title: Terahertz radiation driven by two-color laser pulses at near-relativistic intensities: Competition between photoionization and wakefield effects.
abstract: We numerically investigate terahertz (THz) pulse generation by linearly-polarized, two-color femtosecond laser pulses in highly-ionized argon. Major processes consist of tunneling photoionization and ponderomotive forces associated with transverse and longitudinal field excitations. By means of two-dimensional particle-in-cell (PIC) simulations, we reveal the importance of photocurrent mechanisms besides transverse and longitudinal plasma waves for laser intensities >10(15) W/cm(2). We demonstrate the following. (i) With two-color pulses, photoionization prevails in the generation of GV/m THz fields up to 10(17) W/cm(2) laser intensities and suddenly loses efficiency near the relativistic threshold, as the outermost electron shell of ionized Ar atoms has been fully depleted. (ii) PIC results can be explained by a one-dimensional Maxwell-fluid model and its semi-analytical solutions, offering the first unified description of the main THz sources created in plasmas. (iii) The THz power emitted outside the plasma channel mostly originates from the transverse currents.
authors.affiliation: CEA-DAM, DIF, F-91297 Arpajon, France.
authors.forename: P
authors.initials: P
authors.lastname: González de Alaiza Martínez
authors.affiliation: CEA-DAM, DIF, F-91297 Arpajon, France.
authors.forename: X
authors.initials: X
authors.lastname: Davoine
authors.affiliation: CEA-DAM, DIF, F-91297 Arpajon, France.
authors.forename: A
authors.initials: A
authors.lastname: Debayle
authors.affiliation: CEA-DAM, DIF, F-91297 Arpajon, France.
authors.forename: L
authors.initials: L
authors.lastname: Gremillet
authors.affiliation: CEA-DAM, DIF, F-91297 Arpajon, France.
authors.forename: L
authors.initials: L
authors.lastname: Bergé