I wake up scheming


Mine ice have seen the glory

© D Sharon Pruitt

Anybody who has ever made a wish on the first snowflake of a childhood winter has probably dreamed of someday being able to freeze their enemies using a high-powered laser.  And that day is now a little closer, thanks to some arresting new research from the laboratory of Mansoor Sheik-Bahae of the University of New Mexico (published in the January 2010 issue of the Journal of Nature-Photonics).

Sheik-Bahae’s team has figured out how to cool a crystal down to 155 Kelvin (-180°F), which is more than 50 Kelvin colder than a solid has ever been cooled with a laser.  And it’s actually the coldest that any solid-state device has been able to get; thermoelectric cooling generally quits around 170 Kelvin.

What’s their secret?  Anti-Stokes fluorescence.  Most fluorescent materials demonstrate Stokes fluorescence, absorbing radiation at one wavelength and emitting radiation with a longer wavelength and less energy.  In anti-Stokes fluorescence, a material absorbs radiation and re-emits it at a shorter wavelength with higher energy, drawing the extra energy from the heat of the system.  If you keep exciting a material like this, its heat gets turned into light, and the material gets colder.  Then, once the material is frozen (assuming it is the door on a bank vault), you can crack through it with a hammer and make off with millions of dollars in canvas bags with money signs on them.

The problem, though, is that there aren’t too many materials that display anti-stokes fluorescence.  Ytterbium-doped yttrium lithium fluoride, the crystal used by Sheik-Bahae’s lab, is one of them.  That’s about it.  So it would be almost impossible to use laser cooling on anything practical, like a team of mystery-solving teenagers and their pesky mutt.  Unless, of course, the pesky mutt was made of ytterbium-doped yttrium lithium fluoride crystals.  But alas, I can only dream.

Sheik-Bahae can dream too: he believes that with more careful crystal preparation, purer laser light, and a little elbow grease, he can reach temperatures as low as 10K.  And I’d like to trust him.  After all, every step counts when you’re trying to develop a freeze ray.

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