High harmonic generation in mixed XUV and NIR fields at a free-electron laser

Abstract We present the results of an experiment investigating generation high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in presence extreme ultraviolet (XUV) field provided free-electron (FEL), process referred to as XUV-assisted harmonic (HHG). Our experimental findings show that XUV can lead small enhancement yield when and NIR pulses overlap time, while strong decrease HHG red shift spectrum is observed precedes pulse. The latter observations are qualitative agreement with model calculations consider effect decreased number neutral emitters but at odds predicted correspondingly increased ionization fraction on phase matching. study demonstrates technical feasibility experiments FELs, which may provide new avenues investigate correlation-driven electron dynamics well novel ways control propagation effects matching HHG.