Interference between excitation routes in resonant sum-frequency mixing

We present theory and observation to show that in resonant sum-frequency mixing the two pathways to the excitation of the coherence driving the output may be controlled in relative strength, and combine in antiphase on resonance. These two pathways are, first, a two-step route resulting in the creation of a real intermediate population, and second, a direct two-photon excitation resonantly enhanced by, but not populating, the intermediate level. The former route is shown to be raised over the latter by use of a stronger lower laser field. Experimental observation uses the 3S-3${\mathit{P}}_{1/2}$-3D level scheme in sodium vapor and single-frequency continuous-wave lasers for steady-state conditions and high resolution. A cancellation ratio of 94% is reported. Further, noninterfering incoherent fluorescence and an anti-Stokes Raman process are observed and contrasted.