laser-cooling of molecules through optical pumping

Transcription

1 Ro-vibrational laser-cooling of molecules through optical pumping 2012 Taiwan International Workshop on Ultracold Atoms and Molecules May 18-20, 2012, Taiwan

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7 25 researchers 12 professors or assistants-professors 30 engineers or technicians 25 PhD students 15 visitors or post-docs 20 persons will come from ENS Cachan in 2013 TAIWAN

8 History of Laboratoire Aimé Cotton 1927 : funded by Aimé Cotton ( ) 1928 : big electro-magnet of «Académie des Sciences» located in Meudon 1951 : Pierre Jacquinot ( )gives the name Aimé Cotton to the laboratory Atomic spectroscopy 1967 : Moving in Orsay TAIWAN

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10 High-Resolution SpectromEtEr by Fourier transformation Years 60 - TAIWAN

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12 laser spectroscopy Years 70 - TAIWAN

13 TODAY Many reasearch activities from academic physics researches (cold matter, quantum information, nanosciences, biophysics ) to biomedical and handicap applications and industrial collaborations Cold matter: cold molecules,, cold Rydberg atoms for quantum simulation, ultracold plasmas, monoenergetic ion and electron sources, laser cooling of hydrogen and anti hydrogen atom TAIWAN

14 OUTLINE Motivations for molecules Optical pumping for optical manipulation and cooling Alternatives to laser cooling of molecules Formation of cold molecules via photoassocion Mechanisms of formation REMPI detection: with spectrally narrowband or broadband laser Laser cooling of internal degrees of freedom of molecules Vibration Rotation Perspectives TAIWAN

15 MOTIVATIONS FOR COLD MOLECULES Precision measurements and fundamental tests Quantum information with polar systems Quantum gases Cold collisions, cold chemistry TAIWAN

16 n/cm λ DB ~ distance between particles Quantum properties Quantum gases Many-body How Cold? Control of collisions Quantum chemistry λ DB = h/mv T -1/2 (quantum size) classical size of particles (1nm) ~ small velocity Precision measurement Fundamental test T/K nk µk mk K TAIWAN

17 OPTICAL PUMPING FOR LASER COOLING Alfred Kastler Absorption spontaneous emission of one photon The key of the light or laser manipulation of atoms Laser cooling (dissipative process, need of repumper lasers) Laser cooling of molecules is difficult, because a priori no way to isolate a two-level molecular system F = p 3/2 A(ns)A(np) 2 Cs Energy A(ns)A(ns) 4 6s vibration levels F =3 TAIWAN 2012 Interatomic distance R 17

18 Progresses for Laser Cooling of molecules Difficult to isolate a two-level system, because of the internal degrees of freedom A few polar molecules present good branching ratio due to large Frack-Condon factors SrF D. DeMille et al. TAIWAN

19 FROM 1998, ALTERNATIVE APPROACHES HAVE BEEN INVESTIGATED Starting with pre-formed molecules : Sympathetic cooling with buffer gas (J. Doyle 1998), Slowing supersonic beam (Stark decelerator, G. Meijer ), Velocity filtering an effusive beam (Quadrupole guide, G. Rempe ), Mechanical slower (D. Hershbach 2001), Collisional stop (D.W. Chandler 2003)... but : 1 mk << T < 1K Association of cold atoms: Magneto-association in the vicinity of a Feshbach resonance (with atomic quantum gases), Photoassociation (with atomic thermal gases)... Translationally (T << 1mK) cold but vibrationally excited Use of STIRAP manipulation in quantum gases TAIWAN

20 FORMATION OF COLD MOLECULES VIA PHOTOASSOCIATION OF COLD ATOMS F I R S T D E M O N S T R A T I O N F O R A C O L D M O L E C U L A R G A S TAIWAN

21 Excitation of a Pair of Colliding Cold Atoms via Photoassociation Cs(6s,F=4)Cs(6s,F=4)hν L Cs 2 *(Ω u,g (6s6p 3/2(or 1/2) ;v,j) C 3 /R 3 level f C 6 /R 6 continuum α Formation of an electronically excited long-range molecule in a selected rovibrational level (a resonant process) These excited molecules are cold but have a very short life ~ 10 ns (smaller than the charactistic time to form one molecule ~ 1 µs) Mostly they dissociate; the channel to form ground-state molecules is very often negligible TAIWAN

