Laser cooling and trapping of ytterbium atoms

The experiments on the laser cooling and trapping of ytterbium atoms are reported, including the two-dimensional transversal cooling, longitudinal velocity Zeeman deceleration, and a magneto-optical trap with a broadband transition at a wavelength of 399 nm. The magnetic field distributions along the axis of a Zeeman slower were measured and in a good agreement with the calculated results. Cold ytterbium atoms were produced with a number of about 10⁷ and a temperature of a few milli-Kelvin. In addition, using a 556-nm laser, the excitations of cold ytterbium atoms at ¹S₀-³P₁ transition were observed. The ytterbium atoms will be further cooled in a 556-nm magneto-optical trap and loaded into a three-dimensional optical lattice to make an ytterbium optical clock.      

Cooling and Trapping 88Sr Atoms with 461nm Laser

We report the experimental realization of a ^88Sr magneto-optical trap （MOT） operating at the wavelength of 461 nm. The MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 22. The total laser power available in our experiment is about 300mW. We have trapped 1.6 × 10^8 ^88 Sr atoms with a 679nm and 707nm repumping laser. The two repumping lasers enhance the trap population and trap lifetime by factors of 11 and 7, respectively. The ^88 Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal.     

用于原子干涉仪实验的锂原子的塞曼减速与磁光囚禁

为了制备适于原子干涉仪实验的低温锂原子样品,开展了锂原子的塞曼减速及与磁光阱囚禁相关的实验研究.设计并实现了一种结构紧凑的腔体内冷式多级线圈叠加的塞曼减速器,将速度小于600 m/s的7Li原子减速到60 m/s,磁光阱装载速率为5×108/s,囚禁原子数目1×109个,原子团的最低温度约为220±30μK.研究了光学黏胶中7Li原子的寿命与囚禁光频率失谐量的关系.这些结果为进一步开展7Li原子亚多普勒冷却、光势阱蒸发冷却以及原子干涉仪实验奠定了基础.     

Simultaneous magneto-optical trapping of lithium and ytterbium atoms towards production of ultracold polar molecules

We have successfully implemented the first simultaneous magneto-optical trapping (MOT) of lithium (⁶Li) and ytterbium (¹⁷⁴Yb) atoms towards production of ultracold polar molecules of LiYb. For this purpose, we developed the dual atomic oven which contains both atomic species as an atom source and successfully observed the spectra of the Li and Yb atoms in the atomic beams from the dual atomic oven. We constructed the vacuum chamber including the glass cell with the windows made of zinc selenium (ZnSe) for the CO₂ lasers, which are the useful light sources of optical trapping for evaporative and sympathetic cooling. Typical atom numbers and temperatures in the compressed MOT are 7×10³ atoms, 640 μK for ⁶Li, 7×10⁴ atoms, and 60 μK for ¹⁷⁴Yb, respectively.     

利用补偿线圈提高塞曼减速器效率的理论及实验研究

研究了用于锶原子光晶格光钟原子冷却的塞曼减速器,应用增添补偿线圈的方法可以延长减速器的有效减速距离和增大减速器末端的磁场梯度,进而增加一级冷却俘获锶原子的数目,理论分析采用该方法实现的塞曼减速器较使用单一线圈塞曼减速器可以增加31.17%的俘获原子数目;飞行时间法测量了减速前后原子束中原子的速度分布,原子的最可几速度由380m/s降为43m/s,分布线宽相应变窄。荧光法测量俘获原子数目表明在相同实验条件下,应用补偿线圈后磁光阱俘获原子数目从1.26×106提高到1.81×106,增加30.4%。     

Frequency locking of a 399-nm laser referenced to fluorescence spectrum of an ytterbium atomic beam

The laser cooling of ytterbium(Yb) atoms needs a 399-nm laser which operates on the strong1S0-1P1 transition and can be locked at the desired frequencies for different Yb isotopes.We demonstrate a frequency locking method using the fluorescence spectrum of an Yb atomic beam as a frequency reference.For unresolved fluorescence peaks,we make the spectrum of the even isotopes vanish by using the strong angular-dependence of the fluorescence radiations;the remained closely-spaced peaks are thus clearly resolved and able to serve as accurate frequency references.A computer-controlled servo system is used to lock the laser frequency to a single fluorescence peak of interest,and a frequency stability of 304 kHz is achieved.This frequency-locked laser enables us to realize stable blue magneto-optic-traps(MOT) for all abundant Yb isotopes.     

