Philip H. Jones, Onofrio M. Maragò & Giovanni Volpe
Fig. 24.3 — Sub-Doppler cooling
The combination of multi-level atoms with the polarisation gradients occurring in a standing wave is responsible for sub-Doppler cooling mechanisms. (a) Squares of the Clebsch–Gordan coefficients and polarisations necessary for an ideal |Jg = 1/2⟩ → |Jg = 3/2⟩ transition. (b) Polarisation gradients in a one-dimensional lin⊥lin standing wave. The polarisation changes from linear (+45◦) to σ− to linear (−45◦) to σ+ and back to linear (+45◦) in half the laser wavelength.