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Many of the most impressive demonstrations of the efficient generation of vacuum ultraviolet (VUV) light have made use of two‐photon resonantly enhanced four‐wave mixing to generate light at ωVUV =2ωL1±ωL2. The two‐photon resonance state is coupled to the ground state both by two photons from the first laser, or by a photon from the second laser and one from the generated VUV beam. We show here that these two coherent pathways destructively interfere once the second laser is made sufficiently intense, thereby leading to an important limiting effect on the achievable conversion efficiency.

On the basis of combined experimental and theoretical studies of nonlinear processes associated with two‐photon excitations near 3d and 4d states in Na, we show how resonantly enhanced stimulated hyper‐Raman emission, parametric four‐wave mixing processes and total resonant two‐photon absorption can become severely suppressed through the actions of internally generated fields on the total atomic response in extended media.