First, some historical comments on the Hanle effect and Kastler-type optical pumping are made. Next the recent history of the phenomenon of collision-induced coherences in optical four-wave light mixing experiments is briefly reviewed New experimental results on collision-enhanced Hanle-type resonances in the 3 2S1/2 ground state configuration of Na atoms in a buffer gas at high pressure are presented. The relationship between these optical fourwave experiments and Kastler-type optical pumping is pointed out.
Collision-enhanced Hanle resonances in the 3
2S1/2 electronic ground state of Na atoms have been studied in a phase-conjugate four-wave light-mixing geometry. Narrow resonances in zero magnetic field have been observed at very high inert-buffer-gas pressures, with a width of 21 mG, or less than 10 kHz, full width at half-maximum. The intensity and the width of these resonances has been studied as a function of buffer-gas pressure, of alkali-metal vapor pressure, of detuning from the
D1 and
D2 Na resonances, and of the intensity of the incident light beams. Saturation phenomena and power broadening are observed. The results are explained theoretically in terms of transversely pumped optical gratings. The equivalence of this description with collision-induced coherence gratings in the limit of low light intensities is demonstrated.
The fluorescence light emitted by atoms (Ne) optically pumped with a multimode laser beam exhibits a resonance each time the Zeeman splitting is equal to the frequency difference between two modes. The experimental study shows that these resonances are very sensitive to the relative phase of modes and that their width is approximately equal to the Hanle effect width. A theoretical interpretation is given with the help of a perturbation calculation. Two phenomena produce resonances : 1) A Population Effect (PE) which is resonant when the σ + component of one mode and the σ- component of another interact with atoms of same velocity (hole crossing). The width of these resonances is 2 Γ'ab (natural width broadened by collisions). 2) A Zeeman Coherence Effect (ZCE) : the action of a pair of modes (ν, σ +) and (μ, σ-) produces (at 2nd order) transverse alignment modulated at the beat frequency ων — ω μ and resonant when the Zeeman splitting is equal to that frequency ; by interaction with a second pair of modes (λ, σ-) and (κ, σ+) (ων —ω μ + ωλ — ωκ = 0), this alignment produces unmodulated longitudinal quantities (4th order) which exhibit, as the magnetic field is scanned, a resonance whose width is close to that of the Hanle curve (Γ b(2)). In our experimental conditions (close modes : Γ'ab≳ Δω > Γ b(2)), the resonances of the first kind are not resolved and the observed resonances are only of the second type's which arevery sensitive to the phases of modes. We also discuss the possibility of using these resonances to measure Landé g factors.