High-order harmonic generation in gas jet targets by relativistic irradiance lasers

Pirozhkov, A. S.; Kando, Masaki; Esirkepov, T. Z.; Gallegos, P.*; Ahmed, H.*; Ragozin, E. N.*; Faenov, A. Ya.*; Pikuz, T.; Hayashi, Yukio; Kotaki, Hideyuki; Koga, J. K.; Shimomura, Takuya; Kiriyama, Hiromitsu; Neely, D.*; McKenna, P.*; Borghesi, M.*; Daido, Hiroyuki; Kondo, Kiminori; Kato, Yoshiaki* ; Bulanov, S. V.

We have recently discovered a new high-order harmonic generation mechanism based on relativistic-irradiance ( W/cm) femtosecond lasers interacting with gas jet targets. The comb-like spectra consisting of even and odd orders are generated in the forward direction by either linearly or circularly polarized pulses. In experiments with the 9 TW J-KAREN laser, the spectra extended up to the photon energy of 360 eV, within the "water window" spectral range. We demonstrated the photon number scalability with laser power, reaching photons/sr within the 1200.5 nm spectral region at 120 TW with Astra Gemini laser. In this paper we describe the results concerning angular distribution, new data obtained with J-KAREN laser, and the generation mechanism in more details.