  We report the realization and characterization of parametric amplification in
high harmonic generation around 100 eV using He gas in a double gas jet
arrangement. The delay of the seed XUV pulse with respect to the amplifier gas
jet was scanned by changing the distance between the gas jets. Experiments and
numerical calculations show that parametric amplification occurs within a
temporal window of several optical cycles. Strong correlation between the seed
and amplifier was observed in a shorter, few optical cycles delay range, which
appeared as a modulation of the XUV intensity with an unexpected one optical
cycle periodicity instead of half optical cycle. Simulations revealed that the
strong correlation and also the unusual periodicity was the consequence of the
nonlinear effect produced by plasma dispersion on the parametric amplification
process during propagation in the amplifier jet.