小型化锶光钟物理系统的研制

光钟物理系统的小型化是制约可搬运光钟及空间冷原子光钟发展的重要因素.主要介绍了小型化锶原子光钟物理系统的研制实验.采用真空腔内置反亥姆霍兹线圈,构建一个小电流、低功耗及小体积的磁光阱.实验中测得真空线圈通电电流仅为2 A时,磁光阱中心区域轴向磁场梯度可达到43 Gs/cm,完全满足锶原子多普勒冷却与俘获对磁场梯度的要求.目前已经成功将锶原子光钟物理系统体积缩小至60 cm×20 cm×15 cm,约为实验室原锶光钟物理系统体积的1/10,并且实现了锶原子的一级冷却,测得俘获区冷原子团的直径为1.5 mm,温度约为10.6 mK.锶同位素~(88)Sr和~(87)Sr的冷原子数目分别为1.6×10~6和1.5×10~5.重抽运激光707和679 nm的加入,消除了冷原子在~3P_2和~3P_0两能态上的堆积,最终可将冷原子数目提高5倍以上.     

Correspondence between phenomenological and IBM-1 models of even isotopes of Yb

Energy levels and the reduced probability of E2–transitions for ytterbium isotopes with proton number Z=70 and neutron numbers between 100 and 106 have been calculated through phenomenological(Ph M)and interacting boson(IBM-1)models.The predicted low-lying levels(energies,spins and parities)and the reduced probability for E2–transitions results are reasonably consistent with the available experimental data.The predicted low-lying levels(gr–,β1–andγ1–band)produced in the Ph M are in good agreement with the experimental data compared with those by IBM-1 for all nuclei of interest.In addition,the phenomenological model was successful in predicting theβ2–,β3–,β4–,γ+2–and 1+–band while it was a failure with IBM-1.Also,the 3–band is predicted by the IBM-1 model for172Yb and174Yb nuclei.All calculations are compared with the available experimental data.     

中性汞原子磁光阱装载率的优化

量子投影噪声是影响光晶格钟的一个重要参数,提高磁光阱中装载率有利于降低量子投影噪声,可提升光晶格钟的性能.针对实验所用的汞原子单腔磁光阱,本文分析并计算了磁光阱中汞原子受力情况和一维运动规律,在此基础上用随机数方法对磁光阱中汞原子三维装载进行了数值计算,获得了磁光阱中的稳态原子数,研究了磁光阱的冷却激光的光强、失谐量以及磁场梯度等参数对稳态原子数的影响,得出了获得最优装载率的实验参数.涉及的计算方法和结论对汞原子光晶格钟的实验设计具有参考价值.     

Loading of a krypton magneto-optical trap with two hollow laser beams in a Zeeman slower

A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variable-diameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.     

Enhancement of Transfer Efficiency of Cold Atoms Using an Optical Guiding Laser Beam

We transfer cold ^87 Rb atoms from a vapour cell chamber to a spatially separated UHV magneto-optical trap （MOT） with the assistance of a red-detuned optical guiding beam and a normal push beam. Efficient optical guiding of the cold atoms is observed within a small detuning window. A pulsed optical guiding beam enhances the transfer efficiency and hence allows us to collect more atoms in UHV MOT in a shorter time, which is favourable for our experiment of achieving Bose-Einstein condensates （BEC）. Besides the easy operation, another advantage of this optical guiding technique is also demonstrated such that slower atomic beams may be efficiently transferred along horizontal direction. This study is a direct application of the optical guiding technique as a powerful tool.     

Laser cooling and trapping of Yb from a thermal source

We have successfully loaded a magneto-optic trap for Yb atoms from a thermal source without the use of a Zeeman slower. The source is placed close to the trapping region so that it provides a large flux of atoms that can be cooled and captured. The atoms are cooled on the transition at 398.9 nm. We have loaded all seven stable isotopes of Yb into the trap including the rarest isotope, ¹⁶⁸Yb. For the most abundant isotope (¹⁷⁴Yb), we load more than 10⁸ atoms into the trap within 1 s. We have characterized the source by studying the loading rate and the loss rate for different isotopes and at different trapping powers. We extract values for the loss rate due to collisions and due to branching into low-lying metastable levels. At the highest trap densities, we find evidence of additional loss due to intra-trap collisions.Received: 15 February 2004, Published online: 23 March 2004PACS: 32.80.Pj Optical cooling of atoms; trapping - 42.50.Vk Mechanical effects of light on atoms, molecules, electrons, and ions     

Frequency ratios of Sr,Yb, and Hg based optical lattice clocks and their applications

This article describes the recent progress of optical lattice clocks with neutral strontium (⁸⁷Sr), ytterbium (¹⁷¹Yb) and mercury (¹⁹⁹Hg) atoms. In particular, we present frequency comparison between the clocks locally via an optical frequency comb and between two Sr clocks at remote sites using a phase-stabilized fibre link. We first review cryogenic Sr optical lattice clocks that reduce the room-temperature blackbody radiation shift by two orders of magnitude and serve as a reference in the following clock comparisons. Similar physical properties of Sr and Yb atoms, such as transition wavelengths and vapour pressure, have allowed our development of a compatible clock for both species. A cryogenic Yb clock is evaluated by referencing a Sr clock. We also report on an Hg clock, which shows one order of magnitude less sensitivity to blackbody radiation, while its large nuclear charge makes the clock sensitive to the variation of fine-structure constant. Connecting all three types of clocks by an optical frequency comb, the ratios of the clock frequencies are determined with uncertainties smaller than possible through absolute frequency measurements. Finally, we describe a synchronous frequency comparison between two Sr-based remote clocks over a distance of 15 km between RIKEN and the University of Tokyo, as a step towards relativistic geodesy.     

Laser cooling of thulium atoms

We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10⁶ thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s⁻¹. Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 10⁴ purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 10⁷ atoms at the expense of higher temperature.     

Maximized cooling efficiency for a Zeeman slower operating at optimized magnetic field profile

We build a Zeeman slower with consecutive coils and use it to load an Yb magneto-optical trap(MOTs).Cooling efficiency is measured by the fluorescence intensity of the atomic cloud trapped by the MOT.An optimized magnetic field profile can acquire the maximum cooling efficiency,corresponding to a good compromise between the smaller magnetic field mismatch and the high capture velocity.Our studies provide useful information on how the performance of the Zeeman slower can be improved.     

Ionization Energies of Ions in Hot and Dense Plasma: Beryllium-Like Ions for Z=26-36

Ionization energies of beryllium-like ions for Z = 26 - 36 in hot ana aense plasmas （ne=10^22 -10^24 cm^-3,kT= 500 - 2000 eV） are obtained by using an approach developed for electronic structure and transition property of ions in hot and dense plasmas based on the multi-configuration Dirac-Fock model. Influence of the plasma environment is considered by introducing a correction to the one-electron potential to account for the screening of the ionized electrons. This correction is calculated from the ionized electron micro-space distribution, which is obtained based on an ＂average atom model for the temperature and density-dependent average ionization of atoms in plasmas. Comparison between the present and the ion sphere models is made to display the significance of the ionized electron micro-space distribution.     

Paramagnetic Faraday rotation with spin-polarized ytterbium atoms

We report the observation of paramagnetic Faraday rotation of spin-polarized ytterbium (Yb) atoms. As the atomic samples, we used an atomic beam, released atoms from a magneto-optical trap (MOT), and trapped atoms in a far-off-resonant trap (FORT). Since Yb is diamagnetic and includes a spin-1/2 isotope, it is an ideal sample for spin physics, such as quantum non-demolition measurement of spin (spin QND), for example. From the results of the rotation angle, we confirmed that the atoms were almost perfectly polarized. PACS 32.80.Bx; 32.80.Pj; 42.25.Lc     

Observation and absolute frequency measurements of the 1S0-3P0 optical clock transition in neutral ytterbium

We report the direct excitation of the highly forbidden (6s2) 1S0 <--> (6s6p) 3P0 optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at approximately 70 microK in a magneto-optical trap. The measured frequency in 171Yb (F=1/2) is 518,295,836,591.6 +/- 4.4 kHz. The measured frequency in 173Yb (F=5/2) is 518,294,576,847.6 +/- 4.4 kHz. Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a 10(6)-fold reduction in uncertainty. The natural linewidth of these J=0 to J=0 transitions is calculated to be approximately 10 mHz, making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.     

Controllable Photonic Band Gap and Defect Mode in a 1D CO2-Laser Optical Lattice

We propose a new method to form a novel controfiable photonic crystal with cold atoms and study the photonic band gap （PBG） of an infinite 1D CO2-laser optical lattice of SSRb atoms under the condition of quantum coherence. A significant gap generated near the resonant frequency of the atom is founded and its dependence on physical parameters is also discussed. Using the eigenquation of defect mode, we calculate the defect mode when a defect is introduced into such a lattice. Our study shows that the proposed new method can be used to optically probe optical lattice in situ and to design some novel and controllable photonic crystals.     

Nonlinear Optical Properties of Novel Polymeric Rare Earth Phthalocyanine Studied Using Picosecond Z-Scan Technique

Three novel tri-dimensional phthalocyanine polymers, with lanthanum （LaPPc）, gadolinium （GdPPc） and ytterbium （YbPPc） as centric atoms, have been synthesized from a tetranuclear phthalonitrile. Third-order optical nonlinearities of these compounds in DMF solution are measured by a picosecond Z-sacn technique at 532 nm. It is found that all the compounds show reverse saturation absorption and nonlinear self-focus refraction effect. The second-order molecular hyperpolarizabilities are calculated to be 1.82×10^-23, 1.48×10^-23 and 1.45×10^-23 esu for LaPPc, GdPPc and YbPPc, respectively. The differences among their nonlinear optical properties are attributed to the special tri-dimensional structure and the variation in rare earth atoms.