title: Ultrabroad Terahertz Spectrum Generation from an Air-Based Filament Plasma.
abstract: We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.
authors.forename: V A
authors.initials: VA
authors.lastname: Andreeva
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.|International Laser Center, Lomonosov Moscow State University, 119991 Leninskie gori 1/62, Moscow, Russia.
authors.forename: O G
authors.initials: OG
authors.lastname: Kosareva
authors.affiliation: International Laser Center, Lomonosov Moscow State University, 119991 Leninskie gori 1/62, Moscow, Russia.
authors.forename: N A
authors.initials: NA
authors.lastname: Panov
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.
authors.forename: D E
authors.initials: DE
authors.lastname: Shipilo
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.
authors.forename: P M
authors.initials: PM
authors.lastname: Solyankin
authors.affiliation: Institute on Laser and Information Technologies of the Russian Academy of Sciences (ILIT RAS), 140700 Shatura, Russia.
authors.forename: M N
authors.initials: MN
authors.lastname: Esaulkov
authors.affiliation: CEA-DAM, DIF, 91297 Arpajon, France.
authors.forename: P
authors.initials: P
authors.lastname: González de Alaiza Martínez
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.|International Laser Center, Lomonosov Moscow State University, 119991 Leninskie gori 1/62, Moscow, Russia.
authors.forename: A P
authors.initials: AP
authors.lastname: Shkurinov
authors.affiliation: Faculty of Physics, Lomonosov Moscow State University, 119991 Leninskie gori 1/2, Moscow, Russia.|International Laser Center, Lomonosov Moscow State University, 119991 Leninskie gori 1/62, Moscow, Russia.
authors.forename: V A
authors.initials: VA
authors.lastname: Makarov
authors.affiliation: CEA-DAM, DIF, 91297 Arpajon, France.
authors.forename: L
authors.initials: L
authors.lastname: Bergé
authors.affiliation: Département de Physique, de Génie Physique et d'Optique, Centre d'Optique, Photonique et Laser (COPL), Université Laval, Québec, QC G1V 0A6, Canada.
authors.forename: S L
authors.initials: SL
authors.lastname: Chin