Scattering May Extend Laser Cooling
Daniel S. Burgess
STANFORD, Calif. -- Laser cooling, also known as optical molasses, has found numerous scientific applications since its development 15 years ago, including the production of Bose-Einstein condensates, Fermi degenerate gas and more precise atomic clocks, but its use is limited to particular types of atoms. Now researchers at Stanford University, including Steven Chu, who shared the 1997 Nobel Prize in physics for his contributions in developing the technique, are investigating a more comprehensive method of cooling matter.
The new technique, cooling by coherent scattering, may enable scientists to investigate the low-temperature behavior of atoms, ions and molecules that cannot be cooled with optical molasses.
In laser cooling, laser beams are red-detuned relative to the transition between an atomic target's ground and excited states. An atom moving toward a beam will find the photons blue-shifted into resonance and be able to absorb a photon, while the beams propagating in the same direction will be Doppler-shifted even farther out of resonance.
The atom will re-emit the photon in a random direction and return to its ground state, from which it can absorb another photon. Since each absorption event carries with it the demands of the conservation of momentum of the two-particle system, the net effect is a cooling to within microkelvins of absolute zero.
Not all molecules chill
But the principle behind laser cooling is also its limitation, explained Vladan Vuleti´c, co-author with Chu of a report in the April 24 issue of Physical Review Letters describing cooling by coherent scattering. Atoms may have multiple ground states to which the excited state can decay; when the atoms re-emit their absorbed photons, the lasers may be out of tune and be unable to cool them further. Similarly, molecules cannot be cooled with the technique because they have many vibrational and rotational levels, and the frequency of the lasers cannot be suitable for all of them.
In coherent scattering, however, a beam will be red-detuned relative to an optical resonator and coupled into it, and scattering events will cool the sample inside. Since this mechanism depends on the finesse of the cavity and the tuning of the photons to it -- not to atomic transitions -- it should be able to cool any sort of material. "The detuning relative to the atomic or molecular resonances is essentially arbitrary," said Vuleti´c. "The only requirement is that, at the given laser intensity and detuning, the photon scattering rate is large enough to produce efficient cooling."
He said that the researchers' first plan is to cool cesium atoms as a proof of principle. Subsequent experiments will include the cooling of atoms resistant to optical molasses as well as the cooling of molecules such as I2 and C60. He expects that the first demonstration of cooling by coherent scattering of an atom will be performed within months and that the first cooling of a molecule will come within a year to 18 months.
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