Gaur, S J

Marathe, A P

Pradhan, S

IACS Institutional Repository

Phys. 7 6 B  (4 ) , 5 4 5 - 5 4 7  (2 0 0 2 )UP B— an international journalL a s e r  c o o l i n g  a n d  t r a p p i n g  o f  c e s i u m  a t o i ^ s  i n  a  m a g n e t o - o p t i c a l  t r a pS  P ra d h a n , A  P  M ara th e , S J G aur, A  V enugopalan , IQG M an o h a r a n d  B N  J a g a ta p 'Laser and Plasma Technology Division and Spectroscopy Division, Bhabha |^tomic Research Center, Mumbai-400 085, IndiaE-mail bni@magnum.barc.en|d.inAbstract : Laser cooling and trapping o f cesium atoms using an indigenously developed magneto-optieal trap is reported. The experimental set up o f the trap and laser-cooling procedure adapted for eooling of cesium atoms is discussed.Keywords : Laser cooling, magneto-optical trap, high-resolution spectroscopy.PACS No. 32.80.pjI. Introductionin recent y ears th e re  h a s  b e e n  an  e x te n s iv e  ac tiv ity  in  lase r cooling and tra p p in g  o f  n e u tra l a to m s, an d  a p p lic a tio n s  o f  the cold a to m s fo r  in v e s tig a tio n s  in  a  w id e  v a r ie ty  o f  problems in p h y sic s  [ 1 - 6 ] .  T h e se  in c lu d e , fo r  ex am p le , u ltra  m is io n  m easu rem en ts , m e tro lo g y , n o n lin e a r  o p tic s , a tom  optics, B o se -E in s te in  c o n d e n s a t io n  a n d  n o n -a c c e le ra to r  ^article physics. D e v e lo p m e n ts  in  la s e r  co o lin g  tech n iq u es , vhich are cap ab le  o f  g e n e ra tin g  u ltra  c o ld  a n d  h ig h  d en sity  >tomic sam ples a rc  a t th e  o r ig in  o f  th e se  re c e n t ad v an ces. Magneto-optical t r a p  (M O T ), w h ic h  is b a sed  o n  th e  ligh t icattering fo rce  in  a  sp a tia lly  in h o m o g e n e o u s  m ag n e tic  fie ld  7-10], is c en tra l to  th e s e  e x p e rim e n ts  in  a to m ic  co o lin g , design and d e v e lo p m e n t o f  th is  tra p  is, th e re fo re , a  s ta rtin g  x>int o f any re se a rc h  in  p h y s ic s  o f  c o ld  a to m s. R ecen tly  w e •sve com pleted  d e v e lo p m e n t o f  a  M O T  in  o u r  lab o ra to ry  “id have u sed  it  to  d e m o n s tra te  la s e r  c o o lin g  an d  tra p p in g  >f cesium a to m s. In  th is  p a p e r  w e  re p o r t th e se  d e v e lo p m en ts  ‘long w ith th e  la s e r  c o o lin g  e x p e rim e n t o n  ces iu m  a tom ; '‘hich to  o u r  k n o w le d g e  is  th e  f irs t in  In d ia .Experimentalcooling  o f  c e s iu m  a to m s  w as  ac c o m p lish e d  in  a  M O T , ’hich has b e e n  d e v e lo p e d  in d ig e n o u s ly  in  o iu  labora to ry , fhe d e s ig n  c o n s id e r a t io n s  o f  th i s  d e v ic e  h a v e  b e e n  liscussed in  d e ta ils  in  o u r  r e la te d  p u b lic a tio n s  [9 ,10 ]. O u rCorresponding AuUiorM O T  co n sis ts  o f  an  o c tag o n a l all m e ta l v acu u m  (S S 3 I6 )  ch am b e r f itted  w ith  ten  p o rts . O f  th ese , e ig h t p o rts  a re  loca ted  on  th e  faces  o f  th e  oc tag o n  w hile  tw o  p o rts  a re  loca ted  a t th e  to p  an d  b o tto m  faces a lo n g  th e  cen tra l ax is . S ix  o f  th e  p o rts  a lo n g  th ree  m u tua lly  p e rp e n d ic u la r a x e s  a re  fitted  w ith  o p tica l w in d o w s fo r th e  co o lin g  la se r  beam s, w h ich  c o n sis t o f  th ree  p a irs  o f  c o u n te r-p ro p a g a tin g  b eam s in m u tu a lly  o rth o g o n a l d irec tio n s. TSvo o f  th e  p o rts  o n  th e  ch am b e r se rve  th e  p u rp o se  o f  co n n ec tin g  to  an  a lk a li m e ta l re se rv o ir  an d  an  u ltra  h ig h  vacu u m  (U H V ) p u m p in g  system . T h e  rem ain in g  tw o  p o rts  a re  a lso  p ro v id ed  o p tic a l w in d o w s an d  a re  u sed  fo r d iag n o s tic s  o f  la se r-co o led  a to m s. T h e  M O T  is p ro v id ed  w ith  re ce sse s  a t th e  to p  and  b o tto m  fo r f ittin g  a  p a ir  o f  m ag n e tic  co ils . T h e  p u m p in g  s ta tio n  fo r th e  M O T  ch am b er c o n sis ts  o f  a  tu rb o -m o lecu la r p u m p  an d  an ion  p u m p  o f  p u m p in g  sp eed s  150 1/s  an d  35 1/s respective ly . T h e  system  is c o n d itio n ed  to  o b ta in  a  s tead y  vacu u m  in  th e  ran g e  o f  1 0 ^  to r r  o n  a  co n tin u o u s  b as is . T ra n s fe r  o f  a lk a li m e ta l v jq jour in to  th e  ch am b e r is  a ch ie v ed  b y  a  sp ec ia lly  d e s ig n e d  ce s iu m  re s e rv o ir  th a t  is  a tta c h e d  to  th e  M O T  th ro u g h  a  U H V  co m p a tib le  va lv e . S p h e ric a l q u a d ru p o le  m ag n e tic  f ie ld  [8 - 1 2 ] is  o b ta in ed  b y  tw o  id en tic a l c o ils  in  th e  "an ti-H e lm h o lts"  co n fig u ra tio n  p la c e d  in th e  to p  a n d  b o tto m  rece sse s  o f  th e  M O T . T h ^  c o ils  u sed  in  o u r  ex p e rim en ts  a re  fo rm ed  u s in g  6 0  tu rn s  o f  en am e led  c o p p e r  w ire  o f  2 .3  n u n  d iam eter. T h e  d ia m e te r o f  th e  c o ils  a n d  th e  se p a ra tio n  a re0 2 0 0 2  lA C S5 4 6 S  Pradhan, A P Marathe, S  J  Gaur, A Venugopalan, K G Manohar and B N Jagatapa d ju s te d  to  o b ta in  a  u n ifo rm  m ag n e tic  fie ld  g rad ie n t in  th e  ra n g e  o f  1 0 -4 0  G /cm  in  a  reg io n  o f  ~ 1  cm  a t the  cen tre  o f  th e  M O T  ch am b er. A  low  v o ltag e , h igh  cu rren t reg u la ted  D C  p o w e r  su p p ly  is  u sed  fo r  su p p ly in g  cu rren t to  th ese  co ils .T h e  M O T  assem b ly , w h ich  co n s is ts  o f  th e  o c tag o n a l v a c u u m  ch am b er, o p tic s  an d  o p to -m ech an ica l co m p o n en ts , is  se t u p  o n  an  in d ig en o u sly  b u ilt v ib ra tio n  iso la ted  o p tica l ta b le . T h e  o p tic a l c o n fig u ra tio n  o f  th e  M O T  in c lu d es  th e  d io d e  la se rs , se t-u p  fo r  sa tu ra te d  a b so rp tio n  sp ec tro sco p y , la s e r  b eam  e x p a n s io n  an d  p o la r iz a tio n  o p tic s , beam  sp littin g , th re e -a x is  c o o lin g  b e a m  a rra n g e m e n t an d  p ro v is io n  fo r  h y p e rf in e  re -p u m p in g . F ig u re  1 show s th e  sch em atic s  o f  the  ex p e rim en ta l se t u p  o f  th e  M O T . N o m in a lly  a b o u t 10 %  o f  