H. J. Metcalf, P. Straten, Laser Cooling and Trapping
Beam deceleration with photon recoil
• Absorb a photo to receive momentum kick k
• Spontaneous emission is symmetric
• Random walk theory assumed 1
• Recoil limit k BTrec
• A two-photon process: a simultaneous absorption
and a stimulated emission by an atom
• Two counter-propagating beams
• Resonant excitation condition
L1 k1v (L 2 k2v) 12
2kv 12 (L1 L 2 )
• Linewidth determined by interaction time
• At 600nm, pulse duration 0.6ms v 1mm / s , equivalent to Trec / 900
J. Foot, Atomic Physics
Experimental demonstration by S. Chu et al.
• Using Sodium with hyperfine ground states
• Atoms were pre-cooled using beam
deceleration magneto-optical trap, and
3D polarization gradient molasses.
• Applied a sequence of Raman pulses with
controlled linewidth and line shape.
• Using Blackman pulse envelope to avoid
unwanted off-resonant excitation
• Achieve T=100nK, less than 1/10 recoil limit
M. Kasevich, S. Chu, Laser cooling below a photon recoil with 3-level atoms, Phys. Rev. Lett. 69, 12 (1992)
• In theory one can arbitrarily narrow the linewidth
• However it takes longer time for narrower linewidth, which limit the
Raman cooling in practice.