Theoretical study of the suppression of thermal background in the Raman-enhanced nondegenerate four-wave-mixing spectrum by a time-delayed method.

Abstract

We have studied theoretically the dependence of the Raman-enhanced nondegenerate four-wave-mixing (RENFWM) spectrum on the relative time delay between two pump beams-tau when a thermal effect exists in the sample. Using the chaotic model for the incident laser beams, we give the conditions that the thermal background can be eliminated completely when the relative time delay is much longer than the laser coherence time. In this limit, the theoretical RENFWM spectra show the normal asymmetry due to the interference between the Raman-resonant term and the nonresonant background originating solely from the molecular-reorientational grating. We then turn our attention to the dependence of the RENFWM signal intensity on the relative time delay when the laser frequencies are fixed. Our results indicate that RENFWM is asymmetric about tau = 0 and the maximum of the RENFWM signal does not occur at tau = 0. When the beating frequency between the pump beam and the probe beam is off resonant from the Raman mode, the RENFWM signal exhibits damping oscillation as the time delay increases. We also show that the temporal behavior of the RENFWM signal is drastically different if the laser sources are described by a phase-diffusion model. The different roles of the phase fluctuation and the amplitude fluctuation can be understood in the time domain.