22 Schemes for the formation of cold molecules via photoassociation Scheme: compromise between an efficient photoassociation of cold atoms (long-range molecules) and a good branching ratio of spontaneous emission for stabilizing the molecules in the ground state (most of the time it is difficult to fill both conditions) Detection: REMPI (Resonantly enhanced multi-photon ionization): use of narrow or broad bandwidth lasers Two-color cascading spontaneous emission schemes TAIWAN

23 FORMATION OF COLD ATOMS VIA PHOTOASSOCIATION Cs 2 X 2 Σ g Cs Cs e Ti:Sa Laser REMPI dye laser (2) 3Σ g 6s5d (2) 3 Π g v,j 6s6p 3/ g 0 u 0 g - 1 u 0 g Laser Ti:Sa Σ u Spontaneous emission v,j 1 Σ g 1 g 0 g - 0 u 6s6p 1/2 6s6s R (at. unit) TAIWAN 2012 Dye Laser Photoassociation Formation of cold molecules Detection via REMPI time (ns) dye laser pulse high voltage pulse MCP signal Cs Cs

24 LONG-RANGE MOLECULES Molecules in several ro-vibrational levels (not really optical pumping) Molecules in the triplet state (via 0 g- state) Molecules highly vribrationnaly excited in the singlet sate (via 1 u state) A priori no molecule with low vibrationnal numbers in the ground state TAIWAN

25 TO FORM GROUND-STATE MOLECULE VIA PHOTOASSOCIATION OF COLD ATOMS LAC 1998 Cs 2 0 g- 6s6p 3/2 Double-well configuration (first demonstration of a cold molecular cloud) LAC 2000 Cs 2 0 u 6s6p 1/2 Internal spin-orbit coupling of potentials with the same symmetry Connecticut 2000 K 2 TAIWAN 2012 Two-step excitation Case of heteronuclear systems is different 25

26 PA mechanism for the formation of cold molecules Two-color cascading spontaneous emission scheme En nergy (cm -1 ) Spon. Em. a 3 Σ u X 1 Σ g 6s5d 1 g Internal coupling 6s6p 0 u P.A. 6s6s PA on 1 g (6s6P 3/2 ) level, coupled by fine structure to the 1 g (6S5D 3/2 ). 2-step spontaneous emission to X state via A (0 u ) state R (a 0 ) TAIWAN

27 REMPI DETECTION Resonantly enhanced multiphoton ionization A key element in the cold molecule experiments via photoasociation Gives access to the ro-vibrationnal distribution in the molecular sample TAIWAN

28 FORMATION OF COLD ATOMS VIA PHOTOASSOCIATION Cs 2 X 2 Σ g Cs Cs e Ti:Sa Laser REMPI dye laser (2) 3Σ g 6s5d (2) 3 Π g v,j 6s6p 3/ g 0 u 0 g - 1 u 0 g Laser Ti:Sa Σ u Spontaneous emission v,j 1 Σ g 1 g 0 g - 0 u 6s6p 1/2 6s6s R (at. unit) TAIWAN 2012 Dye Laser Photoassociation Formation of cold molecules Detection via REMPI time (ns) dye laser pulse high voltage pulse MCP signal Cs Cs

29 Detection of ground-state molecules (a 3 Σ u ) 2 photons detection (REMPI) Pulsed dye laser (LDS722) Spectrum when PA on 0 g- (v=79) (2) 3 Π g ( cm -1 ) ion yields (2) 3 Σ g 0 6s5d Io n iz a tio n la s e r w a v e le n g t h c m - 1 6s6s (Very) selective detection of low levels (v = 12-16) of triplet state TAIWAN

30 REMPI DETECTION Resonantly enhanced multiphoton ionization A key element in the cold molecule experiments via photoasociation Gives access to the ro-vibrationnal distribution in the molecular sample Well adapted to the detection of the triplet state Use of spectrally narrowband or broadband lasers? TAIWAN

