| Quantum degenerate gases have been extensively studied in recentyears, including Bose-Einstein condensation(BEC), Quantum degenerateFermi gas (DFG), Bose-Fermi Mixtures(BFM) et. The realization andrapidly development of quantum degenerate gases are due to the followreasons. First, the laser cooling and trapping used as a precooling stage toobtain the starting conditions for quantum degenerate gases; Second, withthe development of vacuum technology, the double-MOTs system providevery clear background for experiments of quantum gases; Third, theFeshbach resonances and optical lattices used as tools provide the possibilityof controlling the atom-atom interaction in ultracold quantum gases whichcan give access to an even broader range of phenomena.DFG and BFM present a new path for understanding high-Tcsuperconductivity and strong interaction, producing ultracold molecules aswell as investigating BEC-BCS crossover regime. DFG have been difficultto realize for two reasons: first, evaporative cooling relies upon elasticrethermalizing collisions, which at the temperatures of interest (＜1mK) areprimarily s-wave in nature and are forbidden for identical fermions; andsecond, the number of fermionic isotopes suitable for laser cooling andtrapping is small. Until now, only four fermions have so far been brought todegeneracy in the dilute gaseous phase: mainly 40K and 6Li, seldom 3He and173Yb. Sympathetic cooling overcomes the limit to evaporative cooling byintroducing a second component (spin state, isotope or element) to the gas,thermalization between the two components then allows the mixture as awhole to be cooled.The purpose of our work is to establish the experimental apparatus for40K DFG and perform the assosiate scientific research. The double-MOTstructure system are adopted by our lab; 40K and 87Rb mixtures are selectedby our lab. For the experimental procession, the first is simultaneous magneto-optical trapping of fermionic 40K and bosonic 87Rb atoms incollection cell. Then push the precooled atoms into the science cell andperform MOT again. The next work is to load the ultracold atoms intomagnetic trap. The last step is to achieve DFG by evaporative cooling of87Rb with sympathetic cooling of 40K.The thesis mainly includes several parts:1, We have developed a semiconductor laser system used in 40K-87Rbtrapping and cooling experiment. The laser diode system is composed ofthree external cavity diode lasers(ECDLs) and four injection diode lasersand a semiconductor tapered amplifier(TA). Four laser beams used as 40Kand 87Rb cooling and repump lights are obtained by using acoustic opticmodulators(AOM) to offset the frequencies from the three ECDL lasers.Four light beams with the different frequencies are injected into four slavelasers for injection locking, then 87Rb cooling light, 40K cooling light and40K repump light are injected into a TA in a master oscillator-powerconfiguration to boost power. This configuration is flexible and steady andcan simultaneously produce the cooling and repump light for 40K and 87Rbtrapping and cooling experiment.In experiment, the ECDLs are locked to certain atomic transition signalby use of saturated absorption spectroscopy and modulation transferspectrum technology.In experiment, we developed a lot of laser sources including ECDL,widely tunable ECDL based on lower temperature and tapered amplifier.2, The natural potassium source is unacceptable for the experimentsbecause the low abundance of the isotope 40K(natural abundance is 93.26%39K, 0.012% 40K, 6.73% 41K) limits the collected atom number in MOT. Wehave developed an enriched 40K source used in 40K-87Rb atomic mixturecooling experiment.The reservoir of our dispenser is made of 0.125mm thin Nichrome(80% nickel, 20% chrome). The Nichrome is chemically inert and behaves well at high temperatures. The design of the reservoir in our experiment isquite different from the dispensers described in the past. It has severaladvantages such as having more space, loading easily and preventing escapeof the reactants. Spot welding method is used for making reservoir.The enriched 40K source release 40K atoms through redox reactionbetween 40K enriched KCl (40K, 6.2%)and calcium (99.99%). Ca and KClneed powder form and are mixed uniformly according to 2:1 volume ratio.The works that convert reaction material into powder form and mix them areaccomplished in a glove box which is pumped by a turbo and then filledwith high purity argon.3, An double-MOT structure vacuum system are designed andestablished using a lot of vacuum compenant, including turbo, ion pump,titanium sublimation pump, capacitorr pump and other things. TheRubidium source and potassium dispensers are cormected to the collectioncell. Typically, the collection cell pressure is about 2×10-7 Pa and thescience cell pressure is about 3×10-9 Pa.4, We realize simultaneous magneto-optical trapping of fermionic 40Kand bosonic 87Rb atoms. This trap is the first step towards quantumdegenerate fermi gas 40K. We obtain up to 107～108 40K and 108～109 87Rbatoms in the steady-state single-species MOT. In the case of two-speciesMOT, the steady state atom numbers of 40K and 87Rb simultaneously trappedare decreased largely due to the decrease of the laser power, the increase ofthe vacuum pressure and the all kinds of lossees caused by collisions.
|
PDF Full Text Request