Abstract

Degenerate four-wave mixing (DFWM) has emerged in recent years as a potentially powerful technique for quantitative measurement of minor species in gas-phase media.^1,2 However, a complicated collision-rate dependence of the DFWM signal is observed when lasers with pulse lengths of several nanoseconds are used.^3,5 On the other hand, picosecond lasers possess pulse widths shorter than the typical collisional time in atmospheric gas-phase media, and use of such lasers for DFWM measurements⁶ could reduce the sensitivity of the technique to the collision rate.^2,7 While pulsed lasers are usually utilized to perform gas-phase DFWM measurements, DFWM analytical models are generally limited to the assumption of continuous wave (cw) laser inputs. These cw models have been applied successfully to modeling gas-phase DFWM with nanosecond-pulse lasers, where the pulse length (~10 nsec) is much longer than the typical collisional time (~100 psec) in atmospheric-pressure flames. In this paper, we use direct numerical integration (DNI) of the density matrix to investigate theoretically the use of lasers with pulse lengths (τ_L) much shorter than characteristic collision times (τ_C) for quantitative DFWM spectroscopy in gas-phase media. Both purely homogeneously broadened resonances and resonances that are both collision- and Doppler-broadened are considered.

© 1996 Optical Society of America