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 |J_g = 1/2⟩ → |J_g = 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.