the  c o o l in g  la s e r  o u tp u t  is  u s e d  fo r  s a tu ra te d  a b s o r p t io n  sp e c tro sc o p y  w h ile  the  rem a in in g  9 0 %  is b eam  ex p an d ed  ( ~ 1 0  m m ) a n d  c ircu la r ly  p o la r iz e d  by  a  q u a rte r-w av e  p la te  b e fo re  se n d in g  to  th e  M O T . T h e  co o lin g  la se r b eam  is then  sp lit  in to  th re e  b eam s o f  n ea rly  eq u a l in ten s itie s  and  sen t in to  th e  M O T  c h a m b e r th ro u g h  th e  o p tic a l p o rts  a lo n g  the  X, y  a n d  z  d irec tio n s . E ach  o f  th e se  b eam s en te rs  th e  M O T  ch am b er, p a sse s  th ro u g h  th e  ce n te r  o f  th e  M O T  an d  ex its  fro m  th e  o p p o s ite  side . A fte r  th e  e x it from  th e  M O T , each  o f  th e se  b eam s is p a sse d  th ro u g h  a  q u a rte r-w av e  p la te  an d  re tro -re f le c te d  b a c k  in to  th e  M O T  in  c o u n te r-p ro p ag a tin g  d irec tion .Figure t .  Schematics of the magneto-optical trap assembly in BARC. Various subsystems and components are designated as follows: C : MOT chamber, QM ; quadrupole magnet, UHV : ultra high vacuum system, R ' aUcati metal reservoir, DLI : cooling diode laser, D1.2 ; hyperfine re­pumping diode laser, SAS ; saturated absorption spectroscopy set up, PC : personal computer, BE ; beam expander. QP ; quarter wave plate, M : minor, B S l : beam splitter 1 0 :90 , B S 2 ' beam splitter 33 : 66 , B S3; beam splitter SO ; 50, PD : photodiode, L ; lens, S : slit, CCD : charge coupled device, VM ; video monitor.T h e  so u rc e  o f  la s e r  ra d ia tio n  fo r  la se r c o o lin g  is a  tu n a b le  e x to n a l  cav ity  d io d e  la se r  in  L ittro w  co n fig tu a tio n  an d  h av in g  a  b a n d w id th  o f  a b o u t 1 M H z . A  c o m p u te r  co n tro l h a s  b een  setiq) fo r  th e  la s e r  tu n in g  t h r o u ^  a  d a ta  a c q u is itio n  a n d  co n tro l c a rd , w h ich  a ls o  s e rv e s  th e  fu n c tio n s  o f  d ig itiz in ga n a lo g  o u tp u t fro m  th e  p h o to d io d e  th a t  d e tec ts  saturated a b so rp tio n  s ig n a ls . T h u s  u n d e r  so ftw a re  com pu ter control it is p o s s ib le  to  s c a n  th e  la s e r  a n d  a c q u ire  saturatec a b so rp tio n  sp ec tru m  o f  c e s iu m  a to m s  a n d  then  lock thi d io d e  la se r  to  th e  sh o u ld e r  (re d  sh ifted  fro m  resonance) o an y  o f  th e  h y p e rfm e  p e a k s  e ith e r  b y  so ftw a re  control or b' a  d e d ica ted  e le c tro n ic  lo ck in g  sy stem . S o ftw are  locking unde p ro g ram  co n tro l h as  th e  ad v an ta g e  o f  ach ie v in g  laser lockini w ith o u t ad d itio n a l h a rd w are , w h ile  th e  elec tron ic  lockini ach ie v es  a  tig h te r  an d  m o re  ac c u ra te  lo ck in g  [ I I ] .  