ac-Stark-effect-enhanced four-wave mixing in a near-resonant four-level system

We report on an ac-Stark-effect-enhanced wave-mixing experiment in ${\mathrm{I}}_2$ vapor in which an output uv signal at frequency 2${\omega }_1$+${\omega }_2$ is maximized when the input frequencies ${\omega }_1$ and ${\omega }_2$ are both detuned in the same direction from the resonance frequencies of the free molecule. A theory based on a configuration interaction between the ‘‘dressed molecular levels’’ is presented to explain the strong enhancement of the uv signal under these conditions. The data presented demonstrate that tunable coherent radiation at ultraviolet and vacuum-ultraviolet frequencies can be generated by the wave-mixing process we describe.