Four-wave mixing with a new phase-matching geometry

Henk F. Arnoldus, Thomas F. George

Thomas A. Reichardt, Robert P. Lucht

Degenerate four-wave mixing (DFWM) spectroscopy is a potentially powerful technique for quantitative measurement of minor species in gas-phase media [1-3]. In this paper we compare the DFWM signal levels and lineshapes obtained using two different phase-matching geometries: the phase-conjugate geometry and the forward phase-matched geometry. DFWM signal levels are calculated by solving the time-dependent density matrix equations using direct numerical integration (DNI) [4, 5] for a two-level system interacting with three laser beams. This numerical analysis includes the effects of molecular motion, intermolecular collisions, and high laser intensities, all of which have been shown to affect the DFWM signal level.

Y. S. Kuo, Y. Fong, C. Wu +2 more

Brillouin enhanced four-wave mixing (BEFWM) is a judicious combination of SBS and FWM processes. In the traditional BEFWM experiments, two cells are used: one generates SBS, the other provides four-wave mixing. We demonstrated the generation of a phase conjugate signal by single cell BEFWM with focusing geometry. A single cell containing the optical nonlinear medium was used simultaneously for conventional SBS and FWM. Experiments were performed by using either CS2 or TiCl4 as the nonlinear medium; both have large Brillouin gain but differ greatly in their nonlinear indices of refraction. Thus the mechanism for four-wave mixing is shown to be Brillouin nonlinearity. In this experiment, the phase matching condition was satisfied and the frequency of the phase conjugate signal was not shifted from that of the master input beam. The operating regime of the experiment was far below the threshold of instability, which accounts for the low reflectivity that we observed. This single cell configuration is more compact and the optical alignment is easier than in the conventional BEFWM, in which two nonlinear cells are used.

Yanpeng Zhang, Zhiqiang Nie, Min Xiao

Thomas A. Reichardt, Robert P. Lucht, Paul M. Danehy +1 more

We examine theoretically the degenerate four-wave mixing (DFWM) signal intensities and line shapes obtained with the forward phase-matched geometry in which all beams propagate in the same direction and compare the results to those of the phase-conjugate geometry with counterpropagating pump beams. To examine the forward phase-matched geometry, we modify a theoretical approach used previously to calculate phase-conjugate DFWM signal intensities. This theoretical approach, which involves 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 experimental data. This methodology is then used to compare the signal intensities and line shapes obtained with the forward phase-matched geometry and the phase-conjugate geometry in the perturbative (low laser power) and saturated (high laser power) regimes. In the perturbative regime the forward phase-matched signal exhibits less sensitivity to the Doppler linewidth. At pump laser intensities approximately equal to the saturation intensity the signal for the forward phase-matched geometry is stronger than that for the phase-conjugate geometry for primarily Doppler-broadened resonances, assuming the same probe volume for both geometries. These advantages warrant further investigations employing the forward phase-matched configuration for DFWM measurements of gas-phase species.

W. J. Firth