Laser-Cooled Atoms: Strontium

Element: Strontium (Sr)

Atomic Number: 38

Mass: Four stable isotopes, ranging from 84 to 88 amu

Laser cooling wavelength: Two different transitions are used in the laser cooling of strontium: a blue line at 461 nm that’s an ordinary sort of transition, and an exceptionally narrow “intercombination” line at 689 nm.

Doppler cooling limit: 770 μK for the blue transition, below a microkelvin for the red. The Doppler limit for the red line turns out not to be all that relevant, as other factors significantly alter the cooling process.

Chemical classification: Alkaline earth, column II of the periodic table. Another greyish metal, but unlike the alkalis, it has not one but two electrons in its outermost shell. This is what leads to the “intercombination” line business above– in two-electron atoms, the atomic states get sorted into two classes based on the relative spins of the two electrons, and transitions between states of different character are strongly suppressed. The red line used for cooling strontium is one such transition, which accounts for the ultra-low Doppler temperature.

Other properties of interest: Unlike most other laser-cooled species, strontium atoms have only a single ground state, which means that Sisyphus cooling doesn’t work, and you’re genuinely limited to the Doppler cooling limit. Which is why the red line is important– a MOT on the blue line produces a cloud of atoms with a temperature around a millikelvin, which isn’t that useful.

History: Laser cooling of strontium started surprisingly early (well, surprisingly to me, anyway), in 1990, in Japan. I’m not quite sure why they initially decided to pursue that in particular, unless they just happened to have a blue laser they were itching to put to use. It has since become moderately popular– it’s no rubidium, mind, but a surprisingly large number of groups are working with it– in large part because of the ultra-low temperatures you can reach with the red line. The arxiv includes a very nice review from an Italian group in 2006.

Interest in strontium really took off in the later part of the 1990’s, when Hidetoshi Katori’s group in Japan came up with the “magic wavelength” idea (at least, I never heard of it before I visited their lab in 1998), which allows efficient loading into an optical trap by using a very particular wavelength that produces an identical light shift in both ground and excited states. This allows you to continue to laser-cool the atoms with the red line while loading them into the dipole trap. That, in turn, lets you create a sample that’s tantalizingly close to BEC purely with laser cooling.

Of course, the word “tantalizingly” is doing a lot of work, there: the density and temeprature are almost what you’d want for BEC, but you can’t quite get there by the normal methods. It turns out that the collisional properties of strontium give the abundant isotopes a very low “scattering length,” which means it takes an exceptionally long time to re-thermalize, and mucks up the evaporative cooling process. Of course, physicists love a challenge, so two different groups nearly simultaneously found a way to Bose condense it anyway, using a rarer isotope with better collisional properties, and sympathetic cooling with a fermionic isotope, which was also cooled to a degenerate Fermi gas, and a mixture of Bose and Fermi gases, both in the quantum degenerate state was made arounf the same time. The idea of directly laser cooling the gas to BEC remained an intriguing possibility, though, and a group in Austria finally managed it earlier this year (the popular article at that link is the source of the middle image at the top of this post).

Random fun things: Strontium is the element responsible for both this “trading card” series of posts and the “cold atom tools” series that preceded it– between the direct laser cooling to BEC paper back in January and the more recent absurdly precise atomic clock (and a quantum simulator in such a clock to boot), I’ve had a bunch of strontium-related papers open in tabs for possible ResearchBlogging write-ups. They didn’t get written, though, because the amount of background required just seemed too daunting. With these series as reference, though, I’ll got back to them and finally do the write-ups. When I have time.

Art: The cartoon version of strontium is a deranged Scotsman waving fireworks, and the Comic Book Periodic Table includes cover illustrations for both “Strontium Dog” and “Strontium Bitch.” Thus satisfying both genders of canid. Or something. I don’t know, comics are weird.