Saturation resonances by magnetic mode crossing in optical pumping with a multimode gas laser

Abstract

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