Degenerate four-wave-mixing line shapes of hydroxyl at high pump intensities

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Abstract
The degenerate four-wave-mixing spectral profile of the R 1(9) transition in the A 2∑+ → X 2Π(0, 0) band of OH has been measured for various combinations of saturating pump beams. With increasing pump-beam intensity the degenerate four-wave-mixing line shape changes dramatically near line center. In phase-conjugate geometry, three distinct spectral line shapes were observed for the cases of (1) equally intense pump beams, (2) a strong forward pump and a weak backward pump, and (3) a weak forward pump and an intense backward pump. A significant saturation dip appears in the spectrum near line center for case (3). The measured spectra have been modeled by the use of nonperturbative numerical solutions of the density matrix equations, and agreement between the calculations and the experimental results is excellent. The differences in the saturated line shapes for cases (2) and (3) are explored theoretically, and the calculated results are compared with previous theoretical work [Bloch and Ducloy, J. Opt. Soc. Am. 73, 635 (1985)] in which the line shapes were calculated in the limit of infinite Doppler broadening.