Two-Step degenerate four-wave mixing as a means to decrease pre- and post-filtering effects in optically thick media

The use of crossed beam 2-Step Degenerate Four-Wave Mixing (2S-DFWM) for decreasing pre- and post-filtering effects under optically thick conditions has been investigated. 2S-DFWM is a technique in which the DFWM process is performed between two excited states of which the lower one is populated by an ordinary laser excitation from a low lying, highly populated state. Experiments were performed on Au in an acetylene/air flame. We have shown that under conditions where ordinary (one-step) DFWM experiments are significantly affected by pre- and post-filtering effects (i.e., partly absorption of the pump, probe or signal beams prior to or after the interaction region) the 2S-DFWM technique can give virtually interference free signals. A variety of different laser beam and flame configurations have been investigated. It was found that the use of a crossed beam geometry, where the first step exciting laser is incident upon the interaction region perpendicular to the DFWM beams, completely eliminated severe pre- and post-filtering effects occurring for an ordinary single-step DFWM scheme in an optically thick medium.