High-order harmonics from relativistic electron spikes

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

We discovered a new high-order harmonic generation regime based on relativistic-irradiance ( 10W/cm) femtosecond lasers interacting with gas jet targets using J-KAREN laser in JAEA and Astra Gemini laser in the CLF, RAL. The comb-like spectra, consisting of several hundred even and odd harmonic orders, are generated by either linearly or circularly polarized pulses. We introduced the new harmonic generation mechanism, where the harmonics are collectively emitted by oscillating electron spikes formed at the joint of the boundaries of a cavity and bow wave created by the relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by 2D and 3D PIC simulations. We describe recent results including the angular distribution, new experimental data obtained with the J-KAREN laser, and the generation mechanism in more details.