Howeve; w h en  the  la se r is tu n e d  n e a r  to  th e  a to m ic  transition. i c a u se s  p o p u la t io n  tr a p p in g  b e c a u se  o f  th e  presence o h y p erf in e  s tru c tu re  in th e  leve ls  o f  a lk a li a to m s [7 - 10]. Fo ex am p le , th e  g ro u n d  leve l o f  cesiu m  a to m , w h ich  is studio in  th is  w ork , has tw o  h y p e rf in e  lev e ls  ( F  =  3 ,4 ). During tb la se r co o lin g  p ro cess , th e  a to m s g e t tra n sfe rre d  to  the ¥  ■ 3 h y p erfin e  leve ls, w h ich  is n o t co n n ec ted  rad ia tive ly  to tb c o o lin g  tra n s it io n . In  o rd e r  to  a v o id  su ch  a  populatioi t r a p p in g ,  a  s e c o n d  la s e r  fo r  h y p e r f in e  re-p u m p in g  i in co rp o ra ted  a lo n g  w ith  its o w n  a ssem b ly  o f  o p tics , compute co n tro l and  sa tu ra te d  a b so rp tio n  sp e c tro sc o p y  setup.3 . R e s u lts  a n d  d is c u s s io n sIn la se r co o lin g  ex p erim en ts , th e  tra p p e d  c lo u d  o f  cold cesiuii a to m s w as o b se rv ed  by  d e te c tin g  th e  flu o rescen ce  emitte by  th e  c lo u d . To th is  e n d  a  C C D  ca m e ra  w as  p laced  on on o f  th e  d iag n o s tic  p o rts  fo r im ag in g  th e  fluo rescence  of th a to m ic  c lo u d . T o  o b ta in  la s e r  c o o le d  ce s iu m  atoms, th fo llo w in g  seq u en ce  o f  s tep s  w as  fo llo w ed . T h e  cooling lase w as scan n ed  u n d e r  c o m p u te r  c o n tro l to  o b ta in  saturate! ab so rp tio n  sp ec tru m  o f  cesium  an d  th en  w as locked  to th sh o u ld e r o f  th e  6 ^Si/2F = 4  ->  6 ^F j/2F '=  5 h y p erf in e  transitioi (cy c lin g  tran s itio n ) . T h e  d e tu n in g  o f  la se r frequency  fror th e  a to m ic  re so n an ce  freq u en cy  w as  - 5 0  M H z  on  the re side. T h e  seco n d  d io d e  la se r d e s ig n a ted  fo r  hyperfine rt p u m p in g  w as lo ck ed  to  th e  6 ^ S \a F  =  3 t ^ P y i F  ~ ‘ h y p e rf in e  trem sition  in  an  id en tic a l fa sh io n . T h e  optica a rra n g e m e n t o f  M O T  en su red  th a t c o o lin g  an d  re-pumpii'i la se rs  o f  c o rre c t p o la r isa tio n s  a n d  b e a m  s ize s  entered th M O T  ch a m b e r an d  in te rsec ted  a t th e  c e n tre  o f  th e  chambfl T h e  co o lin g  b eam s h a d  a  n o m in a l in ten s ity  o f - 5  mW/cm w h ich  w as h ig h e r th an  th e  sa tu ra tio n  in ten s ity  o f  ~ 2  ntW cm ^ fo r th e  re lev an t cesium  a tom ic  tran s itio n  [8 ]. T he intensit o f  th e  h y p erf in e  re -p u m p in g  b eam  w as  p la c e d  a t a  simib leve l. T h e  co o lin g  la se r  b e a m s , p a ss in g  th ro u g h  the MOT ch a m b e r in th e  th re e -a x is  g eom etry , e x c ite d  th e  cesium  atoiw in  th e  v a p o r re su ltin g  in  a  b rig h t f lu o re scen ce  o f  the 0  v apor. In  th e  ab sen ce  o f  th e  m ag n e tic  f ie ld , th e  atom s web u n ifo rm ly  d is t r ib u te d  in  th e  c h a m b e r  v o lu m e  and tl** f lu o re scen ce  w as d iffu se ly  sp re a d  ftm ough o u t th e  padiLaser cooling and trapping o f cesium atoms in a magneto-optical trap 547ilif laser beams. Figure 2a shows the image of cesium jl^ orescence in the MOT without the magnetic field.