XUV-Assisted High-Order Harmonic Generation*

We report on the high-order harmonics generation (HHG) by a near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field. The experiments were performed using the free-electron laser (FEL) in Hamburg (FLASH), Germany. Previous theoretical studies predict an enhancement of the HHG yield when XUV and NIR fields are combined [1]. In our experiments, we were able to observe such an enhancement, albeit very small, in the HHG yield of argon and krypton at different XUV wavelengths. Furthermore, we observe a strong decrease in the HHG yield when the XUV pulse precedes the NIR. [1] C. Buth, M.C. Kohler, J. Ullrich, C.H. Keitel, Opt. Lett. \textbf{36}, 3530--3532 (2011)