Laser Cooling And Trapping Of Strontium Atoms For An Optical Lattice Clock And The Optical Frequency Measurement Of Intercombination Transition

The measurement of time and frequency has always been fundamental in themankind’s everyday life, the foundation of the national economy, even an importantrole in scientific research. New kinds of clocks are revolutionizing time keeping withenormous consequences for science and technology, they are called optical clocks andas a new generation of atomic clocks based on optical transitions. Now, optical clocksbased on strontium neutral atoms are the new generation of frequency standards withstability and accuracy at the10-18level. Accuracy for the SI (International System ofUnits) second is currently defined by the Cs primary standard. However, strontiumoptical clocks have now achieved a lower systematic uncertainty. This systematicuncertainty will become accuracy once the SI second has been redefined. The coolingand trapping of strontium is the basis of the entire optical clock, it provides highquality factor Q of strontium atomic transition reference. This thesis mainly describesthe laser cooling and trapping of strontium: the first-stage broadband Doppler coolingusing the strong dipole allowed1S0→1P1transition at461nm with a natural width of32MHz, strontium atoms in this so called “Blue MOT” are Doppler cooled to a finaltemperature at~5mK level; the second-stage narrow-line cooling using theintercombination transition1S0→3P1which has a natural linewidth of7.6kHz, atomsin this so called “Red MOT” are cooled to a final temperature at~μK level. Thecontents of this thesis can be summarized as following:(1) The detailed experimental measurement of broadband cooling in the “Blue MOT”,radiation at461nm resonant with the dipole allowed transition1S0→1P1which isindeed very robust and for a typical choice of laser, the atoms are cooled reaching afinal temperature at about5mK. The first cooling transition is not perfectly closed,due to a small leakage towards the4d1D2state. In order to recycle the atoms stored inthe metastable states two repumping lasers, respectively at707nm and679nm can beused to pump these atoms. The experimental and theoretical investigations of thefrequency and power fluctuation of repumping lasers, impact on the cooling andtrapping of strontium atoms. More than3.1×108atoms have been trapped with the capture rate2.8×109s-1when the two repumping lasers added. An optical latticeconfinement can be employed to load atoms in so as to cancel Doppler、recoilfrequency shift and broadening resulting from thermal motional effects. However, theconservative force of optical dipole traps is much weaker than dissipative force ofDoppler cooling. The optical dipole trap depth of lattice is about only tens of μK.Owing to the singlet-triplet spin-forbidden narrow transition1S30P1at689nm(27.6kHz), it has a low Doppler cooling limit of180nK, which can be used assecond-stage cooling transition to cool mK atoms down to a few μK. The preliminaryexperimental results of second-stage narrow-line cooling of “Red MOT” have beenimplemented. About20%strontium atoms in the blue MOT are transferred to the redMOT using the release and recapture method. The theoretical calculation of "magic"wavelength with a.c. Stark shift cancellation has been introduced, the blue-detunedlattice “magic” wavelengths values (389.9nm,359.8nm,345.2nm and335.8nm) whilered-detuned lattice “magic” wavelength value813.4nm.(2) A linewidth reduction of the689nm laser from MHz to sub-kHz level is necessary,as a result of its nature linewidth of only7.6kHz. The laser linewidth is reduced lessthan150Hz by locking to the resonance of a high finesse cut-out ULE cavity withstandard Pound–Drever–Hall technique. The finesse of ULE cavity is12000measured in experiment using the cavity ring down technique. Combining afiber-based optical frequency comb, the Allan deviation for beat signal of stablenarrow689nm laser with fiber optical frequency comb is obtained, with a stability of2.8×1013at1s averaging time. The floor of the Allan deviation is about4.4×1014atan averaging time of16s. The finesse was measured using cavity ring downtechniques in different transverse modes of the cavity.(3) The optical frequency measurements are performed for four natural isotopes88Sr,86Sr,87Sr and84Sr, using a fiber frequency comb (Menlo FC1500) generatorreferenced to H maser（sigma-Tau）with the repetition rate and the carrier offsetenvelope frequency are locked to the H maser. The absolute optical frequencymeasurement values of84Sr and87S（rF=9/2→F’=7/2）was also given for the first time. (4) We have demonstrated that Lamb dip holes with velocity selection can be used forvelocimetry. We report measurements of the transverse velocity distribution ofalkaline-earth strontium atoms in a collimated atomic beam, using the stable narrow689nm laser corresponding to1S30P1intercombination transition. The use of anultrastable laser system and the narrow intercombination transition line of Sr atomswith the resolution of the measured velocity can reach0.13m/s, corresponding to90μK in energy units. The velocity or momentum distribution of the thermal atomicbeam is most likely to be Maxwell-Boltmann distribution or Lorentzian distribution.The experimental result of the distribution shape is in agreement well with thetheoretical predictions. The atomic beams have been widely used in the determinationof atomic structures, measurement of physical constants, studies of chemical reactionsand atomic frequency standards. Especially, for the second order Doppler effect andatomic clock frequency error analysis. In all these applications, measurement of thevelocity distribution of the atomic beams is both necessary and highly important.(5) We observe the intercombination transition spectroscopy of alkaline-earthstrontium, the saturation spectroscopy has been conducted with the minimumsub-Doppler width is55kHz, using a stable narrow689nm laser which is locked to ahigh fineness ultralow expansion (ULE) reference cavity. Experimental results andtheoretical explanations of spectra broadening factors have been introduced, such aspower broadening and collisional broadening. In the presence of external magneticfield, we observed a single, triplet or quintuplet spectral line with the differentpolarization angle. A new technique for elimination of crossover resonance is alsodemonstrated by changing the polarization pattern. The EIT effect is also investigatedexperimentally in the strontium atomic beam, using the V-type Zeeman three sublevelssystem of intercombination transition line with the linewidth of transparent window450kHz. It has provided a proposed EIT-based clock scheme using coherent couplingbetween the two states of optical clock transition.