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Physical Review A

Enhancement of degenerate four-wave mixing by atom-wall collisions in atomic vapors

B. Ai, David S. Glassner, R. J. Knize

Published Oct 1, 1994• 21 citations
Publisher

Abstract

Resonant cw degenerate four-wave mixing signals usually are not observed at noncycling transitions in alkali-metal vapors due to population depletion from optical pumping. We demonstrate that frequent atom-wall collisions can overcome the effects of optical pumping when atomic vapors are contained in thin cells. Significantly enhanced degenerate four-wave mixing signals are observed at noncycling transitions, and they are comparable in magnitude to the signals observed at cycling transitions in thin cells. This technique extends the utilization of degenerate four-wave mixing to noncycling transitions in atoms or molecules.

DOI
10.1103/physreva.50.3345

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Physical Review A•Oct 1, 1998•11 citations
Resonant cw degenerate four-wave mixing signals usually are not observed at noncycling transitions in alkali-metal vapors due to population depletion from optical pumping. We demonstrate that frequent atom-wall collisions can overcome the effects of optical pumping when atomic vapors are contained in thin cells. Significantly enhanced degenerate four-wave mixing signals are observed at noncycling transitions, and they are comparable in magnitude to the signals observed at cycling transitions in thin cells. This technique extends the utilization of degenerate four-wave mixing to noncycling transitions in atoms or molecules.
Results of experiments investigating the effects of optical pumping and atomic motion on cw degenerate four-wave mixing in atomic sodium vapor are presented. The signal strength is found to decrease as (sin θ)−2, where θ is the angle between the object and pump waves; however, the linewidth for generation is Doppler free and is independent of θ. Agreement is found with a simple model of the mixing process. Distortions of the atomic velocity distribution (produced in our experiments by optical pumping) are found to be essential for efficient cw generation. A new modulation scheme for the discrimination against background is used.
We have observed a new component in the signals produced by degenerate four-wave mixing of 5-nsec optical pulses by the DDD-line resonances of sodium vapor. This component is comparable in intensity to that previously observed from a grating pattern of excited sodium atoms, but arises rather from a grating of coherently aligned (but not optically pumped) ground-state atoms. This component exists when there are no spatial intensity beats between input waves.
We present experimental observations of phase-conjugate degenerate four-wave mixing in a two-level atomic system (the ${3s}^{2}{S}_{1/2}, F=2, {m}_{F}=\stackrel{\ensuremath{\rightarrow}}{2}{3p}^{2}{P}_{3/2}, F=3, {m}_{F}=3$ transition in a diffuse, collision-free, thermal beam of atomic sodium) driven by cw, narrow-band $(\ensuremath{\sim}250 \mathrm{kHz}),$ stabilized pump and probe fields. The primary measurements that we report are of the peak signal strength of the four-wave-mixing interaction and the bandwidth of the four-wave-mixing spectrum (as the laser frequency is tuned through the resonant frequency) as a function of the intensity of the pump field. We compare these measurements with the predictions of a theoretical analysis in which extension to include the effect of the small transverse velocity $(\ensuremath{\sim}3 \mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c})$ of the atomic beam is critical. These measurements provide a direct verification of the theory of this important nonlinear interaction.