INTERACTION OF CLOSELY SPACED RESONANCES IN DEGENERATE 4-WAVE-MIXING SPECTROSCOPY

The interaction of two closely spaced resonances in degenerate four-wa ve-mixing spectra is investigated theoretically. Degenerate four-wave- mixing spectra for the phase-conjugate geometry (counterpropagating pu mp beams) are calculated by integrating the time-dependent density-mat rix equations at numerous grid points along the phase-matching axis an d summing the polarization contribution from each of these grid points . Both purely homogeneously broadened resonances and resonances that a re both collision and Doppler broadened are considered. The homogeneou sly broadened results are found to agree with the results of the widel y used Abrams-Lind expression for the case of stationary absorbers exp osed to a steady-state excitation. We examine the effects of power bro adening, pressure broadening, transition dipole moment, and population on the spectra of two neighboring homogeneously broadened resonances. As the laser power is increased, the real parts of the resonance susc eptibilities destructively interfere with each other in the region bet ween the lines, resulting in a significant and persistent dip in refle ctivity. When Doppler broadening is included in the calculations, simi lar interference effects are observed, although at first the interfere nce dip begins to disappear until the homogenous part of the line shap es is broadened significantly by saturation. Using this numerical appr oach, we obtain good agreement with experimental results of primarily Doppler-broadened neighbouring resonances. (C) 1997 Optical Society of America.