Sub-Doppler laser cooling of 23Na in gray molasses on the D2 line
Zhenlian Shi, Pengjun Wang, Ziliang Li, Zengming Meng, Lianghui Huang, Jing Zhang
aa r X i v : . [ c ond - m a t . qu a n t - g a s ] D ec Sub-Doppler laser cooling of Na in gray molasses on the D line Zhenlian Shi,
1, 2
Ziliang Li,
1, 2
Pengjun Wang,
1, 2, ∗ Zengming Meng,
1, 2
Lianghui Huang,
1, 2 and Jing Zhang
1, 2, † State Key Laboratory of Quantum Optics and Quantum Optics Devices,Institute of Opto-electronics, Shanxi University, Taiyuan, Shanxi 030006, China Collaborative Innovation Center of Extreme Optics,Shanxi University, Taiyuan, Shanxi 030006, China
We report on the efficient gray molasses cooling of sodium atoms using the D optical transitionat 589.1 nm. Thanks to the hyperfine split about 6Γ between the | F ′ = 2 i and | F ′ = 3 i in theexcited state 3 P / , this atomic transition is effective for the gray molasses cooling mechanism.Using this cooling technique, the atomic sample in F = 2 ground manifold is cooled from 700 µ K to56 µ K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to thelower temperature and high phase space density of atomic cloud after gray molasses. This techniqueoffers a promising route for the fast cooling of the sodium atoms in the F = 2 state. PACS numbers: 37.10.De, 37.10.Gh, 67.85.-d
Ultracold atomic gases offer a remarkably richplatform[1] to enhance our understanding of numerousinteresting phenomenon, such as coherent manipulationof many body states and their dynamics using quantumgases in optical lattices,[2–5] spin orbit coupling to topo-logical matter,[6–11] and ultracold association moleculesto study the long range dipole-dipole interaction and ul-tracold chemistry.[12–14] New cooling techniques are de-veloped continuously to produce an atomic sample witha large atom number and lower temperature, such as3D Raman sideband cooling[15–17] and electromagnet-ically induced transparency (EIT) cooling[18, 19] in site-resolved imaging for Fermi atoms in optical lattice, anddirect laser cooling of rubidium to quantum degeneracyin two-dimensional optical lattice.[20]Cooling neutral atoms to ultracold temperature usu-ally starts with a magnetic-optical trap (MOT), and thentransfers to optical trap or magnetic trap for efficientevaporative cooling. Sub-Doppler laser cooling is a pow-erful tool to decrease the temperature of a three-levelatomic system below the Doppler temperature ~ Γ / k B ,where ~ and k B are the reduced Plank constant andthe Boltzmann constant, and Γ is the natural line widthof atomic transition.[21–23] This technique greatly in-creases the phase space density of atomic cloud, resultingin the higher transfer efficiency to the trap and clearlyleading to a gain in the final atom number after evap-orative cooling. Gray molasses (GM) cooling is a spe-cial method for sub-Doppler laser cooling, which takesadvantage of polarization gradient cooling and velocityselective coherent population trapping.[24]In 1990s, GM cooling was proposed in Ref.[25], inwhich the fluorescence rate of the trapped atoms isstrongly reduced, and then was demonstrated experimen-tally on cesium[26–28] and rubidium[29] within the D transition. More recently, GM cooling was realized on ∗ Corresponding author email:pengjun [email protected] † Corresponding author email:[email protected];[email protected] many atomic species, including alkali atoms K,[30, 31] Li,[32] K,[33, 34] Li,[31, 35] Na,[36] K,[37] andmetastable atom He,[38] which was operating on theblue detuning of the D transition with more resolvedenergy spectrum. The cooling technique was also im-plemented on the D transition to cool K to 50 µ Kwith red-detuned laser,[39] and cool Rb to 4 µ K withblue-detuned laser.[40] It has been proven experimentallythat the GM cooling leads to substantial advantages interms of lower temperature and higher phase-space den-sity. In the GM cooling mechanism, dark and brightstates are the coherent superposition of Zeeman sublevelsin the ground hyperfine manifold.