Phase-locked stimulated photon echo: Coherent control over Liouville-space pathways

We demonstrate coherent control over interference between different Liouville-space pathways that describe the generation of the nonlinear optical polarization in a stimulated photon echo. This control was exercised by phase-locking of the excitation pulses and observed in the heterodyne detected echo signal. A theoretical analysis of the effect is presented and found to be in good agreement with the obtained results. The relation between this novel time-domain interference effect and the pressure-induced extra resonance in four-wave mixing spectroscopy is discussed.