31 REMPI detection of the X ground state 532 nm Selective detection Energy (cm -1 ) nm R (A 0 ) B X V X Power: 1mJ/pulse 1 Bandwith: 0.05 cm laser de d 彋 ection (cm -1 ) TAIWAN

32 Broad band detection Energy (cm -1 ) nm 770 nm R (A 0 ) B X Non-selective detection (25 cm -1 spectral band width) V X ~11730 cm -1 vibrational levels v X >37 ~13000 cm -1 vibrational levels v X <75 Power: 1mJ/pulse 1.0 Bandwith: 25 cm laser de d 彋 ection (cm -1 ) TAIWAN

33 Detection of molecules in X 1 Σ g Broadband laser (FHWM ~ 25 cm -1 ) -> meaning all v levels are detected! Scan PA laser new different lines! Detection in a 3 Σ u 0 g spectrum v = 100 Efficient way to detect all the molecules created (below v=75) in X 1 Σ g TAIWAN

34 Detection of molecules in X 1 Σ g Narrowband laser -> each v level is selectively analyzed - Scan of ionization laser - PA laser on the new line Ion number V C -V X ,1 0,01 1E-3 1E-4 1E Dye laser wavelength (cm -1 ) Franck-Condon = <v X v B > 2 Molecules in v=1-10 (no v=0) TAIWAN

35 LASER COOLING OF THE VIBRATION OF MOLECLES USE OF BROADBAND LASER TAIWAN

36 Broadband optical pumping of molecules Spectrally broadband laser To excite all the levels towards the B state Spontaneous emission Redistribution of the population in the vibrational levels of the X ground state Intensit nombre d'onde (cm -1 ) Energy (cm -1 ) B 1 Π u 1 X 1 Σ g R (A 0 ) 2 3 v= R (A 0 ) R (A 0 ) TAIWAN v=0

37 Optical pumping and laser cooling of the vibration To shape the femtosecond laser To revent the absorption from v=0 dark state Intensit nombre d'onde (cm -1 ) Energy (cm -1 ) B 1 Π u X 1 Σ g R (A 0 ) v= R (A 0 ) v= R (A 0 ) TAIWAN

38 Vibrational laser cooling Accumulation of the population in a dark state v=0 Cooling of the vibration Intensit nombre d'onde (cm -1 ) Energy (cm -1 ) B 1 Π u i X 1 Σ g R (A 0 ) v= R (A 0 ) v= R (A 0 ) v= R (A 0 ) TAIWAN

39 Optical pumping Absorption 1 Emission 2 Shaping the laser pulse V=0 Dark State Energy (cm -1 ) Optical Pumping of molecules 9450 B 1 Π v u B = vx =4 X 1 Σ 1 g v X =0 2 Pulse shaping Remove all the frequency above v=0 Intensity Wavenumber (cm -1 ) R (a 0 ) f f f TAIWAN

40 Use of a femtosecond mode-locked laser 80 MHz, 100 fs, 54 cm -1, 773 nm TAIWAN

41 Optical Pumping and vibrational cooling Transfer 70 % the v (<8) to v=0 when adding the femtosecond laser Accumulation of molecules in the dark state v=0 ( per second) Cs v X v C Science 321, 232 (2008) Wavenumber (cm -1 ) TAIWAN

42 Experimental Control pulses number with AOM 1pulse = 12.5 ns Temporal evolution Simulation >70% in v=0 with <10000 pulses weak field (perturbative) Exact solution of master (rate) equation Kinetic Monte Carlo Very fast (<100 µs), Only 10 absorptions Almost no heating New Journal of Physics 10 (2008) TAIWAN

43 Better amplitude shaping EM E in (ω) CM FM 2 SLM ω G E out (ω) ω cw LASER MCP 9 µm SLM 100 µm 3 µm Slow axis x y REMPI LASER z Liquid Crystal spatial light modulator (SLM) 640 pixel 0.06nm resolution ~ 1 cm -1 Collaboration with Béatrice Chatel, LCAR, Toulouse TAIWAN

