title: Transition to the relativistic regime in high order harmonic generation.
abstract: Experiments and computer simulations on the generation of high order harmonics from steep plasma gradients using intense femtosecond laser pulses are presented. Qualitative changes in the harmonic emission take place when the intensities are increased above 10(19) W/cm2 and/or the plasma scale length is varied. Good agreement between experimental and calculated spectra makes it possible to clearly distinguish between nonrelativistic and relativistic mechanisms of harmonic generation.
authors.affiliation: Institut für Experimentelle Physik, Universität Duisburg-Essen, D-47048 Duisburg, Germany. alexander.tarasevitch@uni-due.de
authors.forename: Alexander
authors.initials: A
authors.lastname: Tarasevitch
authors.forename: Konstantin
authors.initials: K
authors.lastname: Lobov
authors.forename: Clemens
authors.initials: C
authors.lastname: Wünsche
authors.forename: Dietrich
authors.initials: D
authors.lastname: von der Linde