2. J. T. Cusma and L. W. Anderson, "The Polarization of a NaAtom Beam by Laser Optical Pumping," Phys. Rev. A 28,1195(1983).
3. K. W. Giberson, Chu Cheng, M. Onellion, F. B. Dunning, and G.K. Walters, "Optical Pumping of He(2 3 S) Atoms by a Color-Center Laser," Rev. Sci. Instrum. 53, 1789 (1982).
4. K. W. Giberson, L. K. Johnson, M. W. Hart, M. S. Hammond, T.H. Jeys, and F. B. Dunning, "Optical Pumping of a Ne(3P2)Atom Beam with a Multimode Laser," Opt. Commun. 52, 103(1984).
5. K. W. Giberson, M. S. Hammond, M. W. Hart, J. G. Lynn, and F.B. Dunning, "Optical Pumping with a Frequency-ModulatedMultimode Dye Laser," Opt. Lett. 10, 119 (1985).
6. T. W. Riddle, M. Onellion, F. B. Dunning, and G. K. Walters,"Polarized He(2 3S) Thermal Metastable Atom Source," Rev.Sci. Instrum. 52, 797 (1981).
7. L. D. Schearer, "Depolarization Cross Sections for the Metasta-ble Noble Gases by Optical Pumping," Phys. Rev. 188, 505(1969) and references therein.
8. R. D. Rundel, F. B. Dunning, and R. F. Stebbings, "VelocityDistributions in Metastable Atom Beams Produced by CoaxialElectron Impact," Rev. Sci. Instrum. 45, 116 (1974).
9. Purchased from Jodon Laser, Ann Arbor, MI, part 215-002.10. F. B. Dunning, T. B. Cook, W. P. West, and R. F. Stebbings,
"Selective Removal of Either Metastable Species from a mixed3PO,2 Rare-Gas Metastable Beam," Rev. Sci. Instrum. 46, 1072(1975).
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