(C)ifiirc 2. (a) Fluore.scence imaging of cesium atomic cloud in the MOT amber in absence o f a magnetic field (b) Fluorescence imaging of wild and trapped cesium atoms in the MOT (c) Fluoresamce imaging cesium cloud in a MOT in the absence of rc-pumpmg laser.On obtaining the fluorescence from the MOT, a current 130 A was passed through the magnetic coils of the MOT lamber to excite the spherical quadrupole magnetic field, tie combined action of the magnetic field and the laser field ten dissipative and confining forces to cool and trap the oms at the center of the MOT. The effect of this Jnfinement may be seen in Figure 2b, which shows the a^ge of the fluorescence emanating from a smaller volume «ctly at the center of the trap. When the magnetic field as switched on, the fluorescence pattern shown in Figure 1 collapsed into the pattern shown in Figure 2b. This is a rar indication of the cesium atoms being trapped at the Iter of the MOT. Further when the hyperfine re-pumping 'tr was blocked off, the fluorescence at the centre of the OT was weak as shown in Figure 2c. This is to be expectedsince in the absence of the re-pumping laser, atoms get trapped in die ground €^S\n F -  3 hyperfine level and are lost from the trap.In conchision, we have designed and developed a MOT and have used it for laser cooling and trapping of cesium atoms. Wei have started work on the detailed diagnostics o f the co^  atoms, which include measurements of temperatur| and atom density. These results will be reported elsewhere.!Acknowle^mentsAuthors thynk Dr. S K Sikka, Director, Atomic and Condensed Matter Phjisics Group, Dr. N Venkatramani, Head, Laser and Plasma Technology Division and Dr. T K Balasubramaniam, Head, Spectroscopy Division, BARC for their keen interest in this wm'k and continued support.K cfcrencet[ I ] C E Wieinan, D E Pritchard and D J Wincland Rev. Mod Phys. 71 S25.3 (1999)12] M H Anderson, J R Enshcr, M R Matthews, C E Wieman and E A Cornell Science 269 198 (1995)|3 ] K B Davis, M 0  Mewes, M R Andrews, N J Van Drutent, D S Durfee, D M Kitrn and W Ketlerlu Phys. Rev Lett. 75 3969 (1995)|4 ] C C Bradley, C A Sackett, J J Toilet and R G Mulct Phys. Rev ie t l  75 1687 (1995)(51 t; A Hinds and IG  Hughes./. Phys D, Applied Physics 32 R119 (1999)[6 ] C S Wood. S C Bennett, I) ( ho, B I’ Masterson. J I. Roberts, C E Tanner and C E Wieman Science 275 1759 (1997), D J Vieira, S G Crane, K Guckert and X Zhao Hyperfine Interactions 127 387 (2000)J7] E l-  Raab, M Prentiss, A Cable, S Chu and D E Pritchard Phys Rev. Lett 59 2631 (1987)f8 ] C Monroe, W Swann, H Robinson and C H Wieman Phys Rev. Lett 65 1571 (1990)19) B N Jagatap, K G Manohar, S 0  Nakhate, A P Marathc and S A Ahmad Current Science 76 207 (1999)110] B N  Jagatap, A P Marathe, K G Manohar, R C Sethi and S A Ahmad in Trends in Atom ic and M olecular Physics ed K K Sod (New York Kluwer Acadcmic/Plcnum) 299 (2000)(11) S J Gaur, S Pradhan, K G Manohar and B N Jagatap Indian J. Phys (submitted) (2001)

Laser Cooling and Trapping of Cesium Atoms in a Magneto-optical Trap

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/6472/1/LASERC~2.PDF

Laser Cooling and Trapping of Cesium Atoms in a Magneto-optical Trap

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository