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Acta Physica Sinica

Observation of transitions in strontium triplet state and its application in optical clock

Gao Feng, Wang Ye-Bing, Tian Xiao, Peng Xu, Hong Chang

Published Jan 1, 2012• 1 citation
Publisher

Abstract

In this paper we present a method of observing triplet state transitions of strontium. The intercombination transition is employed to pump the atom population from singlet-state (5s2)1S0 to triplet-state (5s5p)3P1 by a laser at 689 nm. Then 688 nm laser is also employed to divide atom population into the two other triplet-state states (5s5p)3P0 and (5s5p)3PP2. We can obtain the absorption signals of triplet-state transition (5s6s)3S1 → (5s5p)3P0 and (5s6p)3S1 → (5s5p)3P3P2. And these atomic absorption signals can be used for stabilizing the repumping light 679 nm and 707 nm directly to transition line of strontium. This method can be used in the Doppler cooling of strontium atoms.

DOI
10.7498/aps.61.173201

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 29, 2007•8 citations
In this paper we present a method of observing triplet state transitions of strontium. The intercombination transition is employed to pump the atom population from singlet-state (5s2)1S0 to triplet-state (5s5p)3P1 by a laser at 689 nm. Then 688 nm laser is also employed to divide atom population into the two other triplet-state states (5s5p)3P0 and (5s5p)3PP2. We can obtain the absorption signals of triplet-state transition (5s6s)3S1 → (5s5p)3P0 and (5s6p)3S1 → (5s5p)3P3P2. And these atomic absorption signals can be used for stabilizing the repumping light 679 nm and 707 nm directly to transition line of strontium. This method can be used in the Doppler cooling of strontium atoms.
We observe the fluorescence and the saturated fluorescence spectra of (5s2)1S0(5s5p)3P1 intercombination transition of thermal strontium atomic beam. Experimental investigation of 88Sr(5s2)1S0(5s5p)3P1 intercombination transition is performed on the different experimental conditions. Our study indicates that experimental parameters affect spectra largely, including temperature, intensity of laser and scanning frequency of laser. Intensity of spectrum is reciprocal of scanning frequency of laser due to long lifetime of (5s5p)3P1 state of Sr atom. Owing to Doppler broadening, transit broadening and other experimental factors, the linewidth of (5s2)1S0(5s5p)3P1 fluorescence spectrum is far more than its natural linewidth and proportional scanning frequency of laser.
At present the optical clock with accuracy and stability achieves to 10-18 level, which could be the next generation of time and frequency standard. This paper gives an introduction of the progress of researching on the optical lattice clock of Strontium atom in NTSC (National Time Service Center). We realize the (5s5s)1S0—(5s5p)1P1 cooling (blue MOT) and (5s5s)1S0—(5s5p)3P1 cooling (red MOT) in succession for the optical lattice clock, and loading these cold atoms in a lattice composed by a magic wave-length is studied. By using our narrow line-width diode laser and the fiber femtosecond optical frequency comb, we precisely measure the absolute frequencies of the inter-combination transitions (5s5s)1S0—(5s5p)3P1 of four isotopes of Strontium referenced to an H maser.
We describe repumping and spectroscopy of laser-cooled strontium (Sr) atoms using the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition. Atom number in a magneto-optical trap is enhanced by driving this transition because Sr atoms that have decayed into the (5s5p) 3 P 2 dark state are repumped back into the (5s 2 ) 1 S 0 ground state. Spectroscopy of 84 Sr, 86 Sr, 87 Sr and 88 Sr improves the value of the (5s5p) 3 P 2 –(5s4d) 3 D 2 transition frequency and determines the isotope shifts for the transition accurately enough to guide laser-cooling experiments with less abundant isotopes.
We demonstrate a spin-polarized clock transition spectrum of the 87Sr optical lattice clock. The clock transition 5s2 1S05s5p 3P0 of isotope 87Sr has a hyperfine structure due to non-zero nuclear spin, inducing ten -polarized transitions from each individual mF state under the condition of a bias magnetic field along the probing polarization axis. In this experiment, atoms are driven to a certain mF state by a circular-polarization pump light to maximize the atomic population, which is beneficial to the stability and uncertainty evaluation of the optical lattice clock. After two stages cooling and trapping, about 3.5106 atoms are trapped in the red magneto-optical trap with a temperature of 3.9 K. A grating-feedback external cavity diode laser with a tapered amplifier is used to build the optical lattice with a magic-wavelength of 813.426 nm. Both waists of the counter-propagating lattice beam along the horizontal direction are overlapped to form a one-dimensional (1D) optical lattice. The lifetime of the atoms trapped in the 1D optical lattice is 1600 ms. The clock laser at 698 nm is a grating-feedback diode laser, which is locked to an ultra-low expansion cavity by the Pound-Drever-Hall technique to stabilize the frequency and phase. As a result, the linewidth of clock laser is narrowed to Hz level. By the normalized shelving method, we obtain a resolved sideband spectrum of 87Sr 5s2 1S05s5p 3P0 transition. According to the spectrum, the lattice temperature along the longitudinal direction is approximately 4.2 K. After that a linewidth of 6.7 Hz of the degenerate clock transition is obtained at a probing time of 150 ms by utilizing a three-dimensional (3D) bias magnetic field, which is used to eliminate the stray magnetic fields. Then a small bias magnetic field of 300 mGs is applied along the polarization axis of the lattice light to achieve the spectrum of Zeeman magnetic sublevels of the clock transition. Furthermore, the mF=+9/2 and mF=-9/2 magnetic sublevels are picked to be respectively pumped by the +-polarized and --polarized light at 689 nm, a variable liquid crystal wave plate is employed to switch on both polarizations. Finally, the spin polarized clock transition spectrum is obtained at the interrogating pulse of 150 ms, and the linewidths of the mF=+9/2, mF=-9/2 magnetic sublevel transitions are 6.8 Hz and 6.2 Hz respectively.
To decrease the Doppler broadening and frequency shift,reducing the atomic velocity by laser cooling is necessary.Noting that through the first stage cooling,atoms can only be slowed down to the temperature at the mK level,which is too high to be loaded to the optical lattice,We made further efforts to slow down the velocity of the atoms,and the second stage cooling was implemented.When the first stage cooling stopped,using the 689 nm laser corresponding to(5s2)1S0→(5s5p)3P1further cooling the atoms could attain the temperature at as low asμK level.Using the timing sequence accurately controlling the time of interaction between the electromagnetic field and atoms.calculated the temperature through timeof-flight method.Dropping time of cold atoms in the MOT was accurately controlled,and the atomic photos were taken at 0ms and 20 ms.By analyzing the data,the atomic temperature after the second stage cooling is 4.39μK,uncertainty is only 0.19μK and the number of the atoms 1.2×107.The temperature and number of the Sr atoms offer the important reference for estimating the SNR of signal,which is also the foundation for achieving the high-precision standard of time and frequency.
We report on our progress toward the realization of a compact optical frequency standard referenced to strontium intercombination lines. Our current setup allows the production of ultracold Sr atoms in hundreds of ms. For high resolution spectroscopy of the 1S0-3P0 doubly forbidden transition we have prepared a 698 nm clock laser stabilized on a high finesse, symmetrically suspended cavity and a high power 813 nm light source for the optical lattice trap at the magic wavelength. Due to their compactness, reliability, and low power consumption, semiconductor laser sources represent the best choice for the development of compact and transportable devices for application both on Earth and in Space. A new Sr trapping and cooling experimental setup is also presented.