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Journal of the Optical Society of America B

Prospects of Doppler cooling on forbidden lines

Uwe Sterr, T. Binnewies, C. Degenhardt, G. Wilpers, J. Helmcke, F. Riehle

Published May 1, 2003• 3 citations
Publisher

Abstract

Doppler cooling on a forbidden transition is studied experimentally and numerically. By quenching the upper level of the cooling transition, the scattering rate is increased, and 106 40Ca atoms have been cooled and trapped in a magneto-optical trap to temperatures of down to 6 μK. A model is developed that describes the cooling method by rate equations. Based on the model, Monte Carlo simulations are performed that show good agreement with the experimental results. Possibilities of reaching high densities and low temperature by optimizing the parameters during the cooling phase are discussed, and the benefit of these ultracold atoms for the accuracy and stability of optical frequency standards is demonstrated.

DOI
10.1364/josab.20.000985

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Conference on Lasers and Electro-Optics•Aug 29, 2005•2 citations
Doppler cooling on a forbidden transition is studied experimentally and numerically. By quenching the upper level of the cooling transition, the scattering rate is increased, and 106 40Ca atoms have been cooled and trapped in a magneto-optical trap to temperatures of down to 6 μK. A model is developed that describes the cooling method by rate equations. Based on the model, Monte Carlo simulations are performed that show good agreement with the experimental results. Possibilities of reaching high densities and low temperature by optimizing the parameters during the cooling phase are discussed, and the benefit of these ultracold atoms for the accuracy and stability of optical frequency standards is demonstrated.
Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of (40)Ca atoms has been cooled and trapped to a temperature as low as 6 microK by operating a magnetooptical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method, more than 10% of precooled atoms from a standard magnetooptical trap have been transferred to the ultracold trap. Monte Carlo simulations of the cooling process are in good agreement with the experiments.
Current dual-frequency magneto-optical traps for ultracold molecules are plagued by sub-Doppler heating effects, making them vastly inferior to standard atomic MOTs. Here we demonstrate theoretically that the sub-Doppler effects in such a MOT can be engineered to provide cooling instead of heating. We give an intuitive picture how to achieve such cooling and show the cooling and trapping force results of the 16 level optical Bloch equations for the case of CaF molecules. From three-dimensional Monte Carlo simulations we estimate the temperature and density of our MOT to be 40μK40 μK40μK and 4×108cm−34 \times 10^8 cm^{-3}4×108cm−3 respectively for a molecule number of 10510^5105.
Sub-Doppler laser cooling in a magneto-optical trap for thulium atoms at a wavelength of 410.6 nm has been experimentally studied. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3 × 106 atoms at a temperature of 25(5) μK was observed. The measured temperature is significantly lower than the Doppler limit of 240 μK for the cooling transition at 410.6 nm. The high efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé g-factors of the lower 4f 136s 2 (J = 7/2) and the upper 4f 125d 3/26s 2 (J = 9/2) cooling levels.
Laser cooling of atoms is the basis of researches on ultracold atoms and molecules. It is of great importance in the study of precision measurement and quantum simulation, etc. In order to obtain ultracold cesium atomic sample with a lower temperature and a higher density, as the starting point to create ultracold cesium ground state molecules, we demonstrate in this article a method which effectively overcomes the heating from the reabsorption of scattered light in magneto-optical trap to further reduce the ultracold atomic temperature by loading atoms into a 3D optical lattices. Based on obtaining the ultracold Cs atoms in a standard magneto-optical trap, we use the compressed magneto-optical trap technology to increase ultracold Cs atomic density. Efficient loading of ultracold Cs atoms is achieved by 3D optical lattices which constructed by four lasers; the temperature of the atoms is measured by time-of-flight method. We observed the cooling result that the temperature drops from ~60.0 μK to ~11.6 μK. We also explore the dependence of the atom numbers in the lattice on the frequency of lattice lasers, and lead to the conclusion that the loading rate of the optical lattices reaches its maximum when the lattices laser’s detuning is –15.5 GHz. In the meantime, this paper investigates the use of optical lattice to suppress the heating of the ultracold atoms due to the radiation photons, in this sense, our research has far-reaching significance. Our scheme is advantageous due to its relaxed starting temperature requirement, meaning that 3D DRSC can be directly accomplished in a standard single cell vapour-loaded MOT without reliance on any additional devices such as a Zeeman slower.
We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10 6 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 × 10 4 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 × 10 7 atoms at the expense of higher temperature.
Summary form only given. All-optical production of quantum degenerate atomic gases has been one of the long-standing targets in laser cooling. We report a novel approach towards this direction based on a narrow-line laser cooling for strontium atoms. By employing two transitions with markedly different dipole moments, i.e., the allowed /sup 1/S/sub 0/-/sup 1/P/sub 1/ (/spl lambda/=461 nm, /spl gamma/=2/spl pi//spl times/32 MHz) and the spin-forbidden /sup 1/S/sub 0/-/sup 3/P/sub 1/ (/spl lambda/=689 nm, /spl gamma/=2/spl pi//spl times/7.6 kHz) transition, thermal strontium atoms were Doppler-cooled down to 400 nK or the photon recoil temperature, in a magneto-optical trap (MOT). Presents the corresponding energy levels for cooling and trapping. The use of a narrow spin-forbidden transition successfully reduced the radiation trapping effects at a high atom density over 10/sup 12//cm/sup 3/, thus enabling us to attain the hitherto high phase space density of 0.01 in a MOT.