[31] The bright state en-ergy is spatially modulated, giving a polarization gradi-ent cooling like a standard red detuned molasses. Atomswith higher energy in dark states could be coupled to thebright states by motion induced coupling and then couldbe cooled down in the polarization gradient cooling, andones with lower energy are trapped in dark space whichprevent heating induced by light-assisted collisions.In this Letter, we report on an experimental study ofsodium atoms cooled in six beams GM on the blue sideof the D transition | F g = 2 i → | F e = 2 i , as shown inFig.1(a). Thanks to the much reduced fluorescence ratein GM compared to the bright molasses cooling, we areable to capture cold and dense atomic cloud of 3 × atoms at temperature of 56 µ K. After the GM phase,atoms in | F = 2 , m F = 2 i are transferred to an opti-cally plugged quadrupole trap, where F is the total an-gular momentum and m F is its projection. We observethat the loading efficiency is increased to 70% due to thelower temperature and high phase space density of atomiccloud using the GM technique. We also show a significantevidence of the atom’s temperature and number after ashort RF-induced evaporative cooling as a function of thetwo-photon Raman detuning in GM phase.In the experiment, the laser light (at 589 nm) is pro-duced by frequency doubling a master oscillator poweramplifier (MOPA) system. First, the laser beam (at 1178nm) of 23 mW from external cavity diode laser (ECDL)is seeded to a Raman fiber amplifier (RFA), to be ampli- λ/4λ/4 λ/4 AOM+382 λ/4
AOM+92.4AOM+82 AOM+375AOM+344
Na vapor
AOM+199
Lock on theF=2 → F'=3 line
3D MOT2D MOTDL Pro 1178 nmRFASHG 589 nm (b)(a)
F=1F=2F'=2F'=3F'=1F'=0 Δδ S P repumpercooler AOMpolarization beam splitter acousto-opticmodulators wave platemirrordetector λ/4
Fig. 1. (a) Sodium cooling scheme on the D line. The coolingbeam (blue) is blue detuned by ∆ from | F g = 2 i → | F e = 2 i transition, the repumping beam (red) is blue detuned by ∆ + δ from | F g = 1 i → | F e = 2 i transition, where the F g ( F e ) isthe ground-excite state level. Here, δ denotes the two-photonRaman detuning. (b) Sketch of the optical setup for coolingsodium, and the dashed box represents the setup for saturatedabsorption spectroscopy. fied to 7 W and transferred to the SHG unit for secondharmonic generation (SHG) modules. We obtain 3.3 Wof output power at 589 nm, which is sufficient for coolingand trapping of Na in experiment. This single lasersource supplies the coherence between the two frequen-cies for generating the long-lived dressed dark states inGM cooling. The frequency of output beam is stabilizedat | F g = 2 i → | F e = 3 i by the saturated absorptionspectroscopy. The cooling and repumping laser beamsare obtained using acousto-optic modulators (AOMs) indouble pass configuration.[41] Here, the AOMs are actingas fast switches and frequency and intensity tuners. Afterthat the laser beams of the cooling and repumping laserare combined by a polarization beam splitter (PBS) anddelivered to the science cell by polarization maintainingoptical fibers; as shown in Fig.1(b). They are then ex-panded to 25 mm for MOT beams and distributed to thethree pairs of σ + - σ − counter-propagating configuration.Sodium atoms are loaded from a two dimensionalmagnetic-optical trap and trapped by a dual-operationthree dimensional magnetic-optical trap (3DMOT) op-erating on the D transition introduced in,[42, 43] inwhich two laser frequencies are used, one is tuned to | F g = 2 i → | F e = 3 i transition with a red detun-ing 34 MHz for cooling and the other one is tuned to | F g = 1 i → | F e = 2 i transition with a red detuning 57MHz for repumping. In the 3DMOT, the almost samenumber of atoms in the F = 1 and F = 2 states and to-tal number is about 7 . × . The temperature is about700 µ K. At the end of 3DMOT loading, the compressedmagnetic optical trap (CMOT) is used to increase thedensity, in which the magnetic field gradient is rampedfrom 4 G/cm to 10 G/cm in 50 ms.To decrease the temperature, the GM phase is appliedfor a duration time 3.5 ms after the magnetic field isswitched off in ≃ µ s. Here, we define the detuning -3 -2 -1 0 10.51.01.52.02.53.0 Molassess time t m (ms)Raman detuning δ (Γ) T e m p e r a t u r e ( µ k ) T e m p e r a t u r e ( µ k ) N u m b e r ( × ) T e m p e r a t u r e ( µ k ) I cool /I repump (a)(b) (c)(d) Δ = 2.48Γ Fig. 2. (a) The temperature (red squares) and (b) number(blue circles) of atomic sample after GM with time t = 3.5ms as a function of two photon Raman detuning in the caseof ∆ = 2.48Γ. The temperature after GM at the fix principaldetuning ∆ = 2.48Γ and Raman detuning δ = 0, as a func-tion of (c) the pulse duration time t and (d) the intensity ratio I cool /I repump of the Raman cooling pulse at a constant inten-sity I repump = 5 I sat , where I sat = 6 .
