THEORETICAL INVESTIGATION OF THE FORWARD PHASE-MATCHED GEOMETRY FOR DEGENERATE 4-WAVE-MIXING SPECTROSCOPY

We examine theoretically the degenerate four-wave mixing (DFWM) signal intensities and line shapes obtained with the forward phase-matched g eometry in which all beams propagate in the same direction and compare the results to those of the phase-conjugate geometry with counterprop agating pump beams. To examine the forward phase-matched geometry, we modify a theoretical approach used previously to calculate phase-conju gate DFWM signal intensities. This theoretical approach, which involve s numerical integration of the time-dependent density-matrix equations , is validated for the forward phase-matched geometry by comparison of our calculated line shapes to both a perturbative solution and to exp erimental data. This methodology is then used to compare the signal in tensities and line shapes obtained with the forward phase-matched geom etry and the phase-conjugate geometry in the perturbative (low laser p ower) and saturated (high laser power) regimes. In the perturbative re gime the forward phase-matched signal exhibits less sensitivity to the Doppler linewidth. At pump laser intensities approximately equal to t he saturation intensity the signal for the forward phase-matched geome try is stronger than that for the phase-conjugate geometry for primari ly Doppler-broadened resonances, assuming the same probe volume for bo th geometries. These advantages warrant fur ther investigations employ ing the forward phase-matched configuration for DFWM measurements of g asphase species. (C) 1998 Optical Society of America [S0740-3224(98)01 819-4].