Parametric amplification and harmonic generation in the VUV and XUV

To overcome limitations in the high harmonic conversion efficiency, the possibility of parametric amplification in the VUV and XUV is considered. Potential approaches, including parametric gain transfer from low- to high-order processes, are discussed. Experimentally, a near two photon resonant parametric gain scheme in xenon, excited by femtosecond KrF laser radiation at ωp=248 nm, was investigated, leading to signal (ωs) and idler (ωi) emissions at 149 nm and 760 nm according to 2ωp=ωs+ωi. For the signal field at 149 nm an exponential increase with density and length is obtained. By tuning the excimer laser radiation slightly towards the two-photon resonance, a macroscopic amplification of about 50 at 149 nm was measured. In addition, a sum-mixing signal at 106 nm according to ωsum=2ωp+ωi, showing a similar exponential increase as the 149-nm signal, is observed, which indicates a coupling of both processes with a gain transfer by the common idler field.