26 mw/cm . The datapoints show the average of three experimental measurements,with error bars corresponding to the standard deviation ofthe mean. of the cooling frequency from the | F g = 2 i → | F e = 2 i transition as the principal (one-photon) laser detuning ∆ in the range of 1Γ −
5Γ (Γ = 2 π × .
79 MHz is the naturallinewidth of the D line), and the frequency differencebetween the cooler f C and the repumper f R as the two-photon Raman detuning δ = f R − f C − . ∆ = 2.48Γ (red detuned 34 MHz on the transitionof | F g = 2 i → | F e = 3 i , which means that it is the bluedetuning 24.3 MHz for | F g = 2 i → | F e = 2 i ), such astwo-photon Raman detuning, duration time of GM andthe intensity ratio, as shown in Fig.2.The atomic sample temperature and number after theGM are observed as a function of two-photon detuning δ and a Fano-like profile at the atom temperature dueto the interference of different excitation processes, ac-companied by a sharp change in the number of cooledatoms, as shown in Figs. 2(a) and 2(b). Firstly, we focuson the case of the zero Raman detuning δ = 0, wherewe find that the temperature minimum of T = 56 µ Kis reached. The maximum number about 3 . × ofthe atomic cloud is obtained by the GM with a phasespace density (PSD) of 1 . × − , which means that T e m p e r a t u r e ( µ k ) T e m p e r a t u r e ( µ k ) Principlal detuning Δ (Γ)Raman detuning δ (Γ) Δ = 2.68Γ (a)( c) -1.0 -0.5 0.0 0.51001000 Raman detuning δ (Γ)Raman detuning δ (Γ) Δ = 3.5Γ Δ = 1.46Γ (b)(d) -0.6 -0.3 0.0 0.3 0.6 0.91001000 -3 -2 -1 0 11001000 Fig. 3. (a) The minimum temperature obtained after GM as afunction of the principal detuning ∆ ; The atom temperatureof Na after t = 3.5 ms of GM cooling as a function of twophoton Raman detuning in the case of (b) ∆ = 3.5Γ, (c) ∆ = 2.68Γ and (d) ∆ = 1.46Γ. in the case the atoms in dark spaces are maximally cou-pled to bright states at the bottom of energy hills andthis induces a peak in the atom number in Fig.2(b). Themotion coupling between dark states and bright statesis velocity selective, thus the atoms with higher veloc-ity could be coupled to bright states and re-cooled bythe Sisyphus cooling, and the coldest atoms will accu-mulate in the dark spaces with less interaction with thelight field. The GM cooling mechanism is working in thiscase and could cool the atoms to lower temperature thanthe bright molasses on the D line. For slightly red de-tuned at δ = − . µ K (the MOTtemperature) to about 2 mK, accompanied by a signif-icant loss of atom number during the molasses phase,which is the inverse of the Sisyphus cooling. In additionto the sharp dip and peak in atom temperature and num-ber close the resonance δ = 0, we notice that the moreatoms with lower temperature are captured in the caseof δ > δ < δ >
0, and more atoms could be trapped indark states with lower temperature.The temperature after the GM is plotted as a functionof the duration time t m in Fig.2(c). The temperature de-creases immediately at about 3 ms and reaches the mini-mum value at about 3.5 ms where a steady state is built,and then increases slowly with a longer duration time.The observation of no noticeable cooling effect at first 3 ms is attributed to the small misplacement between thecenter of magnetic trap and the position of atoms cloud,which is designed for better loading in the MOT phase toavoid atoms loss. Figure 2(d) shows the temperature asa function of the ratio between cooler and repumper in-tensities I cool /I repump . We find a nearly linearly decreaseat the initial regimes and a minimal temperature of ∼ µ K reached at the ratio of 2, and a slightly increase withmore cooling power. This may be understood from theinduced light shift in the dressed state picture.[32] At thecase of fixed frequency, the atoms remained in dark statesdistribute the spatial variation of the standing wave andloss how much energy depending on the difference of lightshift between the entry and departure points, as in thebehavior of bright molasses.We also measure the atom temperature after the GMwith different one-photon detunings for 1Γ < ∆ <
5Γ onthe blue side of the transition of | F g = 2 i → | F e = 2 i , asshown in Fig.3. This shows that the minimal tempera-ture could reach ∼ µ K for ∆ = 2.48Γ, and increaseson both sides. This behavior can be attributed to thechosen structure in GM: the energy separation betweenthe | F e = 2 i and | F e = 3 i in excite states is about 5 . ∆ , the upper state may start to play arole. As mentioned in Ref.[40], the closed transition of | F g = 2 i → | F e = 3 i contributes to the inefficiency of theGM cooling. This phenomenon is different from that ofGM on the D optical transition, in which the reachedtemperature is weakly dependent on the one-photon laserdetuning.The measured temperature after GM with differentone-photon detunings as a function of the two photonRaman detunings δ are shown in Figs. 3(b)–3 (d). Foreach value of ∆ , the minimum temperature is obtained on δ ≈
