Enhancement of third-harmonic generation by Stark-chirped rapid adiabatic passage

We report enhancement of third-harmonic generation in a nonlinear optical medium, prepared in maximum coherence by Stark-chirped rapid adiabatic passage (SCRAP). In this technique a strong off-resonant infrared radiation pulse induces an adiabatic passage process by sweeping the transition frequency of a two-photon transition in Krypton atoms through resonance with a near-resonant pump radiation pulse at 213 nm, appropriately delayed with respect to the infrared radiation. During the adiabatic passage process, a transient two-photon maximum coherence is induced, which beats with the pump laser frequency. Detrimental effects of Doppler broadening and collisional dephasing in the medium are reduced during the SCRAP process. The efficiency of third-harmonic generation, yielding vacuum-ultraviolet radiation at 71 nm is enhanced by more than one order of magnitude with respect to the case of conventional frequency conversion in an unprepared medium.