Doppler Narrowing and Collision-Induced Zeeman Coherence in Four-Wave Light Mixing

Abstract

Consider a three-dimensional four-wave light mixing geometry as schematically shown in Fig. 1. The four beams all travel in the near-forward direction. Two beams have a frequency ω₁, which is offset by a fixed amount Δ = ω₁ - ω_3S,3P from a resonant line of the Na atom. Their wave vectors, k ₁ and k ₁′, respectively, lie in the vertical plane. A third beam, at frequency ω₂, has a wave vector k ₂ in the horizontal plane. In four-wave mixing, the generation of a new beam in the horizontal plane is observed with wave vector k ₁ + k ₁ ′ - k ₂ and frequency 2ω₁ - ω₂. As the frequency ω₂ is varied over an interval of about 40 cm⁻¹ around the value of ω₁, seven resonances in the intensity of the new beam, J(2ω₁ - ω₂), have been observed by Prior et al.¹ in mixtures of Na vapor and helium. These are schematically indicated in Fig. 2. Four of these are familiar one-photon resonant enhanced four-wave mixing processes. The remaining three resonances, at ω₂ = ω₁, and at ω₂ = ω₁ ± 17 cm⁻¹, are only observable in the presence of collisions, which can be controlled by the pressure of helium buffer gas.