Two types of parametric four-wave mixing are observed when a single intense laser beam propagates through Na vapor near the D(1) and D(2) resonance lines. The first is related to three-photon scattering, while the second results from the coherent parametric interaction of the laser with excited-state Raman scattering. Identification of the two processes was greatly facilitated by the use of an optical multichannel detector system to observe the total emission spectrum for each individual laser pulse. The experimental conditions that determine whether stimulated emission or one or both of the four-wave mixing processes dominates the emission spectrum are described.
Two-photon resonant parametric four-wave mixing and a newly developed variant called seeded parametric four-wave mixing are used to detect trace quantities of sodium in a flame. Both techniques are simple, requiring only a single laser to generate a signal beam at a different wavelength which propagates collinearly with the pump beam, allowing efficient signal recovery. A comparison of the two techniques reveals that seeded parametric four-wave mixing is more than two orders of magnitude more sensitive than parametric four-wave mixing, with an estimated detection sensitivity of $5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms/
cm3. Seeded parametric four-wave mixing is achieved by cascading two parametric four-wave mixing media such that one of the parametric fields generated in the first high-density medium is then used to ``seed'' the same four-wave mixing process in a second medium in order to increase the four-wave mixing gain. The behavior of this seeded parametric four-wave mixing is described using semiclassical perturbation theory. A simplified small-signal theory is found to model most of the data satisfactorily. However, an anomalous saturationlike behavior is observed in the large signal regime. The full perturbation treatment, which includes the competition between two different four-wave mixing processes coupled via the signal field, accounts for this apparently anomalous behavior.