Relativistic high harmonic generation in gas jet targets

Pirozhkov, A. S.; Kando, Masaki; Esirkepov, T. Z.; Gallegos, P.*; Ahmed, H.*; Ragozin, E. N.*; Faenov, A. Ya.*; Pikuz, T.; Kawachi, Tetsuya; Sagisaka, Akito; Koga, J. K.; Coury, M.*; Green, J.*; Foster, P.*; Brenner, C.*; Dromey, B.*; Symes, D. R.*; Mori, Michiaki; Kawase, Keigo*; Kameshima, Takashi*; Fukuda, Yuji; Chen, L. M.*; Daito, Izuru; Ogura, Koichi; Hayashi, Yukio; Kotaki, Hideyuki; Kiriyama, Hiromitsu; Okada, Hajime; Nishimori, Nobuyuki; Imazono, Takashi; Kondo, Kiminori; Kimura, Toyoaki*; Tajima, Toshiki*; Daido, Hiroyuki; Rajeev, P.*; McKenna, P.*; Borghesi, M.*; Neely, D.*; Kato, Yoshiaki* ; Bulanov, S. V.

We experimentally demonstrate new regime of high-order harmonic generation by relativistic-irradiance lasers in gas jet targets. Bright harmonics with both the odd and even orders are emitted in the forward direction, while the base harmonic frequency is downshifted. The harmonics are generated by linearly as well as circularly polarized pulses. With the 9 TW laser, the harmonics reach 360 eV, within the "water window" spectral region. Using 120 TW laser producing 40 uJ/sr per harmonic at 120 eV, we demonstrate the photon number scalability. The experimentally demonstrated harmonics cannot be explained by previously known mechanisms. We introduce a novel high-order harmonics generation mechanism, which explains our experimental findings.