Laser cooling

Abhishek Tomar

The first proposal to cool neutral atoms in counter-propagating laser beams was made by T.W. Hänsch and A.L. Schawlow in 1975.

Theodor Wolfgang Hänsch

The Nobel Prize in Physics was awarded in 1997 to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips, for the development of methods to cool and trap atoms with laser light.

Steven Chu

Claude Cohen-Tannoudji

William Daniel Phillips

ATOM TRAPPING

Magneto-optical trap

Doppler effect:The relativistic Doppler effect is the change in frequency (and wavelength) of light, caused by the relative motion of the source and the observer, when taking into account effects described by the special theory of relativity.

LASER COOLING

• A stationary atom sees the laser neither red- nor blue-shifted and does not absorb the photon.

• An atom moving away from the laser sees it red-shifted and does not absorb the photon.

• An atom moving towards the laser sees it blue-shifted and absorbs the photon, slowing the atom.

• The photon excites the atom, moving an electron to a higher quantum state.

• If an atom is moving towards any of the LASER, that particular beam would appear at a slightly higher frequency (Doppler effect).

• The laser light is tuned to a frequency with an energy just below the energy of an electronic transition of an atom to excite the atoms to higher state.

• The atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion.

• This small increase in frequency makes the frequency of LASER exactly equal to the needed one for atomic excitation. The photons are thus absorbed and atom excites.

• The atom recoils back due to the momentum provided by the photons and re-emits a new photon in random direction coming back to its ground state.

• Thus the atom slows down and thus the kinetic energy of the system and thus, the temperature goes down.

KE = 3/2 KT = ½ mv*v So, v α √T• This makes the temperature down to a few milli-Kelvins.

LIMITATIONSMINIMUM TEMPERATURE- The atom performs a random walk in momentum space with steps equal to the photon momentum due to spontaneous emission and photon absorption. This constitutes a heating effect, which counteracts the cooling process and imposes a limit on the amount by which the atom can be cooledMAXIMUM CONCENTRATION- The concentration must be minimal to prevent the absorption of the photons into the gas in the form of heat. This absorption happens when two atoms collide which each other while one of them has an excited electron. There is then a possibility of the excited electron dropping back to the ground state with its extra energy liberated in additionnal kinetic energy to the colliding atoms — which heats the atoms.

ATOMIC STRUCTURE- Only certain atoms and ions have optical transitions susceptible to laser cooling, since it is extremely difficult to generate the amounts of laser power needed at wavelengths much shorter than 300 nm.The more hyperfine structure an atom has, the more ways there are for it to emit a photon from the upper state and not return to its original state, removing it from the cooling process. 

applications

construction of atomic clocks atomic interferometers and atom lasers in the development of instruments for atom optics and atomic

lithography. observation of a Bose-Einstein condensation in a dilute atomic gas

References

http://www.chemeurope.com/en/encyclopedia/Laser_cooling.html http://www.nobelprize.org/nobel_prizes/physics/laureates/1997/

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