0, similar to the report in earlier experiments. In thecase of ∆ = 1.46Γ, there is a very wide heating windowand the temperature is very higher at the red detuning,where the repumper is close resonance with the atomictransition of | F g = 1 i → | F e = 2 i , as shown in Fig.3(d).During optimizing the GM, we find that the stray mag-netic field and misalignment of the laser beams affects theattained minimal temperature of the GM phase. Here,the three pairs of coils are used to produce XY Z biasfields for compensating the stray field.The most evident advantage of the GM is to obtain alarge number of atoms with lower temperature for loadingto magnetic trap. After the GM phase, optical pumpingis applied in 0.8 ms, during which the atoms are pumpedto the stretched low field seeking Zeeman state | F =2 , m F = 2 i , and then are loaded to the optically pluggedmagnetic trap. In the loading magnetic trap process,a magnetic trap with lower gradient about 10 G/cm isfist used and then the trap gradient is increased to 200G/cm in 200 ms. The special designed ramp stage canprevent the atoms in other low field seeking hyperfinestates from loading to the magnetic trap. Lastly, atomsloaded in magnetic trap can be probed after a few ms oftime-of-flight (TOF). The atom number in the magnetic Raman detuning δ (Γ) -1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9150180210240270 T e m p e r a t u r e ( μ k ) N u m b e r ( × ) ( b)(c) N u m b e r ( × ) ( a) Fig. 4. (a) The atom number loaded into the magnetic trap asa function of the two-photon Raman detuning. The number(b) and temperature (c) of atoms after a short RF forcedevaporative cooling as a function of the two-photon Ramandetuning in GM phase at ∆ = 2.48Γ. trap as a function of the two-photon Raman detuning inGM is shown in Fig.4(a). Here the one-photon detuningis ∆ = 2.48Γ. We also observe a Fano-like profile, fromwhich the loading efficiency is increased to 70% by theGM cooling mechanism.After loading to the magnetic trap, an RF forced evap- orative cooling is performed to cool the atomic sampledown to lower temperature for 5 s. We also study the ef-fect of gray molasses on this stage. The atom number andtemperature after evaporative cooling versus the two-photon Raman detuning in gray molasses are shown inFigs. 4(b) and 4(c). After optimizing the parameters onthe gray molasses, an atomic sample containing 5 . × atoms with T = 150 µ K is produced.In conclusion, we have shown an effective GM coolingof sodium on the D transition, thanks to the sufficientlylarge hyperfine split about 6Γ between the | F e = 2 i and | F e = 3 i in the excited state 3 P / , and the phase co-herence between the cooling and repumping beams froma single laser source. We have investigated the proper-ties of GM cooling by studying the dependence of thesample temperature on the one-photon detuning, two-photon Raman detuning, molasses duration time and theintensity ratio. The high phase space density after GMprovides a starting condition for the effective loading ofmagnetic traps, which is the best evidence of the coolingmechanism. The experimental results give us a picture ofGM for D line which could work in the atomic specieswith the hyperfine split about ∼
6Γ in excited states.Our experiment shows that GM cooling on the D lineis a viable route to high phase space density, while it islower than in GM cooling on D line. The requirementof only a single laser for all cooling process is a clearadvantage. ACKNOWLEDGMENTS
This research was supported by National Key Re-search and Development Program of China (Grant No.2016YFA0301602), NSFC (Grants No. 11474188, andNo. 11704234), the Fund for Shanxi “1331 Project” KeySubjects Construction, and the program of Youth SanjinScholar. [1] Bloch I, Dalibard J and Zwerger W 2008 Rev. Mod. Phys. u nas G, ¨ O hberg P and Spielman I B2014 Rep. Prog. Phys. e pel V, Chen W andVuleti´ c V 2017 Science e C, Meacher D R, Verkerk P and Gryn-berg G. 1995 Phys. Rev. A R3425(R)[27] Boiron D, Michaud A, Lemonde P, Castin Y, Salomon C,Weyers S, Szymaniec K, Cognet L and Clairon A 1996Phys. Rev. A R3734(R)[28] Trich´ e C, Verkerk P and Grynberg G 1999 Eur. Phys. J.D a nsch T W,Ritsch H and Weidem¨ u ller M 1996 Opt. Lett. e L, Wang P, Aspect A, Bouyer P andBourdel T 2013 Europhys. Lett. e ment D 2015 Phys.Rev. A46