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Unexpectedly Low Temperatures in an Erbium MOT

One of our research goals is to integrate the Magneto-Optical Trap-based Ion Source (MOTIS) with our erbium Magneto-Optical Trap (MOT) for the development of nanoscale optical devices based on single (or few) rare-earth ions. While pursuing this goal, we discovered that the magneto-optical properties characterizing the rare-earth elements are not only important in the solid state – atomic erbium also has unique properties in free space, making it a potentially important element for applications using cold atoms. By investigating the temperature in our MOT, we found that erbium has useful (and serendipitous) laser-cooling properties allowing us to bring atoms to temperatures as low as 25 µK through sub-Doppler laser cooling and transfer to a magnetic trap.

In atoms with simple (two-level) energy structure, the lowest temperature ordinarily reached by laser cooling is the Doppler temperature, TD=hbarΓ/2kB, which depends only on the laser-cooling transition linewidth Γ. However, atoms with more complicated energy level structure, including erbium, can be cooled to even lower temperatures by "sub-Doppler" processes. For the broad 401 nm transition in erbium, the Doppler temperature is 854 µK, but we observe temperatures as low as 100 µK in our MOT (Fig. 1), a clear indication of sub-Doppler cooling.

Fig 1
Figure 1. Measured MOT temperatures as a function of experimental parameters.


Because erbium has such a large magnetic moment, many atoms remain magnetically trapped even after our MOT trapping lasers are shut off (Fig. 2). After relaxing into this magnetic trap, atoms reach even lower temperatures than in the MOT.

Fig 2
Figure 2. Fluorescence image of magnetically trapped atoms. By fitting the distribution along the x and z directions (bottom and left, respectively) we can determine the temperature.

Based on simple thermodynamic arguments, one would expect these magnetically trapped atoms to have a temperature of at least 1/3 of the initial MOT temperature. However, we find that atoms are even colder than this expectation, as shown in Fig. 3.

Fig 3
Figure 3. MOT temperature TM versus magnetic trap temperature TB. Simple thermodynamic arguments predict that all of the points should lie above the dashed line where TB = TM/3.

These anomalously low temperatures are most likely a result of energy selection during transfer from the MOT to the magnetic trap. If higher energy atoms are preferentially ejected during loading, the remaining atoms will have a lower-than-expected temperature. While the details of the transfer between the traps are not fully understood, the observed temperatures as low as 25 µK are low enough to provide an entryway into the realm of ultracold cooling and its associated wide range of possibilities for applications and scientific discoveries.


Related Publication Listing
Sub-Doppler Laser Cooling and Magnetic Trapping of Erbium

Staff listing
Andrew Berglund - NIST
Jabez J. McClelland - NIST

Former staff listing
Siu Au Lee - Colorado State University


Online: February 2008
Last Updated: February 2008

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