Strontium in Nice

Multiple Scattering of Light with Laser Cooled Strontium
We began construction of a new experiment with laser cooled strontium in the summer of 1999. The experiment is intended to compliment and extend the research being conducted with trapped rubidium. While our cold rubidium experiments have demonstrated the coherent backscattering (CBS) cone, a hallmark of multiple scattering, the results showed significant differences from those observed using other scattering media (particle suspensions, etc.). Specifically, there seems to be a reduction in the strength of the CBS cone for certain polarization channels of the probe light. The hyperfine structure of Rb is suspected to play an important role in this reduction and also complicates a detailed analysis of the process.

Strontium's lack of hyperfine structure contrasts the complexity of rubidium for studying effects of internal atomic structure in multiple scattering phenomena and characterizing relevant features. In addition to a strong (32 MHz) blue atomic transition at 461 nm, strontium offers a narrow (7 kHz) red intercombination transition from the same  J = 0 ground state. The scattering rate of the red transition is just strong enough to support the atoms against gravity in a magneto-optic trap (MOT). We will form a MOT from each transition.

The blue transition, with a scattering rate larger than those used for trapping the alkali elements, is suitable for initial cooling and trapping of hot atoms from an atomic beam or vapor. This Doppler cooled MOT can provide 108 atoms at mK temperatures. The weakness of the red transition, however, gives it a very small capture velocity and demands a cold initial sample for trapping. Therefore we will follow the example of the first two groups who trapped strontium by transferring the mK sample trapped by the blue MOT to a microK red MOT.

There are many interesting and unusual characteristics of this red MOT. The first is that the 7 kHz linewidth is comparable to the Doppler shift from one photon recoil. This is an unusual scattering regime where one scattering event can shift the atom out of resonance. Cooling on this transition has been predicted, and experimentally shown, to reach a few hundred nano-Kelvin: lower than typical temperatures attained by polarization gradient cooling of other elements. This regime could also greatly reduce multiple scattering within the MOT, thus mitigating a major limitation to obtaining large atomic cloud density (radiation trapping). This makes multiple scatting experiments in the red MOT significant for the attainment dense, cold atomic clouds and possibly to an all-optical Bose-Einstein condensate. Beyond studying it's relevance to light scattering, we hope that the low temperatures achievable with this transition will provide sufficiently coherent matter waves for observing coherent multiple scattering of atoms off of a random light potential.


People involved in this project

Permanent staff:
David Wilkowski (project leader)
Robin Kaiser
Christian Miniatura

Students:
Yannick Bidel (PhD)
Thierry Chanelière (PhD)
Gian Luca Gattobigio (Laurea)

Postdocs:
Bruce Klappauf (left to Southampton)
Strontium project
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