Doppler Narrowing and Collision-Induced Zeeman Coherence in Four-Wave Light Mixing
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 - ω3S,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.
KeywordsCentral Resonance Raman Resonance Coherent Model Zeeman Sublevel Collisional Narrowing
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