44 Energy Pumping to a chosen dark state! NJP (2009) X X X a) b) V=2 dark c) d) Internuclear distance Efficiency ~ 60% with SLM Other v coupled Intensity (Arb) Intensity (Arb) Intensity (Arb) Intensity (Arb) V=0 dark % off/on ratio V= V=7 V= Wavenumber (cm -1 ) Cs 2 Cs 2 Cs 2 Cs TAIWAN Wavenumber (cm -1 )

45 Vibrational cooling with diode laser Only need broadband light to cover the desired transitions shaping shaped Free diode fs diode SLM mask 100% off/on ratio Better efficiency Free laser shaped Incoherent light Only 1 W/cm 2 TAIWAN

46 DIFFERENT SHAPING TAIWAN

47 For NaCs, pumping through the B 1 Π state. Cf. N. Bigelow photoassociation For other systems! Simulation Light spectrum Franck-Condon factors V B V X Population transfer Time µs TAIWAN

48 LASER COOLING OF THE ROTATION OF MOLECULES U S E O F B R O A D B A N D L A S E R N A R R O W B A N D L A S E R ( R E C E N T U N P U B L I S H E D R E S U L T S ) TAIWAN

49 ANOTHER SCHEME PA and cold molecule formation mechanism through 2-step decay 0 g below 6s6p 1/2 R 6 asymptotic behavior TAIWAN

50 1 CW DIODE LASER FOR DETECTION 1 CW DIODE LASER FOR THE COOLING R (A0 ) 8 9 v=0 X 1 Σ g v i b r a t i o n a l c o o l i n g I o n i z a t i o n B 1 Π u v=3 v= r o t a t i o n a l c o o l i n g a n d d e t e c t i o n 3 4 TAIWAN

51 DETECTION AFTER VIBRATIONAL COOLING IN V=0 Q(3) 16 PA toward V 26 J 2 nbre d'ions P(5) P(3) Q(5) Q(1) R(1) R(3) fréquences [cm-1] x scan de dét PA V'26 J'1 Q(4) Q(2) R(2) Nbre d'ions P(2) R(0) TAIWAN Fréquence [cm-1] x10 3

52 Rotational cooling population J X P : J reduced by 1 in absorption. SCAN NARROWBAND LASER 8 7 P , , , , , ,1 N bre d'ions Q Wavenumber [cm -1 ] Scan with rotationnel cooling Scan without rotationnel cooling P(4) P(2) Q(4) 5 4 Q(2) R(0) R(2) R R(4) 10 9 See also S. Schiller, M. Drewsen: Nat. Phys. April TAIWAN x Fréquences [cm-1]

53 24 P(4) P(2)Q(4)Q(2) R(0) R(2) R(4) Cs 2 Number V = 0, J = Wavenumber [cm -1 ] P(5) P(3) Q(5) Q(3) Q(1) R(1) R(3) Cs 2 Number V = 0, J = Wavenumber [cm -1 ] TAIWAN

54 Mol. Experiment LAC Matthieu Viteau Amodsen Chotia Dimitris Sofikitis Ridha Horchani Issam Manai Andréa Foretti Daniel Comparat Hans Lignier Pierre Pillet Jacques Robert Nicolas Vanhaecke Visitors Maria Allegrini Emiliya Dimova Marin Pichler Collaboration (Toulouse) Lirong Wang Béatrice Chatel Theory LAC Sébastien Weber Nadia Bouloufa Olivier Dulieu TAIWAN

55 CONCLUSION AND PROSPECTS Laser manipulation and cooling of the internal degrees of freedom of molecules (vibration, rotation) but also electronic (triplet singlet conversion) Can be applied to many systems and extend to external degree of freedom (broadband laser as a repumper) At LAC development of a BaF experiment Lase cooling, Sisyphus cooling Cold collisions, cold chemistry One-way 1 2 TAIWAN

56 24 P(4) P(2)Q(4)Q(2) R(0) R(2) R(4) Cs 2 Number Wavenumber [cm -1 ] THE END THANK YOU FOR YOUR ATTENTION TAIWAN